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Surface mount manufacturing technology quality and automation
Surface mount manufacturing technology quality and automation Robert J. Rowland
Iomega Corporation, Roy, Utah, USA
1. Introduction The electronic assembly industry is basically still in its infancy. Until the last five or six years electronic assembly, for the most part, was done by hand or with dedicated equipment such as DIP inserters. As far as printed circuit assembly was concerned not much thought was given to automation. Indeed, automating a production line to manufacture printed circuit assemblies that use conventional components (DIPs, axials, radials) is a formidable task because these components were not designed for use in an automated production environment, and as a result will not readily adapt to this type of manufacturing process. Surface mount technology (SMT) will not only improve product functionality, size and quality, but it will dramatically affect the methods used to manufacture printed circuit assemblies. The primary reason for automating a production line should be to improve product quality, not reduce labor costs. If the quality of the product can be improved sustantially, a reduction in labor costs per unit will be a direct result. Surface mount technology offers advantages over conventional leaded technology from many view points; design, manufacturing and quality. This article will focus on the manufacture of surface mount printed circuit assemblies that utilize epoxy/glass substrates and standard surface mount components in medium to high volume production. 2. What are the Topics? The following areas of surface mount manufacturing technology will be addressed in this article. Printed circuit boards/substrate materials - Component terminators and leads - Component packaging Designing for manufacturability - Solder creams Screen printing - Auto placement - Solder reflow Cleaning - Rework -
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3. What is SMT? Surface mount technology utilizes micro-miniature leaded or leadless components that are soldered directly to lands on the surface of a printed circuit board. Plated-through holes on the printed circuit board are used only for feed-throughs to connect the various layers of circuitry. MICROELECTRONICS JOURNAL Vol. 17 No. 3 9 1986 Benn Electronics Publications Ltd, Luton 22
4. What are the Basic Types of SMT? Surface mount technology is divided into three basic groups. Type I - All SMT, top or top and bottom Type II - Mixed technology, top or top and bottom Type III - Conventional technology on top, SMT on bottom This paper will address the processes required to manufacture type I and type II surface mount assemblies. Type I is the preferred assembly, but due to component availability at this time a total surface mount assembly is difficult to achieve. Generally 80 to 90% of an assembly can be converted to surface mount components. Refer to Table 1 for a summary of the key features for each type. TABLE I Typel Surface m o u n t c o m p o n e n t s only Requires solder paste Reflow soldering - High to very high population density Single or double sided population -
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Type 11 Surface m o u n t and conventional c o m p o n e n t s Requires solder paste Reflow soldering Wave soldering Moderate to high population density - Passive S M C on bottom or top - Active SMC on top Conventional c o m p o n e n t s on top - May require adhesive
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Type III Surface m o u n t passive and conventional c o m p o n e n t s W a v e soldering - Requires adhesive - Passive S M C on bottom Conventional c o m p o n e n t s on top - Moderate population density
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5. Why Use SMT? Surface mount technology offers many advantages over conventional leaded technology. The more prominent manufacturing advantages of SMT are: -
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Components are designed for automation SMT can be easily automated Decreased capital investment Reduced labor cost Less manufacturing floor space required Sequenced reels not required Component preparation not required Holes for leads not required Lower material/component costs
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Not all of these advantages can be realized today. Some may not be obtainable until the end of this decade, but when dealing with a new technology one must have the foresight to see that new technology is not implemented overnight. Research must be done to ensure that the major problems are solved before production begins. This research takes time, but take the time to do the job right the first time. Generally it is more cost effective to produce printed circuit assemblies today with conventional leaded technology, but in the near future surface mount technology will be the preferred method of manufacturing printed circuit assemblies.j
6. Concerns about SMT Surface mount technology has many advantages. But, at this point in the technology's development there are problems that must be noted and understood. - Component cost/availability - Substrate/component thermal management Coplanarity of components and substrate - Rework techniques Solder joint strength - Component standardization -
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Each of these concerns is a problem that can be overcome, but it is extremely important that each one of them is understood before production begins. Surface mount technology still needs to evolve and mature as a technology. Many areas need to be improved, but during the short time the technology has been considered for commercial applications it has evolved at a tremendous rate.
7. Team Concept Surface mount technology, as with any technology, should be addressed with a team approach. Each group that is involved with the production of a printed circuit assembly must be involved from the day the first circuit path is drawn and at all times during production to ensure that the product is designed with good manufacturability and testability concepts. Manufacturing Engineers must understand what a design engineer needs to do to make a functional design; design engineers must understand manufacturing concepts and how their designs are affected by them. If a product is not designed so it can be easily manufactured and tested it will not be able to compete in today's most cost competitive marketplace. This team must have management support and the freedom and resources do do what is necessary to complete a research program to study and understand all areas of SMT. Problems should be identified and solved before any production is allowed. Do not believe what others say, be a sceptic. Prove to yourself that you are using sound engineering concepts before you begin production.
8. Printed Circuit Boards (PCB) Surface mount printed circuit boards are different from conventional printed circuit boards in several areas. The major features of a surface mount printed circuit board are: - Fewer plated through holes required - Holes are much smaller - Generally fewer layers of circuitry required Track width reduced (0.008 inch maximum) - Track spacing required (0.0085 inch maximum) - Land areas present for mounting SMC
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Many substrate materials are available to manufacture surface mount printed circuit boards, as noted in Table 2. Epoxy/glass is the most popular and common material. It is a suitable material if certain rules are understood and followed. Epoxy/Glass
Substrates
A d v a n t a g e s - Cost effective - G o o d material availability - Proven technology Concerns
- Incorrect T C E for leadless ceramic components - Warpage (causes coplanarity problems)
The biggest problem with epoxy/glass is the thermal mismatch with ceramic. Ceramic has a T C E of 6 t o 8 ppm per degree C and epoxy/glass has a T C E of 14 to 18 ppm per degree C . This mismatch can create extreme pressure on the solder joint when the system is at operating temperature. Do not use leadless ceramic chip carriers or ceramic resistors and capacitors larger than 0.400 inch per side on epoxy/glass substrates.
LCC
8 r'M/~ { CEI LCC-,...~ ~ ~ _v~.,,,~L~
SOLDERJOINT
\ PCB PCB (EPOXY/GLASS} (16 PPM/~ (TCEI Solder mask over bare copper is the preferred method for producing surface mount PCBs. Plated traces can cause solder shorts and bridges during the reflow operation. Also, if the plating expands during reflow it can supply enough pressure to a chip resistor or capacitor to crack it. All of the materials listed in Table 2 can be used to manufacture surface mount printed circuit boards, however cost and availability is a major concern with most of them. TABLE II Substrate Properties Glass Transition Temperature (degree C)
XY Coefficient
Type of P/I Structure Printed Boards . Epoxy Fiberglass Polyimide Fiberglass Epoxy Aramid Fiber Polyimide Aramid Fiber Epoxy Quartz (Fused Silica) Polyimide Quartz (Fused Silica) Fiberglass/Teflon Laminates
125 250 125 250 125 250 75
14 to 18 12to 16 6to8 5to8 6to 12 6to 12 20
of Thermal Expansion (PPM per degree C)
Moisture Absorption (perccnt) 0.10 0.35 0.35 !.50
TBD 0.50 1.1(}
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9. Components Surface mount components are being standardized by the electronics industry. Several common families now exist. No attempt will be made here to discuss surface mount component package types, other than to note what type of package has been evaluated. The important area for the manufacturing engineer to note about surface mount components is the type of lead and terminator configuration being used. The following surface mount components have been utilized. -
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Rectangular chip Small outline transistor (SOT) Small outline (SO) (IC) Small outline large (SOL) (IC) Plastic chip carrier (PCC) Leadless ceramic chip carrier (LCC) Leaded ceramic chip carrier (CCC) Metal electrode face bonding (MELF)
The terminator on a rectangular chip can be manufactured in several ways. Only rectangular chips that use nickel barrier technology should be used. T o produce a terminator silver is deposited onto the ceramic, then the silver is covered with nickel, then a cover of solder, palladium silver or tin is applied. The nickel barrier prevents the silver from leaching away from die ceramic and into the solder joint. Observe the following rules for rectangular chips. Must use nickel barrier technology - Solder Plate, 60Snl40Pb, 63Sn/37Pb or 62Sn/36Pb/2Ag (or) Tin plate, 90Snll0Pb or 95Sn/5Pb (or) Palladium - silver plate Plated terminations only, no dipped terminators
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SILVERlAg) MINATION LAYER . ~ ~ ~ ~ / , , ~ , ~ TERMINATION
~,\\\\\\\\\\\\\\\\\\\\~
- ~
Z_ P.C.SOARD
CHIPSECTION
NICKEl.(Ni) then TIN (Sn) PLATING ORSOLDER(SN62)
Two types of lead configurations are used for the integrated circuit and transistor/diode packages. The first is known as the "gull wing". It is used on the SO, SOL and S O T packages. The second lead configuration is the ".1" lead, so called because it resembles a "J". It is used on the PCC.
CB~
r P
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SO
t_ /GULL-WING~. ~---SOT
h - U ,,SOLDER,.
For additional information on surface motmt components refcr to the following documents. EIA-RS-396, Fixed Film Microelcment EIA-RS-460, Fixed Film P,esistors - EIA-RS-198C, Ceramic Dielectric Capacitors - EIA-RS-228B, Fixed Tantalum Capacitors E I A - J E D E C Publication 95, Integrated Circuits -
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F
J-BEND
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SOLDER
P C ~
LAND
P,,\\\\\\\\\\\
10. Component Packaging Component packaging is used to supply components to the assembly area in an efficient low cost manner. Surface mount components are packaged in one of three methods. Bulk Stick - T a p e and reel
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Bulk packaging is only suitable for low volume manual assembly. For automated assembly a more controlled method of component handling is required, such as the tube or the tape and reel. Sticks (plastic tubes) are uscd to package almost any component type. They are well suited for medium volume production and will support automated equipment. They are similar to DIP sticks. Tape and reel is the preferred packaging method for supplying components to a high volume automated environment. Two types of tape are used; a punched paper and an embossed plastic. The rectangular chip, SO, SOL, PCC and SOT are or will be available in tape and reel. The paper tape is of a pressed construction. Cavities for the components are punched into the paper as well as sprocket holes to align and drive the tape. A transparent mylar tape is placed on both sides of the paper to contain the component. One problem that arises with paper tape is contamination from paper fibres. These fibres can cause problems with mechanical equipment as well as printed circuit assemblies. The plastic tape is embossed with cavities for each component and sprocket holes are punched to align and drive the tape. A transparent mylar tape is placed over the open cavities to contain the components. Embossed plastic is preferred over punched paper, however both are acceptable. Tape is or will be available in the following widths. 8mm Punched paper or embossed plastic - 12mm Punched paper or embossed plastic - 16mm Embossed plastic only 24mm Embossed plastic only - 3 2 m m Embossed plastic only - 44mm Embossed plastic only - 56mm Embossed plastic only -
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(For additional information on taping of surface mount components refer to EIA-481-A.) Component packaging is an important-key to reliable high volume automated placement. It must deliver a continual supply of the correct component to a precise location in the correct orientation.
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1 1 .
M a n u f a e t u r a b i l i t y
Manufacturability is the single most important concept to consider when starting an SMT project. If the printed circuit assembly is not designed to take advantage o f your manufacturing processes you have failed before you start. G o n e is the day when a P C A was designed and then handed over the wall to manufacturing. Design and manufacturing must work together as a team is us important as functionality " , If you can-, 1 . at the start of a layout " Manufacturability . . . . , not bmld your system at a competing price ~t does not matter how much functlonahty ~t has, ~t probably will not sell. Some manufacturability ideas to consider are: -
PCB size Trace width, spacing Land geometry* PCB panel size Trace routing C o m p o n e n t layout Pad size - Hole size - Solder mask type C o m p o n e n t standardization Substrate material*
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Develop a manufacturability handbook that each product designer can use as a reference. W o r k with the product designer. It is important that he/she understands why we need and how important manufacturability is.
12. Why Automate? Generally, most companies automate to reduce labor costs. But, consider this, automate to improve product quality. If, by automating, quality can be improved m a n y direct advantages wilt be achieved, including reduction of labor costs. Automating to improve product quality usually does improve product quality. Automating to reduce labor costs does not necessarily improve product quality.
13. Solder Creams Solder joints form the only means of mechanical and electrical connection of the c o m p o n e n t to the substrate. Determining the correct solder cream is critica ~. The most common alloys for printed circuit assemblies are 62Sn/36Pb/2Ag,63Sn/37Pb and 60Sn/40Pb. When considering which solder cream to select the following questions should be addressed: Particle size Maximum t e m p e r a t u r e during solder reflow - Vehicles Metal content - Flux - H o w will solder cream be applied
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After a solder cream has been selected apply it to a substrate and inspect for the following: -
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Slump - A good solder cream should maintain its screened height Screenability - The solder cream should screen easily and cover the land Adhesion - The solder cream should accept and hold the SMC in place Reflow - The solder cream should not solder ball during reflow C l e a n i n g - The flux residue should be easy to remove with standard cleaning agents
The alloy 62Sn/36Pb/2Ag is tile most common solder cream for surface mounting. The 2% Ag helps to prevent the Ag from leaching from the terminator and into the solder joint, thus weakening the terminator. However, with the advent of the nickel barrier terminator the 2% Ag may not be needed and a standard 60Sn/40Pb solder cream could be used. We have not yet determined whether this is true. Solder cream can be refrigerated (2 to 5 degrees C) to prevent the flux and vehicles from separating from the metal powder. However, the solder cream can be contaminated by moisture if refrigerated. Allow the entire container of solder cream to reach room temperature before opening to prevent condensation on the solder cream. As an alternative to refrigeration, mix the solder cream well before using. Solder creams generaly contain 80 to 90% metal by weight. A solder cream of 85 to 90% metal would be the preferred choice for screen printing. Flux systems are available in the following groups: R -RMA - RA SA -
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Not active Mildly active Active Synthetic activated (limited availability)
An R M A flux should be suitable for most requirements. It is active enough to clean the land, can be easily cleaned and if not all is removed during cleaning the danger of it affecting the PCA is minimal. A note of advice: do a complete chemical analysis on your solder cream so you know what it contains.
14. Surface Mount Manufacturing Techniques The following flow chart illustrates the process required to manufacture a Type I surface mount assembly. The Type II process is similar, except the conventional components are assembled to the printed circtfit board after the surface mount operation is complete.
15. Screen Printing Two methods are generally used to apply solder cream to a substrate in a high volume production environment, screening or stenciling.'Both utilize a standard screen printer. The first method, screening, is done by forcing the solder cream through a patterned screen located directly above the substrate. A squeegee moves over the surface of the screen and deflects the screen to touch the substrate. The solder cream is forced through openings in the screen and deposits itself by surface tension. The screen snaps away from the substrate when not in contact with the squeegee.
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Wetdeposits up to 0.010 inch can be achieved with screens. The following screen will be acceptable for most screening applications. Wet thickness Material Mesh Mesh thickness Open area Emulsion build-up Wire diameter
0.008 inch Stainless steel 80, Plain weave 0.004 inch 70.5% 0.005 inch 0.220 inch
To determine print thickness use the following: Print thickness Where:
T W = (TM • A O ) + T E T W = Wet print thickness TM = Mesh thickness A O = % Open area, decimal T E = Emulsion build-up thickness
If a thicker wet solder joint or longer life is desired, a stencil will be a more effective approach. Metal stencils are made by chemically etching away the metal to form the required pattern. Application of solder cream is similar to methods used for screens. Stencils cost more than screens, but the added life may make this a more cost effective approach for high volume production runs. Stencils will also deposit the solder paste more consistently than a screen. Screen printers that can apply solder creams are manufactured by many companies. When selecting a screen printer some areas to consider are required print area, substrate size, cycle time and the level of automation desired.
16. Placement Systems Automatic placement of surface mounted components is the key to efficient production of printed circuit assemblies. One machine (using several different pick-up heads) can place almost all surface mounted components. Automatic placement machines utilize a vacuum pickup system tn remove the component from its packaging and transfer it to the substrate. The system consists of a vacuum tube (which picks up the component) and alignment jaws which orientate the component. The vacuum tube is narrower than the component and the alignment jaws retract before the component is placed on the substrate, so the tooling does not interfere with neighboring components as is the case with conventional insertion equipment. As a result, population densities can be greatly increased. Also, complex cut and clinch tooling is not required which reduces the cost of the eqipment and eliminates a system that requires extensive maintenance. Placement speed is faster because the cut and clinch operation is eliminated. Automated placement equipment is divided into four categories. -
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In-line placement Sequential placement Simultaneous placement Sequential/simultaneous placement
In-line placement utilizes a group of fixed placement heads. Each head places a component on tile substrate as it passes down the line. Cycle times are from 1.8 to 4.5 seconds per assembly. Sequential placement utilizes a software controlled X-Y table and one or two placement heads. Components are placed on the substrate in sequence, much the same as a conventional 30
DIP inserter. Cycle times are from 0.3 to 1.8 seconds per component. Simultaneous placement equipment places all components on a substrate at one time utilizing one head per component. Cycle times are from 7.0 to 10.0 seconds per assembly. Sequential/simultaneous placement utilizes a software controlled X-Y table with multiple heads placing components in sequence. Cycle time is about 0.2 seconds per component. Many factors govern the selection of automated placement systems. - Number of assemblies per year - Number of different assemblies produced Lot size Dimensions of the assembly - Number of components per assembly Packaging format - Component types -
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Each of these factors must be carefully evaluated before selecting an automated placement system. 17. Solder Cream Precure Several years ago it was a necessity to precure (bake) a solder cream before reflow, especially with a vapour phase reflow system, to eliminate the volatiles. Solder creams have improved to the state where a precure may not be required. This varies from manufacturer to manufacturer, so the end user will have to determine if this operation is still required. The main reasons for a precure are: - Eliminates volatiles from solder cream - Adheres components to lands - Reduces slumping of solder cream Almost any method of heating can be used to achieve a precure. An infrared dryer is the most efficient method. 18. Solder Reflow Vapour phase reflow and infrared are the best methods available to achieve solder reflow on large epoxy/glass substrates. Vapour phase soldering is the most popular method of reflowing soi'ter cream. It utilizes a fluid that vaporizes above the melting temperature of the solder cream. The vapor condenses on the entire assembly when it is placed in the system, quickly elevating it to the correct temperature. This instantaneous heating of the assembly eliminates the need for a preheat zone, as is required with wave soldering. However, a preheat zone may be required to prevent thermal shock to some components. This method does limit the amount of thermal shock to the substrate. T h r e e types of vapour phase system are available. - Dual vapor, batch - Single vapor, batch In-line (single and dual vapor) -
The dual and single vapor batch systems are not recommended for high volume production. The in-line vapor phase system is best stilted for high volume production. The most common vapor phase (primary) fluid is fluorinert. It is available in the following temperature ranges. FC43, boils at I74 degrees C FC70, FC5311, boils at 215 degrees C FC71, boils at 253 degrees C
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NOTE: FC5311 is replacing FC70. FC5311 has improved thermal stability and as a result has reduced levels of trace decomposition products. Infrared ovens with multiple heat zones are available and can be utilized to reflow solder cream. In the past there were problems associated with IR reflow. But, during the past two years major improvements in IR systems have been achieved. Today IR is an acceptable method to achieve solder reflow.
Comparison of Vapor Phase and Infrared Systems (In-line)
Technique
Atmosphere Control
Mininization of Time
Process Control
Double Sided Assembly
Cleaning Ease
Volume Production Capability
Vapour Phase
Yes
Excellent
Excellent
Yes
Excellent
Excellent
Infrared
Yes
Fair"
Fair
Yes
Poor
Good
19. Cleaning The cleaning process to be used will be determined by the types of surface mount components populating the assembly and the type of flux system used. The printed circuit assembly should be cleaned immediately after the solder reflow operation. The longer the flux remains on the assembly the more difficult it will be to remove. The overall amount of flux on an assembly that has been reflowed using solder cream is less than on an assembly that has been wave soldered, however the amount of flux next to the solder joint is greater. A surface mount assembly populated with rectangular chips, MELF, SOT, SO, SOL and PCC packages can be cleaned very well with an in-line solvent system and possibly with an inline aqueous system. More diffcult components, such as the LCC, will require an in-line immersion solvent,system to remove the flux from under the component. Remember also, the next generation of surface mount components will probably be based on a lead pitch of 0.025 inch. This will require an aggressive cleaning system, so purchase a cleaner that will also meet your future needs.
20. Rework Rework techniques and equipment for the removal and replacement of surface mount components are still being developed. With the correct equipment any surface mount component can be reworked easier than its leaded equivalent. All the leads on the surface mount component must be reflowed at the same time to facilitate removal of the component. Two techniques can be used to remove surface mount components. The first, physical contact, reflows the solder by transferring energy from a rework tool to the component lead and solder joint. The problem with this approach is that physical contact must be made with each component lead and solder joint. This can be difficult, especially with the larger PCC packages. The second, and preferred method, is the use of directed hot air to achieve reflow of the solder joint. This method involves surrounding the surface mount component with a collet that will direct hot air onto the solder joint. The hot air will reflow all of the solder joints at once and allow easy removal of the surface mount component. Be aware that too much heating of the component over an extended period of time could damage the component. Also, hot air generates static electricity which could damage the component.
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21. Conclusion Utilizing surface mounted device technology, a more efficient method of producing printed circuit assemblies can be developed. Before implementing surface mount technology be sure you understand the advantages and disadvantages and use the advantages to your advantage.
22. Glossary of Surface M o u n t Terms
Automatic Placement- In surface mounting an a u t o m a t e d system which removes the SMC from its packaging and places it on the surface of the substrate.
Bond S t r e n g t h - In wire bonding, the pull force at rupture of the bond interface measured in the unit gram force.
Bridging - Solder shorting of section that should be open between two or more conductors and/or c o m p o n e n t terminals. Burr- A fragment of excess material attached to a lead. Chase - T h e frame used to hold and support a screen or stencil. Cleaning - The art of removing flux and other contaminates from a printed circuit assembly. Conductor Line Width (Trace) - T h e minimum conductor width on the finished PCB. Conductor Spacing - The minimum space between conductors on a PCB. F e e d - T h r o u g h (Via) - A hole drilled through a series of pads which is then plated through. It is used to connect a conductor from one layer to another. A feed through does not have a c o m p o n e n t lead installed through the holes. F l u o r i n e r t - A chemicl used in wlpor phase reflow systems. It is heated to its boiling point at which time it produces a v a p o r that is at a t e m p e r a t u r e above the melting point of the solder. The assembly is placed into the v a p o r at which time the solder reflows to form the mechanical joint. Flux - Used to clean a contaminated metal surface during soldering operation. F o o r p r i n t - The area on a printed circuit board used for mounting a surface mounted component. Gull Wing Lead- A lead that exits the body of an S O / S O L package parallel to the b o t t o m of the package, then forms a right angle bend which has the lead formed so it is parallel with the surface of the land it is to be mounted on. H e a t D i s s i p a t i o n S u r f a c e - A surface area on a PCB used to dissipate excess heat. This surface area does not have a c o m p o n e n t mounted on it. Infrared T e c h n o l o g y - A system used to dry or reflow solder creams using infrared energy. J - L e a d - A lead used primarily on the plastic chip carrier. It exits the side of the package and is then bent down the side and under the package. So called because the lead resembles a capital "J". Land - The surface area on a PCB onto which the lead of a surface mount component is placed and then soldered in place. Lands do not have holes drilled through them. Land/Pad Layer- A printed circuit board in which only lauds and pads are on the two outer layers. All of the circuitry is on the inner Payers. This protects fine line circuitry from handling damage. Leaching - In soldering, the dissolving of the material to be soldered into the molten solder. Leaded Ceramic Chip Carrier (CCC) - Same as the leadless ceramic chip carrier, but leads have been attached to allow mounting on PCB that do not have a T C E that matches ceramic (such as epoxy/glass).
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Leadless Ceramic Chip Carrier (LCC) - A surface mount package that contains an integrated circuit. The leads are part of and flush with the bottom side of the chip carrier. The package material is ceramic. M a n u f a e t u r a b i l i t y - T h e art of designing a product so that it can be efficiently manufactured using available manufacturing technology. Melf - Metal Electrode Face Bonding. A cylindrical leadless component with end metalliq terminals. It is a popular package for fixed film resistors. Microelectronics - That area of electronic technology associated with electronic systems that use extremely small electronic components or elements. Pad - T h e surface area on a PCB in which a hole is drilled and plated through. Pitch - The distance between a point and an adjacent point on the same item. Plastic Chip Carrier {PCC) - A surface mount package that contains an integrated circuit. It has leads on all four sides and is either rectangular or square. Leads are usually on a fifty mil pitch. PPM - Parts per million. How many parts per million are defective. Determined by multiplying the percent defective by 10,000. R e c t a n g u l a r Chip - A surface mount resistor or capacitor. It is made from ceramic, an inorganic non-metallic material. S c r e e n - A contact print method for applying solder cream to a substrate. Screens, usually made from stainless steel, are covered with an emulsion that has open areas that match the land geometry on the substrate. The solder cream is forced through these openings onto tile substrate. S c r e e n P r i n t i n g - In surface mounting, the art of applying solder cream to a substrate surface using a screen printing process. Small Outline {SO) - A surface mount package that contains an integrated circuit. It is a rectangular package with leads on two sides, usually on a pitch of fifty mils and a body width of 0.150 inch. Package material is plastic. Small Outline Large {SOL) - Same as the small outline (SO) but with a body width of 0.300 inch to accommodate larger die. Small Outline T r a n s i s t o r {SOT) - A surface mount package that contains transistors and diodes. It is a rectangular package that has either three or four leads. Package material is plastic. S M C - Surface Mount Component. Any component that is mounted on the surface of a PCB. SMD - Surface Mounted Device. Any device that is mounted on the surface of a PCB. S M T - Surface Mount Technology. The technology in which components are mounted on the surface of a PCB. Solder Balls- Small balls of solder formed during the reflow process. High concentrations of oxides and contaminants on the solder powders surface leads to the generation of solder balls. Solder C r e a m - A homogenous mixture of a powdered metal alloy, a flux system and a viscosity modifier. During a solder reflow operation the metal alloy melts to form the solder joint, while the viscosity modifier evaporates. The flux is removed during a cleaning operation. S o l d e r Mask - A chemical system which forms a protective cover over the circuitry on a printed circuit board.
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Stencil - Metal stencils are used in the same method as a screen. The openings are made by chemically milling the land geometry into a thin metal such as brass, nickel or stainless steel. Stencils are used to apply a thicker deposit of solder cream. They also wear better than screens, and thus last longer. S u r f a c e M o u n t A s s e m b l y (SMA) - A printed circuit assembly that is populated with a majority of surface mount components. S u r f a c e T e n s i o n - During the solder reflow process, while the solder is in the liquidus state, the surface mount component will center itself on the land due to the surface tension of the solder (if the land was designed correctly). T e r m i n a t o r - Part of the interconnect structure of the rectangular chip. It consists of a layer of silver (forms a mechanical bond with the ceramic), then a nickel barrier (to prevent leaching of silver away from the ceramic during reflow) and then a solderable plated surface (tin, solder alloy, palladium-silver). T e s t P r o b e N o d e - Any pad, land or exposed conductor that would allow access for probing. T e s t a b i l i t y - The art of designing a product so that it can be efficiently tested on existing test equipment. Thermal Coefficient of Expansion (TCE) -The fractional change in the dimensions or volume of a material per unit in change of temperature. Vapor Phase Reflow - A system used to reflow solder creams. It utilizes a gas (vapour) that is at a temperature above the melting point of the solder alloy being used.
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Iomega Corporation, Roy, Utah, USA
1. Introduction The electronic assembly industry is basically still in its infancy. Until the last five or six years electronic assembly, for the most part, was done by hand or with dedicated equipment such as DIP inserters. As far as printed circuit assembly was concerned not much thought was given to automation. Indeed, automating a production line to manufacture printed circuit assemblies that use conventional components (DIPs, axials, radials) is a formidable task because these components were not designed for use in an automated production environment, and as a result will not readily adapt to this type of manufacturing process. Surface mount technology (SMT) will not only improve product functionality, size and quality, but it will dramatically affect the methods used to manufacture printed circuit assemblies. The primary reason for automating a production line should be to improve product quality, not reduce labor costs. If the quality of the product can be improved sustantially, a reduction in labor costs per unit will be a direct result. Surface mount technology offers advantages over conventional leaded technology from many view points; design, manufacturing and quality. This article will focus on the manufacture of surface mount printed circuit assemblies that utilize epoxy/glass substrates and standard surface mount components in medium to high volume production. 2. What are the Topics? The following areas of surface mount manufacturing technology will be addressed in this article. Printed circuit boards/substrate materials - Component terminators and leads - Component packaging Designing for manufacturability - Solder creams Screen printing - Auto placement - Solder reflow Cleaning - Rework -
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3. What is SMT? Surface mount technology utilizes micro-miniature leaded or leadless components that are soldered directly to lands on the surface of a printed circuit board. Plated-through holes on the printed circuit board are used only for feed-throughs to connect the various layers of circuitry. MICROELECTRONICS JOURNAL Vol. 17 No. 3 9 1986 Benn Electronics Publications Ltd, Luton 22
4. What are the Basic Types of SMT? Surface mount technology is divided into three basic groups. Type I - All SMT, top or top and bottom Type II - Mixed technology, top or top and bottom Type III - Conventional technology on top, SMT on bottom This paper will address the processes required to manufacture type I and type II surface mount assemblies. Type I is the preferred assembly, but due to component availability at this time a total surface mount assembly is difficult to achieve. Generally 80 to 90% of an assembly can be converted to surface mount components. Refer to Table 1 for a summary of the key features for each type. TABLE I Typel Surface m o u n t c o m p o n e n t s only Requires solder paste Reflow soldering - High to very high population density Single or double sided population -
-
-
-
Type 11 Surface m o u n t and conventional c o m p o n e n t s Requires solder paste Reflow soldering Wave soldering Moderate to high population density - Passive S M C on bottom or top - Active SMC on top Conventional c o m p o n e n t s on top - May require adhesive
-
-
-
-
-
-
Type III Surface m o u n t passive and conventional c o m p o n e n t s W a v e soldering - Requires adhesive - Passive S M C on bottom Conventional c o m p o n e n t s on top - Moderate population density
-
-
-
5. Why Use SMT? Surface mount technology offers many advantages over conventional leaded technology. The more prominent manufacturing advantages of SMT are: -
-
-
-
-
-
-
-
-
Components are designed for automation SMT can be easily automated Decreased capital investment Reduced labor cost Less manufacturing floor space required Sequenced reels not required Component preparation not required Holes for leads not required Lower material/component costs
2
3
Not all of these advantages can be realized today. Some may not be obtainable until the end of this decade, but when dealing with a new technology one must have the foresight to see that new technology is not implemented overnight. Research must be done to ensure that the major problems are solved before production begins. This research takes time, but take the time to do the job right the first time. Generally it is more cost effective to produce printed circuit assemblies today with conventional leaded technology, but in the near future surface mount technology will be the preferred method of manufacturing printed circuit assemblies.j
6. Concerns about SMT Surface mount technology has many advantages. But, at this point in the technology's development there are problems that must be noted and understood. - Component cost/availability - Substrate/component thermal management Coplanarity of components and substrate - Rework techniques Solder joint strength - Component standardization -
-
Each of these concerns is a problem that can be overcome, but it is extremely important that each one of them is understood before production begins. Surface mount technology still needs to evolve and mature as a technology. Many areas need to be improved, but during the short time the technology has been considered for commercial applications it has evolved at a tremendous rate.
7. Team Concept Surface mount technology, as with any technology, should be addressed with a team approach. Each group that is involved with the production of a printed circuit assembly must be involved from the day the first circuit path is drawn and at all times during production to ensure that the product is designed with good manufacturability and testability concepts. Manufacturing Engineers must understand what a design engineer needs to do to make a functional design; design engineers must understand manufacturing concepts and how their designs are affected by them. If a product is not designed so it can be easily manufactured and tested it will not be able to compete in today's most cost competitive marketplace. This team must have management support and the freedom and resources do do what is necessary to complete a research program to study and understand all areas of SMT. Problems should be identified and solved before any production is allowed. Do not believe what others say, be a sceptic. Prove to yourself that you are using sound engineering concepts before you begin production.
8. Printed Circuit Boards (PCB) Surface mount printed circuit boards are different from conventional printed circuit boards in several areas. The major features of a surface mount printed circuit board are: - Fewer plated through holes required - Holes are much smaller - Generally fewer layers of circuitry required Track width reduced (0.008 inch maximum) - Track spacing required (0.0085 inch maximum) - Land areas present for mounting SMC
-
24
Many substrate materials are available to manufacture surface mount printed circuit boards, as noted in Table 2. Epoxy/glass is the most popular and common material. It is a suitable material if certain rules are understood and followed. Epoxy/Glass
Substrates
A d v a n t a g e s - Cost effective - G o o d material availability - Proven technology Concerns
- Incorrect T C E for leadless ceramic components - Warpage (causes coplanarity problems)
The biggest problem with epoxy/glass is the thermal mismatch with ceramic. Ceramic has a T C E of 6 t o 8 ppm per degree C and epoxy/glass has a T C E of 14 to 18 ppm per degree C . This mismatch can create extreme pressure on the solder joint when the system is at operating temperature. Do not use leadless ceramic chip carriers or ceramic resistors and capacitors larger than 0.400 inch per side on epoxy/glass substrates.
LCC
8 r'M/~ { CEI LCC-,...~ ~ ~ _v~.,,,~L~
SOLDERJOINT
\ PCB PCB (EPOXY/GLASS} (16 PPM/~ (TCEI Solder mask over bare copper is the preferred method for producing surface mount PCBs. Plated traces can cause solder shorts and bridges during the reflow operation. Also, if the plating expands during reflow it can supply enough pressure to a chip resistor or capacitor to crack it. All of the materials listed in Table 2 can be used to manufacture surface mount printed circuit boards, however cost and availability is a major concern with most of them. TABLE II Substrate Properties Glass Transition Temperature (degree C)
XY Coefficient
Type of P/I Structure Printed Boards . Epoxy Fiberglass Polyimide Fiberglass Epoxy Aramid Fiber Polyimide Aramid Fiber Epoxy Quartz (Fused Silica) Polyimide Quartz (Fused Silica) Fiberglass/Teflon Laminates
125 250 125 250 125 250 75
14 to 18 12to 16 6to8 5to8 6to 12 6to 12 20
of Thermal Expansion (PPM per degree C)
Moisture Absorption (perccnt) 0.10 0.35 0.35 !.50
TBD 0.50 1.1(}
25
9. Components Surface mount components are being standardized by the electronics industry. Several common families now exist. No attempt will be made here to discuss surface mount component package types, other than to note what type of package has been evaluated. The important area for the manufacturing engineer to note about surface mount components is the type of lead and terminator configuration being used. The following surface mount components have been utilized. -
-
-
-
Rectangular chip Small outline transistor (SOT) Small outline (SO) (IC) Small outline large (SOL) (IC) Plastic chip carrier (PCC) Leadless ceramic chip carrier (LCC) Leaded ceramic chip carrier (CCC) Metal electrode face bonding (MELF)
The terminator on a rectangular chip can be manufactured in several ways. Only rectangular chips that use nickel barrier technology should be used. T o produce a terminator silver is deposited onto the ceramic, then the silver is covered with nickel, then a cover of solder, palladium silver or tin is applied. The nickel barrier prevents the silver from leaching away from die ceramic and into the solder joint. Observe the following rules for rectangular chips. Must use nickel barrier technology - Solder Plate, 60Snl40Pb, 63Sn/37Pb or 62Sn/36Pb/2Ag (or) Tin plate, 90Snll0Pb or 95Sn/5Pb (or) Palladium - silver plate Plated terminations only, no dipped terminators
-
-
SILVERlAg) MINATION LAYER . ~ ~ ~ ~ / , , ~ , ~ TERMINATION
~,\\\\\\\\\\\\\\\\\\\\~
- ~
Z_ P.C.SOARD
CHIPSECTION
NICKEl.(Ni) then TIN (Sn) PLATING ORSOLDER(SN62)
Two types of lead configurations are used for the integrated circuit and transistor/diode packages. The first is known as the "gull wing". It is used on the SO, SOL and S O T packages. The second lead configuration is the ".1" lead, so called because it resembles a "J". It is used on the PCC.
CB~
r P
26
SO
t_ /GULL-WING~. ~---SOT
h - U ,,SOLDER,.
For additional information on surface motmt components refcr to the following documents. EIA-RS-396, Fixed Film Microelcment EIA-RS-460, Fixed Film P,esistors - EIA-RS-198C, Ceramic Dielectric Capacitors - EIA-RS-228B, Fixed Tantalum Capacitors E I A - J E D E C Publication 95, Integrated Circuits -
~
F
J-BEND
-
-
SOLDER
P C ~
LAND
P,,\\\\\\\\\\\
10. Component Packaging Component packaging is used to supply components to the assembly area in an efficient low cost manner. Surface mount components are packaged in one of three methods. Bulk Stick - T a p e and reel
-
-
Bulk packaging is only suitable for low volume manual assembly. For automated assembly a more controlled method of component handling is required, such as the tube or the tape and reel. Sticks (plastic tubes) are uscd to package almost any component type. They are well suited for medium volume production and will support automated equipment. They are similar to DIP sticks. Tape and reel is the preferred packaging method for supplying components to a high volume automated environment. Two types of tape are used; a punched paper and an embossed plastic. The rectangular chip, SO, SOL, PCC and SOT are or will be available in tape and reel. The paper tape is of a pressed construction. Cavities for the components are punched into the paper as well as sprocket holes to align and drive the tape. A transparent mylar tape is placed on both sides of the paper to contain the component. One problem that arises with paper tape is contamination from paper fibres. These fibres can cause problems with mechanical equipment as well as printed circuit assemblies. The plastic tape is embossed with cavities for each component and sprocket holes are punched to align and drive the tape. A transparent mylar tape is placed over the open cavities to contain the components. Embossed plastic is preferred over punched paper, however both are acceptable. Tape is or will be available in the following widths. 8mm Punched paper or embossed plastic - 12mm Punched paper or embossed plastic - 16mm Embossed plastic only 24mm Embossed plastic only - 3 2 m m Embossed plastic only - 44mm Embossed plastic only - 56mm Embossed plastic only -
-
(For additional information on taping of surface mount components refer to EIA-481-A.) Component packaging is an important-key to reliable high volume automated placement. It must deliver a continual supply of the correct component to a precise location in the correct orientation.
27
1 1 .
M a n u f a e t u r a b i l i t y
Manufacturability is the single most important concept to consider when starting an SMT project. If the printed circuit assembly is not designed to take advantage o f your manufacturing processes you have failed before you start. G o n e is the day when a P C A was designed and then handed over the wall to manufacturing. Design and manufacturing must work together as a team is us important as functionality " , If you can-, 1 . at the start of a layout " Manufacturability . . . . , not bmld your system at a competing price ~t does not matter how much functlonahty ~t has, ~t probably will not sell. Some manufacturability ideas to consider are: -
PCB size Trace width, spacing Land geometry* PCB panel size Trace routing C o m p o n e n t layout Pad size - Hole size - Solder mask type C o m p o n e n t standardization Substrate material*
-
-
-
-
-
Develop a manufacturability handbook that each product designer can use as a reference. W o r k with the product designer. It is important that he/she understands why we need and how important manufacturability is.
12. Why Automate? Generally, most companies automate to reduce labor costs. But, consider this, automate to improve product quality. If, by automating, quality can be improved m a n y direct advantages wilt be achieved, including reduction of labor costs. Automating to improve product quality usually does improve product quality. Automating to reduce labor costs does not necessarily improve product quality.
13. Solder Creams Solder joints form the only means of mechanical and electrical connection of the c o m p o n e n t to the substrate. Determining the correct solder cream is critica ~. The most common alloys for printed circuit assemblies are 62Sn/36Pb/2Ag,63Sn/37Pb and 60Sn/40Pb. When considering which solder cream to select the following questions should be addressed: Particle size Maximum t e m p e r a t u r e during solder reflow - Vehicles Metal content - Flux - H o w will solder cream be applied
-
-
-
After a solder cream has been selected apply it to a substrate and inspect for the following: -
28
Slump - A good solder cream should maintain its screened height Screenability - The solder cream should screen easily and cover the land Adhesion - The solder cream should accept and hold the SMC in place Reflow - The solder cream should not solder ball during reflow C l e a n i n g - The flux residue should be easy to remove with standard cleaning agents
The alloy 62Sn/36Pb/2Ag is tile most common solder cream for surface mounting. The 2% Ag helps to prevent the Ag from leaching from the terminator and into the solder joint, thus weakening the terminator. However, with the advent of the nickel barrier terminator the 2% Ag may not be needed and a standard 60Sn/40Pb solder cream could be used. We have not yet determined whether this is true. Solder cream can be refrigerated (2 to 5 degrees C) to prevent the flux and vehicles from separating from the metal powder. However, the solder cream can be contaminated by moisture if refrigerated. Allow the entire container of solder cream to reach room temperature before opening to prevent condensation on the solder cream. As an alternative to refrigeration, mix the solder cream well before using. Solder creams generaly contain 80 to 90% metal by weight. A solder cream of 85 to 90% metal would be the preferred choice for screen printing. Flux systems are available in the following groups: R -RMA - RA SA -
-
Not active Mildly active Active Synthetic activated (limited availability)
An R M A flux should be suitable for most requirements. It is active enough to clean the land, can be easily cleaned and if not all is removed during cleaning the danger of it affecting the PCA is minimal. A note of advice: do a complete chemical analysis on your solder cream so you know what it contains.
14. Surface Mount Manufacturing Techniques The following flow chart illustrates the process required to manufacture a Type I surface mount assembly. The Type II process is similar, except the conventional components are assembled to the printed circtfit board after the surface mount operation is complete.
15. Screen Printing Two methods are generally used to apply solder cream to a substrate in a high volume production environment, screening or stenciling.'Both utilize a standard screen printer. The first method, screening, is done by forcing the solder cream through a patterned screen located directly above the substrate. A squeegee moves over the surface of the screen and deflects the screen to touch the substrate. The solder cream is forced through openings in the screen and deposits itself by surface tension. The screen snaps away from the substrate when not in contact with the squeegee.
29
Wetdeposits up to 0.010 inch can be achieved with screens. The following screen will be acceptable for most screening applications. Wet thickness Material Mesh Mesh thickness Open area Emulsion build-up Wire diameter
0.008 inch Stainless steel 80, Plain weave 0.004 inch 70.5% 0.005 inch 0.220 inch
To determine print thickness use the following: Print thickness Where:
T W = (TM • A O ) + T E T W = Wet print thickness TM = Mesh thickness A O = % Open area, decimal T E = Emulsion build-up thickness
If a thicker wet solder joint or longer life is desired, a stencil will be a more effective approach. Metal stencils are made by chemically etching away the metal to form the required pattern. Application of solder cream is similar to methods used for screens. Stencils cost more than screens, but the added life may make this a more cost effective approach for high volume production runs. Stencils will also deposit the solder paste more consistently than a screen. Screen printers that can apply solder creams are manufactured by many companies. When selecting a screen printer some areas to consider are required print area, substrate size, cycle time and the level of automation desired.
16. Placement Systems Automatic placement of surface mounted components is the key to efficient production of printed circuit assemblies. One machine (using several different pick-up heads) can place almost all surface mounted components. Automatic placement machines utilize a vacuum pickup system tn remove the component from its packaging and transfer it to the substrate. The system consists of a vacuum tube (which picks up the component) and alignment jaws which orientate the component. The vacuum tube is narrower than the component and the alignment jaws retract before the component is placed on the substrate, so the tooling does not interfere with neighboring components as is the case with conventional insertion equipment. As a result, population densities can be greatly increased. Also, complex cut and clinch tooling is not required which reduces the cost of the eqipment and eliminates a system that requires extensive maintenance. Placement speed is faster because the cut and clinch operation is eliminated. Automated placement equipment is divided into four categories. -
-
-
-
In-line placement Sequential placement Simultaneous placement Sequential/simultaneous placement
In-line placement utilizes a group of fixed placement heads. Each head places a component on tile substrate as it passes down the line. Cycle times are from 1.8 to 4.5 seconds per assembly. Sequential placement utilizes a software controlled X-Y table and one or two placement heads. Components are placed on the substrate in sequence, much the same as a conventional 30
DIP inserter. Cycle times are from 0.3 to 1.8 seconds per component. Simultaneous placement equipment places all components on a substrate at one time utilizing one head per component. Cycle times are from 7.0 to 10.0 seconds per assembly. Sequential/simultaneous placement utilizes a software controlled X-Y table with multiple heads placing components in sequence. Cycle time is about 0.2 seconds per component. Many factors govern the selection of automated placement systems. - Number of assemblies per year - Number of different assemblies produced Lot size Dimensions of the assembly - Number of components per assembly Packaging format - Component types -
-
-
Each of these factors must be carefully evaluated before selecting an automated placement system. 17. Solder Cream Precure Several years ago it was a necessity to precure (bake) a solder cream before reflow, especially with a vapour phase reflow system, to eliminate the volatiles. Solder creams have improved to the state where a precure may not be required. This varies from manufacturer to manufacturer, so the end user will have to determine if this operation is still required. The main reasons for a precure are: - Eliminates volatiles from solder cream - Adheres components to lands - Reduces slumping of solder cream Almost any method of heating can be used to achieve a precure. An infrared dryer is the most efficient method. 18. Solder Reflow Vapour phase reflow and infrared are the best methods available to achieve solder reflow on large epoxy/glass substrates. Vapour phase soldering is the most popular method of reflowing soi'ter cream. It utilizes a fluid that vaporizes above the melting temperature of the solder cream. The vapor condenses on the entire assembly when it is placed in the system, quickly elevating it to the correct temperature. This instantaneous heating of the assembly eliminates the need for a preheat zone, as is required with wave soldering. However, a preheat zone may be required to prevent thermal shock to some components. This method does limit the amount of thermal shock to the substrate. T h r e e types of vapour phase system are available. - Dual vapor, batch - Single vapor, batch In-line (single and dual vapor) -
The dual and single vapor batch systems are not recommended for high volume production. The in-line vapor phase system is best stilted for high volume production. The most common vapor phase (primary) fluid is fluorinert. It is available in the following temperature ranges. FC43, boils at I74 degrees C FC70, FC5311, boils at 215 degrees C FC71, boils at 253 degrees C
31
NOTE: FC5311 is replacing FC70. FC5311 has improved thermal stability and as a result has reduced levels of trace decomposition products. Infrared ovens with multiple heat zones are available and can be utilized to reflow solder cream. In the past there were problems associated with IR reflow. But, during the past two years major improvements in IR systems have been achieved. Today IR is an acceptable method to achieve solder reflow.
Comparison of Vapor Phase and Infrared Systems (In-line)
Technique
Atmosphere Control
Mininization of Time
Process Control
Double Sided Assembly
Cleaning Ease
Volume Production Capability
Vapour Phase
Yes
Excellent
Excellent
Yes
Excellent
Excellent
Infrared
Yes
Fair"
Fair
Yes
Poor
Good
19. Cleaning The cleaning process to be used will be determined by the types of surface mount components populating the assembly and the type of flux system used. The printed circuit assembly should be cleaned immediately after the solder reflow operation. The longer the flux remains on the assembly the more difficult it will be to remove. The overall amount of flux on an assembly that has been reflowed using solder cream is less than on an assembly that has been wave soldered, however the amount of flux next to the solder joint is greater. A surface mount assembly populated with rectangular chips, MELF, SOT, SO, SOL and PCC packages can be cleaned very well with an in-line solvent system and possibly with an inline aqueous system. More diffcult components, such as the LCC, will require an in-line immersion solvent,system to remove the flux from under the component. Remember also, the next generation of surface mount components will probably be based on a lead pitch of 0.025 inch. This will require an aggressive cleaning system, so purchase a cleaner that will also meet your future needs.
20. Rework Rework techniques and equipment for the removal and replacement of surface mount components are still being developed. With the correct equipment any surface mount component can be reworked easier than its leaded equivalent. All the leads on the surface mount component must be reflowed at the same time to facilitate removal of the component. Two techniques can be used to remove surface mount components. The first, physical contact, reflows the solder by transferring energy from a rework tool to the component lead and solder joint. The problem with this approach is that physical contact must be made with each component lead and solder joint. This can be difficult, especially with the larger PCC packages. The second, and preferred method, is the use of directed hot air to achieve reflow of the solder joint. This method involves surrounding the surface mount component with a collet that will direct hot air onto the solder joint. The hot air will reflow all of the solder joints at once and allow easy removal of the surface mount component. Be aware that too much heating of the component over an extended period of time could damage the component. Also, hot air generates static electricity which could damage the component.
32
21. Conclusion Utilizing surface mounted device technology, a more efficient method of producing printed circuit assemblies can be developed. Before implementing surface mount technology be sure you understand the advantages and disadvantages and use the advantages to your advantage.
22. Glossary of Surface M o u n t Terms
Automatic Placement- In surface mounting an a u t o m a t e d system which removes the SMC from its packaging and places it on the surface of the substrate.
Bond S t r e n g t h - In wire bonding, the pull force at rupture of the bond interface measured in the unit gram force.
Bridging - Solder shorting of section that should be open between two or more conductors and/or c o m p o n e n t terminals. Burr- A fragment of excess material attached to a lead. Chase - T h e frame used to hold and support a screen or stencil. Cleaning - The art of removing flux and other contaminates from a printed circuit assembly. Conductor Line Width (Trace) - T h e minimum conductor width on the finished PCB. Conductor Spacing - The minimum space between conductors on a PCB. F e e d - T h r o u g h (Via) - A hole drilled through a series of pads which is then plated through. It is used to connect a conductor from one layer to another. A feed through does not have a c o m p o n e n t lead installed through the holes. F l u o r i n e r t - A chemicl used in wlpor phase reflow systems. It is heated to its boiling point at which time it produces a v a p o r that is at a t e m p e r a t u r e above the melting point of the solder. The assembly is placed into the v a p o r at which time the solder reflows to form the mechanical joint. Flux - Used to clean a contaminated metal surface during soldering operation. F o o r p r i n t - The area on a printed circuit board used for mounting a surface mounted component. Gull Wing Lead- A lead that exits the body of an S O / S O L package parallel to the b o t t o m of the package, then forms a right angle bend which has the lead formed so it is parallel with the surface of the land it is to be mounted on. H e a t D i s s i p a t i o n S u r f a c e - A surface area on a PCB used to dissipate excess heat. This surface area does not have a c o m p o n e n t mounted on it. Infrared T e c h n o l o g y - A system used to dry or reflow solder creams using infrared energy. J - L e a d - A lead used primarily on the plastic chip carrier. It exits the side of the package and is then bent down the side and under the package. So called because the lead resembles a capital "J". Land - The surface area on a PCB onto which the lead of a surface mount component is placed and then soldered in place. Lands do not have holes drilled through them. Land/Pad Layer- A printed circuit board in which only lauds and pads are on the two outer layers. All of the circuitry is on the inner Payers. This protects fine line circuitry from handling damage. Leaching - In soldering, the dissolving of the material to be soldered into the molten solder. Leaded Ceramic Chip Carrier (CCC) - Same as the leadless ceramic chip carrier, but leads have been attached to allow mounting on PCB that do not have a T C E that matches ceramic (such as epoxy/glass).
33
Leadless Ceramic Chip Carrier (LCC) - A surface mount package that contains an integrated circuit. The leads are part of and flush with the bottom side of the chip carrier. The package material is ceramic. M a n u f a e t u r a b i l i t y - T h e art of designing a product so that it can be efficiently manufactured using available manufacturing technology. Melf - Metal Electrode Face Bonding. A cylindrical leadless component with end metalliq terminals. It is a popular package for fixed film resistors. Microelectronics - That area of electronic technology associated with electronic systems that use extremely small electronic components or elements. Pad - T h e surface area on a PCB in which a hole is drilled and plated through. Pitch - The distance between a point and an adjacent point on the same item. Plastic Chip Carrier {PCC) - A surface mount package that contains an integrated circuit. It has leads on all four sides and is either rectangular or square. Leads are usually on a fifty mil pitch. PPM - Parts per million. How many parts per million are defective. Determined by multiplying the percent defective by 10,000. R e c t a n g u l a r Chip - A surface mount resistor or capacitor. It is made from ceramic, an inorganic non-metallic material. S c r e e n - A contact print method for applying solder cream to a substrate. Screens, usually made from stainless steel, are covered with an emulsion that has open areas that match the land geometry on the substrate. The solder cream is forced through these openings onto tile substrate. S c r e e n P r i n t i n g - In surface mounting, the art of applying solder cream to a substrate surface using a screen printing process. Small Outline {SO) - A surface mount package that contains an integrated circuit. It is a rectangular package with leads on two sides, usually on a pitch of fifty mils and a body width of 0.150 inch. Package material is plastic. Small Outline Large {SOL) - Same as the small outline (SO) but with a body width of 0.300 inch to accommodate larger die. Small Outline T r a n s i s t o r {SOT) - A surface mount package that contains transistors and diodes. It is a rectangular package that has either three or four leads. Package material is plastic. S M C - Surface Mount Component. Any component that is mounted on the surface of a PCB. SMD - Surface Mounted Device. Any device that is mounted on the surface of a PCB. S M T - Surface Mount Technology. The technology in which components are mounted on the surface of a PCB. Solder Balls- Small balls of solder formed during the reflow process. High concentrations of oxides and contaminants on the solder powders surface leads to the generation of solder balls. Solder C r e a m - A homogenous mixture of a powdered metal alloy, a flux system and a viscosity modifier. During a solder reflow operation the metal alloy melts to form the solder joint, while the viscosity modifier evaporates. The flux is removed during a cleaning operation. S o l d e r Mask - A chemical system which forms a protective cover over the circuitry on a printed circuit board.
34
Stencil - Metal stencils are used in the same method as a screen. The openings are made by chemically milling the land geometry into a thin metal such as brass, nickel or stainless steel. Stencils are used to apply a thicker deposit of solder cream. They also wear better than screens, and thus last longer. S u r f a c e M o u n t A s s e m b l y (SMA) - A printed circuit assembly that is populated with a majority of surface mount components. S u r f a c e T e n s i o n - During the solder reflow process, while the solder is in the liquidus state, the surface mount component will center itself on the land due to the surface tension of the solder (if the land was designed correctly). T e r m i n a t o r - Part of the interconnect structure of the rectangular chip. It consists of a layer of silver (forms a mechanical bond with the ceramic), then a nickel barrier (to prevent leaching of silver away from the ceramic during reflow) and then a solderable plated surface (tin, solder alloy, palladium-silver). T e s t P r o b e N o d e - Any pad, land or exposed conductor that would allow access for probing. T e s t a b i l i t y - The art of designing a product so that it can be efficiently tested on existing test equipment. Thermal Coefficient of Expansion (TCE) -The fractional change in the dimensions or volume of a material per unit in change of temperature. Vapor Phase Reflow - A system used to reflow solder creams. It utilizes a gas (vapour) that is at a temperature above the melting point of the solder alloy being used.
35