A knowledge-base for electronics soldering

Journal of Materials Processing Technology 97 (2000) 1±9

A knowledge-base for electronics soldering S.M. Darwisha,*, S. Al-Habdanb, A. Al-Tamimia a

Mechanical Engineering Department, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia b Advanced Electronics Company, PO Box 90916, Riyadh 11623, Saudi Arabia Received 24 December 1996; received in revised form 17 March 1999

Abstract Soldering is associated mainly with the electronics and military industries, which always prefer to preserve their `know-how' on this subject. Thus, the main objective of the present work is to establish a knowledge-base for electronics soldering. This includes: materials for soldering, soldering processes, cleaning for soldering, and trouble shooting for soldering. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Soldering; Electronics; Knowledge base

1. Introduction Soldering is a joining process in which a ®ller metal with a melting point not exceeding 4508C is melted and distributed by capillary action between the faying surfaces of the metal parts being joined. No melting of the base metals occurs, but the ®ller metal wets and combines with the base metal to form a metallurgical bond. The surfaces to be soldered must be pre-cleaned so that they are free of oxides, oils, etc. An appropriate ¯ux must be applied to the faying surfaces, and the surfaces are heated. Filler metal, called solder, is added to the joint, and distributes itself between the closely ®tting parts. After solidi®cation, the ¯ux residue must be removed. As an industrial process, soldering is most closely associated with electronics assembly. It is also used for mechanical joints, but not for joints subjected to elevated stresses or temperatures. Advantages attributed to soldering include: low energy input relative to brazing and fusion welding, good electrical and thermal conductivity in the joint, ease of repair and rework; added to their capacity to make air-tight and liquid-tight seams for containers. The biggest disadvantages of soldering are: low joint strength unless reinforced by mechanical means, and possible weakening or melting of the joint in elevated temperature service [1]. Today's manufacturing industries need to produce at reduced cost and improved quality, in order to compete

* Corresponding author. Tel.: ‡966-1-4675614; fax: ‡966-1-4674254 E-mail address: [email protected] (S.M. Darwish)

effectively in an area of shrinking product life cycle, and rapid advances in technology. This calls for, more specialist expertise and a concurrent engineering approval [2±9]. Recently, knowledge-bases for welding have been evolved in order to improve the decision-making process within the different factors required in the welding technology. The evolved expert systems include welding process selection, welding process control, welding default diagnosis, and welding material selection [10±15]. A knowledgebase for metal welding process selection has been developed [16]. Not only is the selection of the welding process important for the manufacturer, but also the data knowledge for implementing the selected welding process is of prime importance [17]. Since soldering is associated mainly with the electronics and military industries, which always prefer to preserve their `know-how' on this subject, few publications concerning metal soldering are in existence. Therefore the aim of the present work is to establish a knowledge-base for soldering. Automation of knowledge through a ``knowledge-base system'' will greatly enhance the decision-making process. The bene®ts of building a knowledge-base system for metal soldering, can be summarized as follows [2±9]: (i) capturing the scarce expertise and making it available for effective use; (ii) dealing with a large amount of data and responding quickly; (iii) standardizing the conclusions for a given set of data; (iv) allowing the problem-solving skills of several people to be combined; and (v) updating existing knowledge when new information is available.

0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 3 7 8 - 7

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2. Soldering knowledge The soldering knowledge involved in this work contains the most practical knowledge that is expected to be needed by engineers working with soldering. This knowledge has been divided into four sections [18±23]. Materials used for soldering: Solders and ¯uxes are the materials used for soldering, both being critically important in the joining process. Most solders are alloys of tin and lead, since both metals have a low melting point. Their alloys possess a range of liquidus and solidus temperatures to achieve good control of the soldering process for a variety of applications. Soldering ¯uxes should be molten at soldering temperatures, remove oxide ®lms and tarnish from the base part surfaces, prevent oxidation during heating, promote wetting of the faying surfaces, and be readily displaced

by the molten solder during the process. This section contains information on soldering ¯uxes (¯ux purpose, ¯uxing techniques, operation instructions, and ¯ux selection) and solders (solder types, and solder selection) [17±22]. Soldering processes: Many of the methods used in soldering are the same as those used in brazing, except that less heat and lower temperatures are required for soldering. The present work concentrates on the soldering techniques that are heavily used in the electronics industry namely; hand soldering and wave soldering. Wave soldering is a mechanized technique that allows multiple lead wires to be soldered to a printed circuit board (PCB) as it passes over a wave of molten solder. The typical set-up is one in which a PCB, on which electronic components have been placed with their leadwires extending through the holes in the board, is loaded onto a conveyor. The conveyor supports the PCB on its sides,

Fig. 1. Knowledge organization. Table 1 Depth of wave Board type and thickness Flexible Single sided Double sided Ð thickness 1.6 mm Double sided Ð thickness 2.4 mm Multilayer Ð thickness 1.6 mm Multilayer Ð thickness 2.4 mm Multilayer Ð thickness 3.2 mm a

N/A: Not applicable.

Depth of wave Kiss Kiss 1/3 1/2 1/2 5/8 3/4

a

N/A 1/3 2/3 3/4 3/4 3/4 7/8

Table 2 Pre-heating temperature Board type and thickness

Temperature range (8C)

Single sided or flexible Double sided Multilayer (up to four layers) Multilayer (over four layers)

80±90 100±110 105±120 110±130

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Fig. 2. Adjustment of a wave solder machine. Table 3 Wave soldering machine selection No.

1 2 3 4 5 6 7

Wave Soldering Machine

Board Type

Minipak 300 Econpak 400 SV Econopak/EUROPAK I Econopak/EUROPAK Plus Ultrapak 600 C Ultra 2000 Atoms 2000

X X

Single sided

Assembly Type Multi sided

Through-hole X

X X X X X

X

Through-hole and SMT X X X X X

Production volume

Maximum board width (mm)

Low±Medium Low±Medium Medium±High High High High High

300 400 400 400 600 600 500

Table 4 Trouble-shooting summary for wave-soldered boards High wave length Iciling Bridging Poor wetting PCB Poor wetting base metal Dewetting Poor wetting of leads Blowholes Cold joints Gravity joints Yellow joints Inclusions Dull joints Cracks Excess solder Lack of solder

Low wave temp

High conveyor speed

X X

X

Low conveyor speed

High Low High Low Flux Interference preheating preheating contact contact density temperature temperature time time X X

X X

Amount of oil

Solder Abnormal pot fluxing contamination

Turbulent wave

Shape Conveyor of the wave

Soldering direction

Dross on the wave

Type of flux

Protection lacquers

Solder resist

Base laminate

Coating type

Coating contamination

X

X X X X

X

X X

X X X

X X

X

X X

X

X

X X

X

X

X

X

X

Non-activated base metal

X

X

X

Coating excessive porosity

X X X

X

Coating wrong composition

X X

X X

X X

X X

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Table 5 Glossary of electronics soldering terminology Terminology

Interpretation

Wave soldering Kiss Depth of wave

Mass soldering process in which assembled boards are dragged across the surface of the wave of molten solder The bottom surface of the board touches the solder wave The distance between the machine nozzle and the top of the solder wave measured as a percentage of the board thickness Occurrence during soldering, where an initial bond is formed, followed by a withdrawal of solder from the joint, leaving irregularly shaped mounds of solder separated by areas covered with a thin solder film A yellowish color of the solder fillet Formation of spikes of solder after soldering, if excess liquid solder is not removed from a joint quickly enough Where solder joins two or more conductive parts which are not meant to be connected After cleaning, residues are often retained on the printed board because of the diverse chemical nature Electronics assembly in which leadless components are mounted directly on to the board surface

Dewetting Yellow joints Iciling Bridging White residues SMT (surface mount technology)

so that its under-side is exposed to the processing steps. This section includes information on manual soldering (applications, bits for soldering irons, and suggestions for manual soldering) and wave soldering (the adjustment of wave soldering machines) [17±22]. Cleaning processes: This section includes information on cleaning solvents, and cleaning equipment [15±20]. Quality control: This section includes information on soldering defects, and trouble-shooting for soldering [15±20]. Fig. 1 shows the organization of soldering knowledge. 3. Formatting the knowledge The soldering knowledge-base was developed using the Visual BASIC programming tool. Part of the knowledge was formatted in the form of direct advice (see for example Tables 1 and 2 [18±22] for the depth of the wave and preheating temperatures). The rest of the knowledge was formatted, to interact with the user in acquiring a certain input, in order to issue the appropriate advice (see for example Fig. 2 the adjustment of wave soldering machines and Tables 3 and 4 [18±23] for the selection of wave soldering machines and soldered board trouble-shooting, respectively). Two types of screens were designed for presenting and retrieving the knowledge. A set of pictures has been incorporated into the knowledge-base using a scanner, such as: ¯uxing techniques, wave soldering machines, and cleaning equipment. Table 5 provides an interpretation of the most important soldering terminology used through the present work.

facturing, using wave soldering machines, and a board thickness of 3 mm. The following information is required: (a) How to adjust the wave soldering machine to solder a double sided boards? (b) The wave soldering machine that satisfies the following conditions: board type is a multi layer board; assembly type is a through hole ‡ SMT; production volume is medium to high; board width is from 400± 600 mm. (c) The usual causes of some soldering defects such as: excess solder, dewetting, blow holes, and bridging. Fig. 3 demonstrates the run session concerned with this enquiry. It is worth mentioning that the system response has been validated and approved by the Advanced Electronics Company (the major electronics company working in Saudi Arabia).

5. Conclusions This research resulted in the development of a knowledgebase for electronics soldering. Documented knowledge supported with practical experience have been used throughout the present work. The proposed knowledge-base is hoped to compensate for the lack of practical information on electronics soldering.

4. The knowledge-base for electronics soldering/ example of application of the system

Acknowledgements

An electronics company produces military products, where soldering represents a major issue in board manu-

The cooperation of the Advanced Electronics Company, Saudi Arabia, is highly appreciated.

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Appendix 1, Part A.

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Appendix 1, Part B. Appendix 1, Part A (Contd.)

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Appendix 1, Part C.

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Appendix 1, Part C (Continued )

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