- Email: [email protected]
Tin, lead, and tin-lead plating
TIN, LEAD, AND TIN-LEAD PLATING by Stanley Hirsch Leeam Consultants Ltd., New Rochelle, N. Y,
and Charles Rosenstein She#Case Ltd., Jerusalem, Israel Tin and tin-lead alloys are solderable and, therefore, are used extensively in the electronics industry to bond electronic components. This precludes the need for strong fluxes to wet the deposit. Tin and lead can be codeposited easily because of the closeness of their standard electrode potentials. Tin and tin-lead deposits must possess all of the following characteristics: good solderability and reflowability; low porosity; good corrosion resistance; and uniformity of alloy composition, thickness, smoothness, and appearance, over a wide current density range.
ADDITIVES Organic additives are required in all tin, lead, and tin-lead electroplating solutions to produce a useful deposit. In the absence of additives, treed, nonadherent, and nodular deposits result; therefore, additives are absolutely essential in order to yield smooth, uniform deposits and to impart good throwing power. Additives are depleted during plating and must be routinely replenished. Electrolysis causes some decomposition of the organics, resulting in their occlusion in the deposit. If present in sufficiently large amounts in the deposit, organics can cause solderability and reflow problems. Accelerated aging tests are performed on tin and tin-lead deposits to help predict their shelf life, as components are sometimes used long after they are plated.
TIN, LEAD, AND TIN-LEAD PLATING BATHS Tin, lead, and tin-lead electroplating solutions are used to plate components in numerous engineering, communications, military, and consumer product applications.
Tin Barrel, Still, and High-Speed Baths These baths are well suited for electrolytic tin plate and are used to plate transistors, semiconductors, various electronic parts, refrigerator parts, and kitchenware. Higher temperatures permit higher current densities and wire speeds.
Lead Barrel a n d Still B a t h s Lead baths are used in the plating of bearings, connectors, internal and conforming anodes for chromium plating, valves, seals, and parts for storage batteries. Because lead deposits are soft, stow barrel speeds are recommended in order to prevent heavy parts from bonding to one another.
291
Table I. Concentrations of Fluoboric Acid, Lead Fluoborate, Stannous Fluoborate, and Copper Fluoborate Used in Tin, Lead, and Tin-Lead Alloy Plating Product
Concentration (g/L)
48% Fluoboric Acid Fluoboric Acid, 100% Boric Acid 5 t % Lead Fluoborate Lead Fluoboric Acid, 100% Boric Acid 50% Stannous Fluoborate Stannous Tin Fluoboric Acid, 100% Boric Acid 45% Copper Fluoborate Copper Fluoboric Acid, 100% Boric Acid
Specific Gravity (20°C)
1.365 656 19 1.710 480 11 48 1.600 320 48 48 1.550 187 30 30
60 Tin/40 Lead Solder Barrel, Still, and High-Speed Baths The high throw of the baths and the excellent solderability and shelf life of the deposit make these baths suitable for use on printed circuit boards, connectors, and other specialized electrical devices. The high-speed bath is used for reel-to-reel applications, such as wire and strip plating.
90 Tin/10 Lead Barrel, Still, and High-Speed Baths This bath provides a uniform, smooth matte deposit, which is used for various engineering applications. Higher temperatures permit higher current densities, which are required in wire and strip plating.
93 Lead/7 Tin Barrel and Still Baths Deposits plated from 93 lead/7 tin baths are harder than those obtained from lead baths. The 93 lead/7 fin bath is used to plate bearings and seals. Since 93 lead/7 tin alloys are soft, slow barrel speeds are recommended in order to prevent heavy parts from bonding to one another.
10 Tin/88 Lead/2 Copper Ternary Alloy Barrel and Still Baths The operation of the ternary alloy bath is similar to that of the 93 lead/7 tin bath. The ternary alloy deposit results in bearing metals that exhibit a greater resistance to fatigue than do the 93 lead/7 tin alloys. The alloy is used to plate linings in sleeve bearings.
FLUOBORATE PLATING Tin fluoborate, lead fluoborate, and fluoboric acid, in various proportions (see Tables I and II), can be used for plating all percentages of tin-lead, 100% lead, and 100% tin. Fhioborate baths require boric acid for stability. Anode bags filled with boric acid are hung in the plating tanks. Stabilized liquid peptone, gelatin, resorcinol, or other liquid organic additives must be added to tin, lead, and tin-lead baths to produce smooth, nontreeing deposits
292
!
c,D 4:x
10-20 15
93 Lead/7 Tin Barrel and Still Baths: Range Optimum 135-150 143
195-239 217
8-12 10
8-14 11
23-30 26
195-239 218
--
--
Lead (g/L)
1.5 2.5 2
--
--
--
--
--
--
Copper (g/L)
aAgitation required, mechanical or barrel; vigorous for high-speed plating, Periodic filtration through 1-t~m Dynel or polypropylene cartlidges. Tanks, pumps, and filters made from polyethylene, polyvinyl chloride (PVC), or Koroseal Heating and cutting equipment made from Karbate or Teflon.
lO Tird88 Lead/2 Copper Barrel and Still Baths." Range 20-40 Optimum 30 8-12 10
70-80 75
90 Tin/lO Lead Barrel and Still Baths." Range 150-200 Optimum 175 15-30 23
12-20 16
350 500 425
High Throw: Range Optimum
53-60 56
98-150 124
60 Tin~40 Lead Solder Bath: Barrel and still: Range Optimum
45-60 53
15-30 23
225-300 263
Wire and strip: Range Optimum
3045 37
Tin (g/L)
Lead Barrel and Still Baths: Range Optimum
188-263 225
Tin Bath." Barrel and still: Range Optimum
HBF4 (g/L)
Table II. Fluoborate Bath Compositions and Operating Parametersa
As required
As required
As required
As required
As required
As required
As required
As required
Additive
70-100 (21-38)
70-100 (21-38)
70-100 (21-38)
70-100 (21-38)
70-85 (21-29)
70-100 (21-38)
90-t30 (32-54)
90-120 (3249)
Temperature T (°C)
20-70
20-70
1-80
15 25
25-35
20-70
1-300
1 80
Current Density (A/ft 2)
1:1
1:1
1:1
2:1
2:1
1:t
1:1
1:1
Anode to Cathode Ratio
10 tin/88/lead/2 copper, or 90 lead/10 tin, or pure lead
93 lead/7 tin
90 tirdl0 lead
60 tird40 lead
60 tin/40 lead
Pure lead
Pure tin
Pure tin
Anodes
Electrop/ating Chemicals & Services
®
~:s~ ...and y o u were led to believe we only
TM
~irl~t~
TM
supply palladium products! i
Electroptating Chemicals and Services Technical Information: 1-800-221-6316 Customer Service: 1-800-3(
Table III. Concentrations of Methane Sulfonic Acid (MSA) and MSA Salts Used in Tin, Lead, and Tin-Lead Alloy Plating Product
Concentration (g/L)
Specific Gravity (20°C)
50% MSA 70% MSA Stannous methane sulfonate
MSA 100% = 613 MSA 100% = 945 Sn = 120 Sn = 150 Sn = 200 Sn = 240 Sn = 300 Pb = 450 Pb = 500 Cu = 100
1.225 1.350 1.230 1.285 1.390 1.450 1.560 1.630 1.700 1.260
Lead methane sulfonate Copper methane sulfonate
and to increase the throwing power of the plating bath. These additives are mandatory, as fluoborate baths cannot function without them.
METHANE-SULFONIC-ACID-BASED PLATING Methane sulfonic acid (MSA)-based electroplating systems gained commercial acceptability during the early 1980s. Various plating baths and additives have been developed. Each MSA bath requires a custom-made additive for that specific content of tin, lead, and MSA in order to obtain a useful deposit. As with fluoborates, metal salts of MSA are very soluble and various concentrations of MSA and its metal salts are available (see .Tables III and IV). Troubleshooting guidelines for fluoborate- and MSA-based tin, lead, and tin-lead plating baths are given in Tables V to VII. The troubleshooting table for acid-tin-based baths (Table V) is also applicable to stannous sulfate baths.
TIN PLATING FROM STANNOUS SULFATE BATHS The stannous sulfate bath contains stannous sulfate, sulfuric acid, and additives (see Table VIII). Advantages of this bath are its good throwing power, high cathode efficiency, operation at room temperature, and ease of control. Addition agents are essential for the operation of this bath. They inhibit the oxidation of stanuous tin, produce smooth, dense deposits, and prevent treeing. Among those in use are phenol- or cresol-sulfonic acid, gelatin, /?-naphthol, and resorcinol. Several proprietary additives are also available. Bright tin deposits are obtained through the addition of proprietary additives. Solderability, corrosion resistance, and leveling characteristics are essentially the same in bright or matte plating. It is questionable whether bright tin plating is superior to matte in resistance to whisker growth and tin pest, as has been claimed. Refer to Table V for troubleshooting.
TIN PLATING FROM STANNATE BATHS Alkaline baths are also used for tin plating, with sodium stannate and potassium stannate as the two available baths (see Table IX). Both will produce similar satisfactory results. The criteria for choosing one over the other are related to cost and speed of plating.
296
General Chemical, the North American pioneer in fluoborates, is "basic" in these important metal finishing chemicals. We still control quality and consistency, from raw material to finished product delivery. It's what sets us apart from our competition. For additional information and the location o f youJ nearest distributor, call us toll-free at 800-631-8050. Ammonium Fluoborate, Sodium Fluoborate • Fluoboric Acid Stannous Fluoborate • Lead Fluoborate ° Peptone
General Chemical 90 East HalseyRoad [] Parsippany,NJ 07054 ©Genera/Chemical Corp. 1998
www.genchemcorp,com
0o
I'o co
Tin Barrel Still and High-Speed Baths:
200-250 225
High speed: Range Optimum
30-50 40
200-250 225
30-50 40
4-5 4.5
4-5 4.5
17-25 21
12-20 16
12-20 16
--
35-55 45
Tin (g/L)
65-75 70
56-72 64
2~3
8-12 10
6-10 8
56-72 64
--
Lead (g/L)
0.8-1.2 1.0
--
--
Copper (g/L)
and Operating
aAgitation required, mechanical or barrel; vigorous for high-speed plating. ]?latinized titanium anodes can be substituted for the soluble anodes. Periodic filtration through 1-/xm Dynel or polypropylene cartridges. Anode to cathode ratios for all baths is 1:1. Tanks, pumps, and filters made from polyethylene, polyvinyl chloride (PVC), or Koroseal. Heating and cooling equipment made from Karbate or Teflon.
Range Optimum
10 Tiru'88 Lead~2 Copper Barrel and Still Baths:
Range Optimum
93 Lead/7 Tin Barrel and Still Baths:
Range Optimum
90 Tin/lO Lead Barrel, Still, and High-Speed Baths:
200-250 225
30-50 40
200-250 225
Barrel and still: Range Optimum
60 Tin/40 Lead Solder Bath:
Range Optimum
Lead Barrel and Still Baths:
Range Optimum
MSA (g/L)
T a b l e IV. M e t h a n e S u l f o n i c A c i d ( M S A ) B a t h C o m p o s i t i o n s
As required
As required
As required
As required
As required
As required
As required
Additive
Parameters "
70-100 (21-38)
70-85 (21-29)
70-130 (21-54)
70-100 (21-38)
70-85 (21-29)
70-100 (21-38)
70-130 (21-54)
Temperature °F (°C)
1-40
1-40
1-250
1-250
1-40
1-40
1-250
Current Density (A/ft2)
10 tin/88 lead/2 copper, or 90 lead/10 tin, or pure lead
93 lead/7 tin
90 tirdl0 lead
60 tin/40 lead
60 tird40 lead
Pure lead
Pure tin
Anodes
TIN-LEAD
AND
PLATING
LEAD
FREE
PROCESSES
SPECIALISTS FOR
BRIGHT
AND
MATTE
FINISHES
•
•
MACLEE
CHEMICAL
C O . INC.
2065 N. Southport, Chicago, IL 60014 • (773) 9 2 9 - 9 7 9 2 • F a x ( 7 7 3 ) 9 2 9 - t 6 2 2 E-mail: [email protected]
* Website:
http://www.macleechemical.com
Mason
A
THECLEAR A N D ODOR FREE SOLUTION IN M S A
HECTRIO MANUFACTURERSOF:
•.Tin (II) Methane Sulfonate • Lead (II) Methane Sulfonate •.Purified Methane Sulfonic Acid (MSA) • 13% Stannous Sulfate solution - highest concentration available worldwide 400 First Avenue, Royersford, PA 19468 Tel: 1-610-948-0734, Fax: 1-610-948-0744
1-800-784-9533
• Stannic Chloride (Tin Tetrachloride) (Tin IV Chloride), (Tin Chloride) • Stannic Chloride Solution ~ m Stannous Chl~fide Anhy~rou s (Tin |ii~Chlori~),
:,
• sto88~us C~i~ide Solu, o~ ~,. ............
• ~$~ium
......................
• Potassium • Potassium Stannate S~'tio~ ' ;'~:!:;:::~' Mason Corporation chemicals' unique features include specialized service, the highest quality and purity in tin metals, and flexibilty in packaging of the chemicals to meet the customers needs.
Mason Corporation P.O. Box 38 • Schererville, IN 46375 1-219-865-8040 • Fax: 1-219-322-3611 800-326-3075 • Email: [email protected]
Table V. Troubleshooting Acid-Tin-Based Baths Problem
Cause
Remedy
Treeing
Low additive Low acid Organic contamination Solids in bath Metallic contamination Stannic tin Low additive Low tin Low acid Too high current density Low additive Low additive Low additive Low acid Low tin Too high current density Low additive Organic contamination Low tin Low temperature Poor cleaning Organic contamination Low temperature Low additive Organic contamination
Add additive. Increase acid. Carbon treat. Filter. Dummy bath. Filter and exclude air from bath. Add additive. Increase tin. Increase acid. Lower current density. Add additive. Add additive. Add additive. Increase acid. Increase tin. Lower current density. Add additive. Carbon treat. Increase tin. Increase temperature. Correct cleaning cycle. Carbon treat. Increase temperature. Add additive. Carbon treat.
Roughness
Graininess Bumhlg
Streaking or pitting Poor throw
Gassing
Lack of adhesion Brittle deposits Dark deposit
Table VI. Troubleshooting Lead-Based Baths Problem
Cause
Remedy
Treeing
Low lead Low additive Low lead Low additive Metallic contamination Solids in bath Too high current density Low agitation Low additive Low acid Chloride/sulfate contamination
Increase lead. Add additive. Increase lead. Add additive. Dummy the bath. Filter bath. Lower current density. Increase agitation. Add additive. Increase acid. Filter bath and check cleaning cycle. Correct cleaning cycle. Carbon treat. Increase lead. Increase current density. Analyze and adjust copper.
Poor throw Roughness
Lack of adhesion Dark deposit Thin deposit Mossy deposit (ternary alloy only)
300
Poor cleaning Organic contamination Low lead Too low current density Excess copper in bath
~imply The Best •
ll
~ I
-
-.
.
.
.
Guaranteed Q~,ality * O~-Time Delivery * Ex~Mtent Prices
, x
TITAN ""-)~:,_.S-" INTERNATIONAL, INC, Plating Supply Specialist
•
-
~_
•
- ~
O
MSA Salts-Tin, Lead, Copper Sodium Hypophosphite, High Purity
ll
(IIEMICALS: StannousMethaneSulfonate 300 g/L to 400 g/L, & Crystals Lead Methane5ulfonate Methane SulfonicAdd 70%
MSA ElectroplcttingAdditives Pure Tin, Tin-Leod, Tin-Copper Motte & Bright Finishes
PotassiumStannate 5tannousSuffate Sodium Stannate
C A , E4, NJ~
Other Metal & Chemical Lines: Brass, Bronze, Cadmium, Copper, Chrome, Nickel & Zinc! Call Toll Free: 800-435-4644 w w w . titanin tl. corn
Rotterdam, Japan, Tatwan+China
Building a ~ ~ . ~ | PARINEK~HIP( ~k/ | ~ t with y o u n o w and i ikto thefuture [
Varsa! [nstrarm, nts, Ir~c, [
i
[
F',."~ Che~*dcal,~" D~:¢si:m
[
| |
[[
363 [vy~ea~dR&~Warrrda~tel;PA lg974][
| |
Tel: (215)957-5880 Fax: (2~5}957-9~ 1 l | /gmail: hffo(~'arszl,e~m t
| ~
a
t
.
c
o
m
The Electrodeposition of Tin and Its Alloys by M. Jordan 409 pages $130.00 This is the definitive book on every conceivable aspect of the electrodeposition (and electroless deposition) of tin and its alloys. All bath types are discussed in detail. Analysis of baths and testing of deposits using both simple methods and more advanced instrumental techniques are covered. National, international, military, and other standards and specifications are also included. Send Orders to: METAL FINISHING 650 Avenue of the Americas, New York, NY 10011 For faster service, call (212) 633-3199 or FAX your order to (212) 633-3140 All book orders must be prepaid.. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and $20.00 for each book shipped express to all other countries.
_i
Table VII. Troubleshooting 60 Tin/40 Lead Solder Baths Problem
Cause
Remedy
Poor throw
Low tin and lead Low acid Low additive High anode to cathode ratio Organic contamination Too high current density Organic contamination Low additive Low acid Poor cleaning Low tin and lead Low acid Too high current density Organic contamination Organic contamination Solids in bath Stannic tin Chloride/sulfate contamination
Increase tin and lead. Increase acid. Add additive. Remove several anodes. Carbon treat. Lower current density. Carbon treat. Add additive. Increase acid. Correct cleaning cycle. Increase tin and lead. Increase acid. Lower current density. Carbon treat. Carbon treat. Filter. Filter and exclude air from bath. Filter bath and check cleaning cycle. Dummy the bath. Increase acid. Use correct current density. Use proper agitation. Use anodes with correct composition. Analyze bath and correct ratio. Use correct plating time.
Gassing Treeing Lack of adhesion Burning
Graininess Roughness
Incorrect alloy
Unsatisfactory reflow
Metallic contamination Low acid Wrong current density Improper agitation Incorrect anodes Incorrect tin to lead ratio Incorrect thickness Dewetting caused by: Poor cleaning Organic contamination High reflow temperature Incorrect alloy
Correct cleaning cycle. Carbon treat. Use proper temperature. See Problem: "Incorrect alloy" above.
Table VIII. Stannous Sulfate Bath Composition and Operating Conditions
Tin Sulfuric acid, 100% Additive Anodes Current density, Mft 2 Anode to cathode ratio Temperature, °F (°C) Agitation Filtration Pumps and tanks Heaters and coolers
302
Range (g/L)
Optimum (g/L)
30-50 100-140 As recommended by manufacturer Pure tin 1-25
40 120
l:l 70-85 (21-29) Cathode and solution 1 /J,m Dynel or polypropylene cartridges Polyethylene, polypropylene, rubber Karbate
Table IX. Stannate Baths Composition and Operating Conditions Rack (g/L)
Barrel (g/L)
90 40 12 15-20 25 3-4 170-180 (77-82)
180 80 23 5-15 15-25 3-4 165-175 (74-79)
100 40 15 30-100 30-40 4-8 150-180 (66-82)
200 80 23 1-100 10-30 4-14 150-180 (66-82)
Sodium Stannate Bath:
Sodium stannate Tin Free sodium hydroxide Cathode current density, A/ft 2 Anode current density, A/ft ~ Voltage Temperature, °F (°C) Potassium Stannate Bath:
Potassium stannate Tin Free potassium hydroxide Cathode current density, A/ftz Anode current density, A/ft2 Voltage Temperature, °F (°C) Sodium and Potassium Stannate Baths:
Anode to cathode ratio Filtration Agitation Pumps and tanks Heaters and coolers
1:1 25/zm Dynel or polypropylene cartridges Cathode and solution Stainless steel, low-carbon steel, polypropylene, polyethylene Low-carbon steel, polypropylene, polyethylene
Potassium stannate is generally preferred over sodium stannate for the following reasons: 1. Higher plating rates. 2. Greater conductivity, especially in barrel plating. 3. Less potential for sludge formation. The plater is cautioned to choose one or the other bath, and not to mix them. Potassium hydroxide should be used with the potassium stannate bath and sodium hydroxide with the sodium stannate bath. Although the "mixed" bath is operable and satisfactory results can be obtained, analytical control is quite difficult.
A n o d e s in Stannate Baths Anodes used in stannate baths are made of pure tin, tin alloyed with 1% aluminum (known as "high-speed" anodes), or inert materials such as steel, nickel, or stainless steel. Advantages of the high-speed anode are its greater current density range and its consistent efficiency at higher current density. The formation and maintenance of the anodic film is critical to the operation of the stannate bath. The primary cause of poor tin deposits is the improper filming of the anodes. To film tin anodes, a "surge" of current, at higher than normal current density, must be
Table X. Stannate Anode Film Characteristics
Color of film formed Anode current density, Alft 2
Pure Tin
Tin + 1% AI (High Speed)
Inert Anode
Yellowish 15-40
Olive green 30-80
No film No limits
303
impressed on them for a few seconds to a minute, after which the current is reduced to its regular value (see Table X). The anodes must be refilmed after each shut-down period prior to plating, as the film dissolves rather quickly. Practically all problems in the operation of a stannate bath are resolved when proper anode operation is achieved. Inert anodes are advantageous in that they avoid the difficulty of filming and they inhibit the formation of harmful stannous tin by producing oxygen at the anode. Furthermore, they do not change shape in use. Their use has previously been somewhat limited because of the need for chemical replenishment of the bath as more tin is consumed. Replenishment of the tin content of the bath by a tin oxide solution has made the use of inert anodes practical in potassium stannate systems only.
Operation of Stannate Baths Stannate solutions should be light straw or light gray in color. Black solutions indicate the presence of stannous tin, and they should be treated with hydrogen peroxide (2 ml 35% hydrogen peroxide per gallon of solution). If frequent additions of peroxide are necessary, make sure the hydroxide concentration is not too high and/or the anode current density is not too low. A 10% solution of acetic acid is used to lower the free hydroxide content of the bath; one gallon of 10% acetic acid will neutralize 9.25 ounces of sodium hydroxide or 12.75 ounces of potassium hydroxide. Acetic acid must be added slowly, with constant stirring, so that the stannic acid formed will redissolve. Filtration of stannate baths is very difficult. Sludge removal should be done when parts being plated are rough from occluded dirt. Sludge usually consists of hydrated tin oxide and carbonates. Allow the bath to cool and settle overnight, decant the clear solution to another tank, and shovel out the remaining sludge. Refer to Table XI for troubleshooting.
REFLOWING TIN DEPOSITS Tin can be plated either matte or bright. Although the characteristics of each are comparable, bright tin plating has its proponents and is quite successful, commercially. Converting the matte finish of tin plate to a bright one can be accomplished by means of a process called reflowing, flow melting, or flow brightening. The tin coating is heated momentarily to a temperature slightly above its melting point of 450°F, and then quickly quenched to produce the bright finish. Among the methods used in heating the plated material are induction, conduction, radiant heating, and immersion. The most commonly used is immersion in hot oil or fat for a short time. Typical operating conditions are given in Table XII. The oil is usually a long chain fatty acid ester of glycerin such as tallow, palm oil, or partially hydrogenated oil, which has a sufficiently high flash point for the temperature used. It should also have some free fatty acid to serve as a flux. It is best to reflow as soon as possible after plating. The heating to the molten stage Should be completed within 2 to 10 seconds before removing and quenching. For best results, the time to melt the tin coating should be preciseIy calculated. Unsatisfactory results, in the form of dewetting or bailing-up, can occur if the parts are left in the oil too long after the coating melts. The size of the oil bath is also important so that the immersion of cold parts does not reduce the oil temperature to the point that heating time must be prolonged. A suitable quench to use is 4 to 6 in. of kerosene (acid-free) over water. The parts should be lowered into the quench slowly so that the molten tin coating solidifies in the kerosene layer. A spangled appearance will result if the molten tin comes in contact with the water. The water further lowers the temperature of the parts and also keeps the kerosene from being heated too near its flash point. The kerosene also serves to remove some of the oil from the tin surface.
304
Table XI. Troubleshooting Stannate Baths Problem
Cause
Remedy
Anodes gray or white
Initial current density too low to form film.
Remove anodes and replace one at a time when current is on.
Anodes brown or black, film passive
Low free hydroxide Low temperature High anode current density
Add KOH or NaOH. Increase temperature. Add anodes. If above does not remove film, dip anodes in a 20% hydrochloric acid solution and rinse well before using.
Excessive anodic gassing
Low temperature High anode current density
Increase temperature. Add anodes.
Film lost after first forming
High free hydroxide Low anode current density High temperature Poor anode contact
Add 10% acetic acid. Remove some anodes. Lower temperature. Clean contact.
No anodic film
Initial current density too low to form film
Remove anodes and replace one at a time when current is on.
Low cathode efficiency
Low temperature Low tin High free hydroxide High current density
Increase temperature. Add stannate. Add 10% acetic acid. Lower current density.
Low anode efficiency
Low temperature Low free hydroxide High current density
Increase temperature. Add KOH or NaOH. Lower current density or add mere anodes.
Narrow plating range
Low temperature Low tin Potassium bath Sodium bath
Increase temperature. Add stannate Add stannate. Switch to potassium bath.
Low conductivity
Low temperature Low free hydroxide Low tin
Increase temperature. Add KOH or NaOH. " Add stannate.
Nonadherent deposits
Initial current density too low to form film
Remove anodes and replace one at a time when current is on.
Solution crystallizes
High carbonates
Freeze out carbonates and then filter.
Rough, dark, or spongy deposits
Stannous tin in solution
Treat with hydrogen peroxide.
Small parts generally are i m m e r s e d in the hot oil in bulk in a suitable basket. T h e y are then separated b y p o u r i n g t h e m slowly into the q u e n c h or b y using a breaker, w h i c h separates t h e m before entering tile quench. F o r f l o w - m e l t i n g large parts, a c o m p a r t m e n t basket is r e c o m m e n d e d to prevent disfiguration o f parts t o u c h i n g while the tin is molten. The b r e a k d o w n o f free fatty acid content will g r a d u a l l y reduce the effectiveness o f the oil, relative to w h e t h e r it drains freely a n d evenly f r o m the molten surface or w h e t h e r a
305
Table XII. Operating Conditions for Tin Reflowing
Plating thickness, in. Immersiontime, see. Temperature of oil bath, °F Acid neutralization value, mg KOH/g of oil
Optimum
Range
0.00025 6 490 2.7
0.00014).0003 2-10 480-510 2.3~5.6
Table XIII. Troubleshooting for Tin Reflowing Problem
Cause
Remedy
Etching
Acid neutralization value too high
Dewetting
Tin coating too thick Part not cleaned well before plating Acid neutralization value too low Tin coating too thin Molten tin comes in contact with water in quench
Remove part of oil bath and replace with new oil. Plate for shorter time. Check cleaning cycle. Add fatty acid flux. Plate for longer period of time. Increase in quench. Lower parts more slowly.
Streaky Not fully bright Spangling
streaked appearance occurs. To maintain the oil suitably for proper draining, part of it (10-25%) must be replaced frequently. Residual oil must be removed from the tin surface after flow melting. Any ordinary degreasing method may be used, but the preferred method is vapor degreasing. There are a number of factors that are critical in successful reflowing. Good cleaning, good plating, and good rinsing practices are vital. Dewetting is more likely to occur on large flat areas, rather than with rounded shapes. Troubleshooting hints are offered in Table XIII.
Determination of Acid Neutralization Value 1. Weigh 6 to 10 grams of oil in a 250-ml Erlenmeyer f a s k and add approximately 50 ml of 3A alcohol (a methanol-ethanol mixture) previously neutralized to a pH of 7.0. 2. Carefully heat the solution on a hot plate while stirring until one or two bubbles appear on the surface of the solution. 3. Remove the solution from the hot plate, add five drops phenolphthalein indicator, and immediately titrate with 0.1 N sodium hydroxide from clear to a pink endpoint that persists for 10 seconds. AcidNeutralizationValuemg KOH/g oil = mlNa/OH × 5.611/weightoilsample,g
306
and Charles Rosenstein She#Case Ltd., Jerusalem, Israel Tin and tin-lead alloys are solderable and, therefore, are used extensively in the electronics industry to bond electronic components. This precludes the need for strong fluxes to wet the deposit. Tin and lead can be codeposited easily because of the closeness of their standard electrode potentials. Tin and tin-lead deposits must possess all of the following characteristics: good solderability and reflowability; low porosity; good corrosion resistance; and uniformity of alloy composition, thickness, smoothness, and appearance, over a wide current density range.
ADDITIVES Organic additives are required in all tin, lead, and tin-lead electroplating solutions to produce a useful deposit. In the absence of additives, treed, nonadherent, and nodular deposits result; therefore, additives are absolutely essential in order to yield smooth, uniform deposits and to impart good throwing power. Additives are depleted during plating and must be routinely replenished. Electrolysis causes some decomposition of the organics, resulting in their occlusion in the deposit. If present in sufficiently large amounts in the deposit, organics can cause solderability and reflow problems. Accelerated aging tests are performed on tin and tin-lead deposits to help predict their shelf life, as components are sometimes used long after they are plated.
TIN, LEAD, AND TIN-LEAD PLATING BATHS Tin, lead, and tin-lead electroplating solutions are used to plate components in numerous engineering, communications, military, and consumer product applications.
Tin Barrel, Still, and High-Speed Baths These baths are well suited for electrolytic tin plate and are used to plate transistors, semiconductors, various electronic parts, refrigerator parts, and kitchenware. Higher temperatures permit higher current densities and wire speeds.
Lead Barrel a n d Still B a t h s Lead baths are used in the plating of bearings, connectors, internal and conforming anodes for chromium plating, valves, seals, and parts for storage batteries. Because lead deposits are soft, stow barrel speeds are recommended in order to prevent heavy parts from bonding to one another.
291
Table I. Concentrations of Fluoboric Acid, Lead Fluoborate, Stannous Fluoborate, and Copper Fluoborate Used in Tin, Lead, and Tin-Lead Alloy Plating Product
Concentration (g/L)
48% Fluoboric Acid Fluoboric Acid, 100% Boric Acid 5 t % Lead Fluoborate Lead Fluoboric Acid, 100% Boric Acid 50% Stannous Fluoborate Stannous Tin Fluoboric Acid, 100% Boric Acid 45% Copper Fluoborate Copper Fluoboric Acid, 100% Boric Acid
Specific Gravity (20°C)
1.365 656 19 1.710 480 11 48 1.600 320 48 48 1.550 187 30 30
60 Tin/40 Lead Solder Barrel, Still, and High-Speed Baths The high throw of the baths and the excellent solderability and shelf life of the deposit make these baths suitable for use on printed circuit boards, connectors, and other specialized electrical devices. The high-speed bath is used for reel-to-reel applications, such as wire and strip plating.
90 Tin/10 Lead Barrel, Still, and High-Speed Baths This bath provides a uniform, smooth matte deposit, which is used for various engineering applications. Higher temperatures permit higher current densities, which are required in wire and strip plating.
93 Lead/7 Tin Barrel and Still Baths Deposits plated from 93 lead/7 tin baths are harder than those obtained from lead baths. The 93 lead/7 fin bath is used to plate bearings and seals. Since 93 lead/7 tin alloys are soft, slow barrel speeds are recommended in order to prevent heavy parts from bonding to one another.
10 Tin/88 Lead/2 Copper Ternary Alloy Barrel and Still Baths The operation of the ternary alloy bath is similar to that of the 93 lead/7 tin bath. The ternary alloy deposit results in bearing metals that exhibit a greater resistance to fatigue than do the 93 lead/7 tin alloys. The alloy is used to plate linings in sleeve bearings.
FLUOBORATE PLATING Tin fluoborate, lead fluoborate, and fluoboric acid, in various proportions (see Tables I and II), can be used for plating all percentages of tin-lead, 100% lead, and 100% tin. Fhioborate baths require boric acid for stability. Anode bags filled with boric acid are hung in the plating tanks. Stabilized liquid peptone, gelatin, resorcinol, or other liquid organic additives must be added to tin, lead, and tin-lead baths to produce smooth, nontreeing deposits
292
!
c,D 4:x
10-20 15
93 Lead/7 Tin Barrel and Still Baths: Range Optimum 135-150 143
195-239 217
8-12 10
8-14 11
23-30 26
195-239 218
--
--
Lead (g/L)
1.5 2.5 2
--
--
--
--
--
--
Copper (g/L)
aAgitation required, mechanical or barrel; vigorous for high-speed plating, Periodic filtration through 1-t~m Dynel or polypropylene cartlidges. Tanks, pumps, and filters made from polyethylene, polyvinyl chloride (PVC), or Koroseal Heating and cutting equipment made from Karbate or Teflon.
lO Tird88 Lead/2 Copper Barrel and Still Baths." Range 20-40 Optimum 30 8-12 10
70-80 75
90 Tin/lO Lead Barrel and Still Baths." Range 150-200 Optimum 175 15-30 23
12-20 16
350 500 425
High Throw: Range Optimum
53-60 56
98-150 124
60 Tin~40 Lead Solder Bath: Barrel and still: Range Optimum
45-60 53
15-30 23
225-300 263
Wire and strip: Range Optimum
3045 37
Tin (g/L)
Lead Barrel and Still Baths: Range Optimum
188-263 225
Tin Bath." Barrel and still: Range Optimum
HBF4 (g/L)
Table II. Fluoborate Bath Compositions and Operating Parametersa
As required
As required
As required
As required
As required
As required
As required
As required
Additive
70-100 (21-38)
70-100 (21-38)
70-100 (21-38)
70-100 (21-38)
70-85 (21-29)
70-100 (21-38)
90-t30 (32-54)
90-120 (3249)
Temperature T (°C)
20-70
20-70
1-80
15 25
25-35
20-70
1-300
1 80
Current Density (A/ft 2)
1:1
1:1
1:1
2:1
2:1
1:t
1:1
1:1
Anode to Cathode Ratio
10 tin/88/lead/2 copper, or 90 lead/10 tin, or pure lead
93 lead/7 tin
90 tirdl0 lead
60 tird40 lead
60 tin/40 lead
Pure lead
Pure tin
Pure tin
Anodes
Electrop/ating Chemicals & Services
®
~:s~ ...and y o u were led to believe we only
TM
~irl~t~
TM
supply palladium products! i
Electroptating Chemicals and Services Technical Information: 1-800-221-6316 Customer Service: 1-800-3(
Table III. Concentrations of Methane Sulfonic Acid (MSA) and MSA Salts Used in Tin, Lead, and Tin-Lead Alloy Plating Product
Concentration (g/L)
Specific Gravity (20°C)
50% MSA 70% MSA Stannous methane sulfonate
MSA 100% = 613 MSA 100% = 945 Sn = 120 Sn = 150 Sn = 200 Sn = 240 Sn = 300 Pb = 450 Pb = 500 Cu = 100
1.225 1.350 1.230 1.285 1.390 1.450 1.560 1.630 1.700 1.260
Lead methane sulfonate Copper methane sulfonate
and to increase the throwing power of the plating bath. These additives are mandatory, as fluoborate baths cannot function without them.
METHANE-SULFONIC-ACID-BASED PLATING Methane sulfonic acid (MSA)-based electroplating systems gained commercial acceptability during the early 1980s. Various plating baths and additives have been developed. Each MSA bath requires a custom-made additive for that specific content of tin, lead, and MSA in order to obtain a useful deposit. As with fluoborates, metal salts of MSA are very soluble and various concentrations of MSA and its metal salts are available (see .Tables III and IV). Troubleshooting guidelines for fluoborate- and MSA-based tin, lead, and tin-lead plating baths are given in Tables V to VII. The troubleshooting table for acid-tin-based baths (Table V) is also applicable to stannous sulfate baths.
TIN PLATING FROM STANNOUS SULFATE BATHS The stannous sulfate bath contains stannous sulfate, sulfuric acid, and additives (see Table VIII). Advantages of this bath are its good throwing power, high cathode efficiency, operation at room temperature, and ease of control. Addition agents are essential for the operation of this bath. They inhibit the oxidation of stanuous tin, produce smooth, dense deposits, and prevent treeing. Among those in use are phenol- or cresol-sulfonic acid, gelatin, /?-naphthol, and resorcinol. Several proprietary additives are also available. Bright tin deposits are obtained through the addition of proprietary additives. Solderability, corrosion resistance, and leveling characteristics are essentially the same in bright or matte plating. It is questionable whether bright tin plating is superior to matte in resistance to whisker growth and tin pest, as has been claimed. Refer to Table V for troubleshooting.
TIN PLATING FROM STANNATE BATHS Alkaline baths are also used for tin plating, with sodium stannate and potassium stannate as the two available baths (see Table IX). Both will produce similar satisfactory results. The criteria for choosing one over the other are related to cost and speed of plating.
296
General Chemical, the North American pioneer in fluoborates, is "basic" in these important metal finishing chemicals. We still control quality and consistency, from raw material to finished product delivery. It's what sets us apart from our competition. For additional information and the location o f youJ nearest distributor, call us toll-free at 800-631-8050. Ammonium Fluoborate, Sodium Fluoborate • Fluoboric Acid Stannous Fluoborate • Lead Fluoborate ° Peptone
General Chemical 90 East HalseyRoad [] Parsippany,NJ 07054 ©Genera/Chemical Corp. 1998
www.genchemcorp,com
0o
I'o co
Tin Barrel Still and High-Speed Baths:
200-250 225
High speed: Range Optimum
30-50 40
200-250 225
30-50 40
4-5 4.5
4-5 4.5
17-25 21
12-20 16
12-20 16
--
35-55 45
Tin (g/L)
65-75 70
56-72 64
2~3
8-12 10
6-10 8
56-72 64
--
Lead (g/L)
0.8-1.2 1.0
--
--
Copper (g/L)
and Operating
aAgitation required, mechanical or barrel; vigorous for high-speed plating. ]?latinized titanium anodes can be substituted for the soluble anodes. Periodic filtration through 1-/xm Dynel or polypropylene cartridges. Anode to cathode ratios for all baths is 1:1. Tanks, pumps, and filters made from polyethylene, polyvinyl chloride (PVC), or Koroseal. Heating and cooling equipment made from Karbate or Teflon.
Range Optimum
10 Tiru'88 Lead~2 Copper Barrel and Still Baths:
Range Optimum
93 Lead/7 Tin Barrel and Still Baths:
Range Optimum
90 Tin/lO Lead Barrel, Still, and High-Speed Baths:
200-250 225
30-50 40
200-250 225
Barrel and still: Range Optimum
60 Tin/40 Lead Solder Bath:
Range Optimum
Lead Barrel and Still Baths:
Range Optimum
MSA (g/L)
T a b l e IV. M e t h a n e S u l f o n i c A c i d ( M S A ) B a t h C o m p o s i t i o n s
As required
As required
As required
As required
As required
As required
As required
Additive
Parameters "
70-100 (21-38)
70-85 (21-29)
70-130 (21-54)
70-100 (21-38)
70-85 (21-29)
70-100 (21-38)
70-130 (21-54)
Temperature °F (°C)
1-40
1-40
1-250
1-250
1-40
1-40
1-250
Current Density (A/ft2)
10 tin/88 lead/2 copper, or 90 lead/10 tin, or pure lead
93 lead/7 tin
90 tirdl0 lead
60 tin/40 lead
60 tird40 lead
Pure lead
Pure tin
Anodes
TIN-LEAD
AND
PLATING
LEAD
FREE
PROCESSES
SPECIALISTS FOR
BRIGHT
AND
MATTE
FINISHES
•
•
MACLEE
CHEMICAL
C O . INC.
2065 N. Southport, Chicago, IL 60014 • (773) 9 2 9 - 9 7 9 2 • F a x ( 7 7 3 ) 9 2 9 - t 6 2 2 E-mail: [email protected]
* Website:
http://www.macleechemical.com
Mason
A
THECLEAR A N D ODOR FREE SOLUTION IN M S A
HECTRIO MANUFACTURERSOF:
•.Tin (II) Methane Sulfonate • Lead (II) Methane Sulfonate •.Purified Methane Sulfonic Acid (MSA) • 13% Stannous Sulfate solution - highest concentration available worldwide 400 First Avenue, Royersford, PA 19468 Tel: 1-610-948-0734, Fax: 1-610-948-0744
1-800-784-9533
• Stannic Chloride (Tin Tetrachloride) (Tin IV Chloride), (Tin Chloride) • Stannic Chloride Solution ~ m Stannous Chl~fide Anhy~rou s (Tin |ii~Chlori~),
:,
• sto88~us C~i~ide Solu, o~ ~,. ............
• ~$~ium
......................
• Potassium • Potassium Stannate S~'tio~ ' ;'~:!:;:::~' Mason Corporation chemicals' unique features include specialized service, the highest quality and purity in tin metals, and flexibilty in packaging of the chemicals to meet the customers needs.
Mason Corporation P.O. Box 38 • Schererville, IN 46375 1-219-865-8040 • Fax: 1-219-322-3611 800-326-3075 • Email: [email protected]
Table V. Troubleshooting Acid-Tin-Based Baths Problem
Cause
Remedy
Treeing
Low additive Low acid Organic contamination Solids in bath Metallic contamination Stannic tin Low additive Low tin Low acid Too high current density Low additive Low additive Low additive Low acid Low tin Too high current density Low additive Organic contamination Low tin Low temperature Poor cleaning Organic contamination Low temperature Low additive Organic contamination
Add additive. Increase acid. Carbon treat. Filter. Dummy bath. Filter and exclude air from bath. Add additive. Increase tin. Increase acid. Lower current density. Add additive. Add additive. Add additive. Increase acid. Increase tin. Lower current density. Add additive. Carbon treat. Increase tin. Increase temperature. Correct cleaning cycle. Carbon treat. Increase temperature. Add additive. Carbon treat.
Roughness
Graininess Bumhlg
Streaking or pitting Poor throw
Gassing
Lack of adhesion Brittle deposits Dark deposit
Table VI. Troubleshooting Lead-Based Baths Problem
Cause
Remedy
Treeing
Low lead Low additive Low lead Low additive Metallic contamination Solids in bath Too high current density Low agitation Low additive Low acid Chloride/sulfate contamination
Increase lead. Add additive. Increase lead. Add additive. Dummy the bath. Filter bath. Lower current density. Increase agitation. Add additive. Increase acid. Filter bath and check cleaning cycle. Correct cleaning cycle. Carbon treat. Increase lead. Increase current density. Analyze and adjust copper.
Poor throw Roughness
Lack of adhesion Dark deposit Thin deposit Mossy deposit (ternary alloy only)
300
Poor cleaning Organic contamination Low lead Too low current density Excess copper in bath
~imply The Best •
ll
~ I
-
-.
.
.
.
Guaranteed Q~,ality * O~-Time Delivery * Ex~Mtent Prices
, x
TITAN ""-)~:,_.S-" INTERNATIONAL, INC, Plating Supply Specialist
•
-
~_
•
- ~
O
MSA Salts-Tin, Lead, Copper Sodium Hypophosphite, High Purity
ll
(IIEMICALS: StannousMethaneSulfonate 300 g/L to 400 g/L, & Crystals Lead Methane5ulfonate Methane SulfonicAdd 70%
MSA ElectroplcttingAdditives Pure Tin, Tin-Leod, Tin-Copper Motte & Bright Finishes
PotassiumStannate 5tannousSuffate Sodium Stannate
C A , E4, NJ~
Other Metal & Chemical Lines: Brass, Bronze, Cadmium, Copper, Chrome, Nickel & Zinc! Call Toll Free: 800-435-4644 w w w . titanin tl. corn
Rotterdam, Japan, Tatwan+China
Building a ~ ~ . ~ | PARINEK~HIP( ~k/ | ~ t with y o u n o w and i ikto thefuture [
Varsa! [nstrarm, nts, Ir~c, [
i
[
F',."~ Che~*dcal,~" D~:¢si:m
[
| |
[[
363 [vy~ea~dR&~Warrrda~tel;PA lg974][
| |
Tel: (215)957-5880 Fax: (2~5}957-9~ 1 l | /gmail: hffo(~'arszl,e~m t
| ~
a
t
.
c
o
m
The Electrodeposition of Tin and Its Alloys by M. Jordan 409 pages $130.00 This is the definitive book on every conceivable aspect of the electrodeposition (and electroless deposition) of tin and its alloys. All bath types are discussed in detail. Analysis of baths and testing of deposits using both simple methods and more advanced instrumental techniques are covered. National, international, military, and other standards and specifications are also included. Send Orders to: METAL FINISHING 650 Avenue of the Americas, New York, NY 10011 For faster service, call (212) 633-3199 or FAX your order to (212) 633-3140 All book orders must be prepaid.. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and $20.00 for each book shipped express to all other countries.
_i
Table VII. Troubleshooting 60 Tin/40 Lead Solder Baths Problem
Cause
Remedy
Poor throw
Low tin and lead Low acid Low additive High anode to cathode ratio Organic contamination Too high current density Organic contamination Low additive Low acid Poor cleaning Low tin and lead Low acid Too high current density Organic contamination Organic contamination Solids in bath Stannic tin Chloride/sulfate contamination
Increase tin and lead. Increase acid. Add additive. Remove several anodes. Carbon treat. Lower current density. Carbon treat. Add additive. Increase acid. Correct cleaning cycle. Increase tin and lead. Increase acid. Lower current density. Carbon treat. Carbon treat. Filter. Filter and exclude air from bath. Filter bath and check cleaning cycle. Dummy the bath. Increase acid. Use correct current density. Use proper agitation. Use anodes with correct composition. Analyze bath and correct ratio. Use correct plating time.
Gassing Treeing Lack of adhesion Burning
Graininess Roughness
Incorrect alloy
Unsatisfactory reflow
Metallic contamination Low acid Wrong current density Improper agitation Incorrect anodes Incorrect tin to lead ratio Incorrect thickness Dewetting caused by: Poor cleaning Organic contamination High reflow temperature Incorrect alloy
Correct cleaning cycle. Carbon treat. Use proper temperature. See Problem: "Incorrect alloy" above.
Table VIII. Stannous Sulfate Bath Composition and Operating Conditions
Tin Sulfuric acid, 100% Additive Anodes Current density, Mft 2 Anode to cathode ratio Temperature, °F (°C) Agitation Filtration Pumps and tanks Heaters and coolers
302
Range (g/L)
Optimum (g/L)
30-50 100-140 As recommended by manufacturer Pure tin 1-25
40 120
l:l 70-85 (21-29) Cathode and solution 1 /J,m Dynel or polypropylene cartridges Polyethylene, polypropylene, rubber Karbate
Table IX. Stannate Baths Composition and Operating Conditions Rack (g/L)
Barrel (g/L)
90 40 12 15-20 25 3-4 170-180 (77-82)
180 80 23 5-15 15-25 3-4 165-175 (74-79)
100 40 15 30-100 30-40 4-8 150-180 (66-82)
200 80 23 1-100 10-30 4-14 150-180 (66-82)
Sodium Stannate Bath:
Sodium stannate Tin Free sodium hydroxide Cathode current density, A/ft 2 Anode current density, A/ft ~ Voltage Temperature, °F (°C) Potassium Stannate Bath:
Potassium stannate Tin Free potassium hydroxide Cathode current density, A/ftz Anode current density, A/ft2 Voltage Temperature, °F (°C) Sodium and Potassium Stannate Baths:
Anode to cathode ratio Filtration Agitation Pumps and tanks Heaters and coolers
1:1 25/zm Dynel or polypropylene cartridges Cathode and solution Stainless steel, low-carbon steel, polypropylene, polyethylene Low-carbon steel, polypropylene, polyethylene
Potassium stannate is generally preferred over sodium stannate for the following reasons: 1. Higher plating rates. 2. Greater conductivity, especially in barrel plating. 3. Less potential for sludge formation. The plater is cautioned to choose one or the other bath, and not to mix them. Potassium hydroxide should be used with the potassium stannate bath and sodium hydroxide with the sodium stannate bath. Although the "mixed" bath is operable and satisfactory results can be obtained, analytical control is quite difficult.
A n o d e s in Stannate Baths Anodes used in stannate baths are made of pure tin, tin alloyed with 1% aluminum (known as "high-speed" anodes), or inert materials such as steel, nickel, or stainless steel. Advantages of the high-speed anode are its greater current density range and its consistent efficiency at higher current density. The formation and maintenance of the anodic film is critical to the operation of the stannate bath. The primary cause of poor tin deposits is the improper filming of the anodes. To film tin anodes, a "surge" of current, at higher than normal current density, must be
Table X. Stannate Anode Film Characteristics
Color of film formed Anode current density, Alft 2
Pure Tin
Tin + 1% AI (High Speed)
Inert Anode
Yellowish 15-40
Olive green 30-80
No film No limits
303
impressed on them for a few seconds to a minute, after which the current is reduced to its regular value (see Table X). The anodes must be refilmed after each shut-down period prior to plating, as the film dissolves rather quickly. Practically all problems in the operation of a stannate bath are resolved when proper anode operation is achieved. Inert anodes are advantageous in that they avoid the difficulty of filming and they inhibit the formation of harmful stannous tin by producing oxygen at the anode. Furthermore, they do not change shape in use. Their use has previously been somewhat limited because of the need for chemical replenishment of the bath as more tin is consumed. Replenishment of the tin content of the bath by a tin oxide solution has made the use of inert anodes practical in potassium stannate systems only.
Operation of Stannate Baths Stannate solutions should be light straw or light gray in color. Black solutions indicate the presence of stannous tin, and they should be treated with hydrogen peroxide (2 ml 35% hydrogen peroxide per gallon of solution). If frequent additions of peroxide are necessary, make sure the hydroxide concentration is not too high and/or the anode current density is not too low. A 10% solution of acetic acid is used to lower the free hydroxide content of the bath; one gallon of 10% acetic acid will neutralize 9.25 ounces of sodium hydroxide or 12.75 ounces of potassium hydroxide. Acetic acid must be added slowly, with constant stirring, so that the stannic acid formed will redissolve. Filtration of stannate baths is very difficult. Sludge removal should be done when parts being plated are rough from occluded dirt. Sludge usually consists of hydrated tin oxide and carbonates. Allow the bath to cool and settle overnight, decant the clear solution to another tank, and shovel out the remaining sludge. Refer to Table XI for troubleshooting.
REFLOWING TIN DEPOSITS Tin can be plated either matte or bright. Although the characteristics of each are comparable, bright tin plating has its proponents and is quite successful, commercially. Converting the matte finish of tin plate to a bright one can be accomplished by means of a process called reflowing, flow melting, or flow brightening. The tin coating is heated momentarily to a temperature slightly above its melting point of 450°F, and then quickly quenched to produce the bright finish. Among the methods used in heating the plated material are induction, conduction, radiant heating, and immersion. The most commonly used is immersion in hot oil or fat for a short time. Typical operating conditions are given in Table XII. The oil is usually a long chain fatty acid ester of glycerin such as tallow, palm oil, or partially hydrogenated oil, which has a sufficiently high flash point for the temperature used. It should also have some free fatty acid to serve as a flux. It is best to reflow as soon as possible after plating. The heating to the molten stage Should be completed within 2 to 10 seconds before removing and quenching. For best results, the time to melt the tin coating should be preciseIy calculated. Unsatisfactory results, in the form of dewetting or bailing-up, can occur if the parts are left in the oil too long after the coating melts. The size of the oil bath is also important so that the immersion of cold parts does not reduce the oil temperature to the point that heating time must be prolonged. A suitable quench to use is 4 to 6 in. of kerosene (acid-free) over water. The parts should be lowered into the quench slowly so that the molten tin coating solidifies in the kerosene layer. A spangled appearance will result if the molten tin comes in contact with the water. The water further lowers the temperature of the parts and also keeps the kerosene from being heated too near its flash point. The kerosene also serves to remove some of the oil from the tin surface.
304
Table XI. Troubleshooting Stannate Baths Problem
Cause
Remedy
Anodes gray or white
Initial current density too low to form film.
Remove anodes and replace one at a time when current is on.
Anodes brown or black, film passive
Low free hydroxide Low temperature High anode current density
Add KOH or NaOH. Increase temperature. Add anodes. If above does not remove film, dip anodes in a 20% hydrochloric acid solution and rinse well before using.
Excessive anodic gassing
Low temperature High anode current density
Increase temperature. Add anodes.
Film lost after first forming
High free hydroxide Low anode current density High temperature Poor anode contact
Add 10% acetic acid. Remove some anodes. Lower temperature. Clean contact.
No anodic film
Initial current density too low to form film
Remove anodes and replace one at a time when current is on.
Low cathode efficiency
Low temperature Low tin High free hydroxide High current density
Increase temperature. Add stannate. Add 10% acetic acid. Lower current density.
Low anode efficiency
Low temperature Low free hydroxide High current density
Increase temperature. Add KOH or NaOH. Lower current density or add mere anodes.
Narrow plating range
Low temperature Low tin Potassium bath Sodium bath
Increase temperature. Add stannate Add stannate. Switch to potassium bath.
Low conductivity
Low temperature Low free hydroxide Low tin
Increase temperature. Add KOH or NaOH. " Add stannate.
Nonadherent deposits
Initial current density too low to form film
Remove anodes and replace one at a time when current is on.
Solution crystallizes
High carbonates
Freeze out carbonates and then filter.
Rough, dark, or spongy deposits
Stannous tin in solution
Treat with hydrogen peroxide.
Small parts generally are i m m e r s e d in the hot oil in bulk in a suitable basket. T h e y are then separated b y p o u r i n g t h e m slowly into the q u e n c h or b y using a breaker, w h i c h separates t h e m before entering tile quench. F o r f l o w - m e l t i n g large parts, a c o m p a r t m e n t basket is r e c o m m e n d e d to prevent disfiguration o f parts t o u c h i n g while the tin is molten. The b r e a k d o w n o f free fatty acid content will g r a d u a l l y reduce the effectiveness o f the oil, relative to w h e t h e r it drains freely a n d evenly f r o m the molten surface or w h e t h e r a
305
Table XII. Operating Conditions for Tin Reflowing
Plating thickness, in. Immersiontime, see. Temperature of oil bath, °F Acid neutralization value, mg KOH/g of oil
Optimum
Range
0.00025 6 490 2.7
0.00014).0003 2-10 480-510 2.3~5.6
Table XIII. Troubleshooting for Tin Reflowing Problem
Cause
Remedy
Etching
Acid neutralization value too high
Dewetting
Tin coating too thick Part not cleaned well before plating Acid neutralization value too low Tin coating too thin Molten tin comes in contact with water in quench
Remove part of oil bath and replace with new oil. Plate for shorter time. Check cleaning cycle. Add fatty acid flux. Plate for longer period of time. Increase in quench. Lower parts more slowly.
Streaky Not fully bright Spangling
streaked appearance occurs. To maintain the oil suitably for proper draining, part of it (10-25%) must be replaced frequently. Residual oil must be removed from the tin surface after flow melting. Any ordinary degreasing method may be used, but the preferred method is vapor degreasing. There are a number of factors that are critical in successful reflowing. Good cleaning, good plating, and good rinsing practices are vital. Dewetting is more likely to occur on large flat areas, rather than with rounded shapes. Troubleshooting hints are offered in Table XIII.
Determination of Acid Neutralization Value 1. Weigh 6 to 10 grams of oil in a 250-ml Erlenmeyer f a s k and add approximately 50 ml of 3A alcohol (a methanol-ethanol mixture) previously neutralized to a pH of 7.0. 2. Carefully heat the solution on a hot plate while stirring until one or two bubbles appear on the surface of the solution. 3. Remove the solution from the hot plate, add five drops phenolphthalein indicator, and immediately titrate with 0.1 N sodium hydroxide from clear to a pink endpoint that persists for 10 seconds. AcidNeutralizationValuemg KOH/g oil = mlNa/OH × 5.611/weightoilsample,g
306