By Mr. R.V. Sudhakar
Director, Electronics Test & Development Centre, Government of India, Hyderabad.

Zero defect soldering requires consistent solder quality. Contaminants and high oxide levels in solder can result in soldering defects such as non-wetting, dull and granular structures and also bridges and icicles formation in case of wave soldering. In order to avoid these problems, the assembly industry shall check the impurities content of solder alloy regularly as per standards prevailing in the industry.

The main concern of solder impurities is in the context of wave soldering and solder baths (used for pre-tinning of component leads, and removal of Gold), since initially pure solder alloy is liable to be contaminated gradually during usage. Periodic testing/checking of impurity levels is mandatory for solder baths and wave soldering machines.

Solder wire/Solder paste – with degree of purity (with impurities within limits) as received from the vendor/manufacturer is acceptable. The vendor/manufacturer should be reliable, reputed and experienced.

In this article the following points pertaining to impurities in solder alloys are discussed.

  • Potential defects in soldered joints due to impurities in solder alloys
  • Effects of critical impurities
  • Periodicity of analysis
  • What standards say about impurities

Potential Defects in soldered joints due to impurities in solder alloys

The amount of impurities in the solder is related to observed defects or solder conditions. These defects, specifically caused by contaminated solder, are noted below with some discussion about the impurities which caused the problem.

Icicles, Shorts, Bridges: Cadmium, zinc and aluminum in trace amounts increase the surface tension of the solder to cause this defect. Copper and gold increase the solder viscosity to cause the same problem.

Large Solder Fillets

Copper, gold and antimony increase the melting point of the solder and the intermetallic compounds with tin or lead make the solder more sluggish. The result is larger fillets and more solder is consumed to create the solder joint.

Unfilled Holes

The speed of wetting is reduced by the presence of copper, gold, antimony and cadmium. Though no such instance occurred with zinc and aluminum, these metals are also likely to affect wetting speed because of their ability to increase the surface tension of the solder.

Dull Solder, Gritty Solder

Cadmium and zinc in trace amounts make the solder surface dull. Gold also dulls the surface but is quite often indicated by a sparkling, crystalline surface condition. Bismuth or antimony in large amounts above 2.5% also dull the surface. Copper and aluminum contamination results in a gritty-looking solder surface. Both phosphorous and sulfur can cause gritty solder though rarely are these two impurities found in solder samples.

Dross Inclusions

Dross inclusions in the solder show up as visible particulate grit or hidden inside a bump or pimple in the otherwise shiny solder surface. Quite often the source of this problem is an unusual amount of iron in the solder.

Cracked Joints

Inclusions in the solder such as intermetallics of tin or lead with copper, gold and antimony can provide the nucleus for crack propagation. Excess amounts of Aluminum and Zinc also causes cracks on soldered joints.

De-wetting

Zinc, aluminum, cadmium, antimony and phosphorous can cause solder to de-wet in soldered joints.

Now let us see the effects of some critical impurities and its sources in the solder alloy

Effects of Critical impurities

  1. Copper:If the level rises above solubility level at the temperature used, needle shaped Cu6Sn5 crystals form in molten solder thereby reducing mobility. Gives rise to incomplete filling of joints. Joint appears to be rough (grainy) and dull. Formation of dross increases.
  2. Iron:Less soluble in molten solder. Beyond solubility iron forms small globular crystal of Fe2Sn, gives a gritty appearance
  3. Aluminium:Very low trace forms skin over molten solder. Makes gritty looking joint and increases dross formation. Solder won’t have bright shiny surface.
    Source:Aluminium foils, pallets, fixtures, abrasion of worn conveyor rails.
  4. Zinc:Very small quantity creates skin on molten solder. Solder joints will have rough, frosty appearance, causes excessive dross formation.
    Sources:Brass parts and terminals, pins, fasteners, galvanized steel implements.
  5. Cadmium:Causes reduction in wetting power and produces dull looking joints.
    Sources:Cadmium plated hardware for large components. Today Cadmium has been restricted as per ROHS guidelines
  6. Gold:Dissolves extremely quickly in molten solder. On solidification large brittle crystals of AuSn4 are formed which weakens the joint structure. Joints are frosty and gritty.
    Source:Pins, connectors, gold plated edge connectors/contacts and component leads, footprints.
  7. Antimony: Reduces wetting power of solder and slightly increases wetting time. A minimum (0.2%) amount improves low temperature characteristics of solder joints. Too much Antimony makes the joint brittle. Solder becomes sluggish causing shorts.
    Sources:Found in some solder additives
  8. Silver:Creates dull appearance and with high concentration reduces mobility.
    Sources:Plated component leads, single strand jumpers
  9. Nickel:Forms insoluble components and creates blisters.
    Sources:Component leads or plated hardwires.
  10. Arsenic:Produces dull finish for soldered joints

Periodicity of analysis of contamination in solder alloys:

  • Solder bath used for solderability tests: As per IPC J-STD-003C, the solder in solder baths used for solderability testing shall be chemically or spectrographically analyzed or replaced each 30 operating days (An operating day consists of 8 hours period). Deviations to the 30 day limit shall be supported by historical data.
  • Solder bath used for pre-tinning: Once in every six months
  • Solder bath used for Gold removal: The solder in the solder pot used for tinning of leads should be changed as frequently as possible to prevent gold buildup in the solder
  • Solder bath in wave soldering machine: As per pre-determined schedule. The periodicity depends on the usage of the machine (typically if you find excess dross formation).

What standards say about impurities in solder alloy.

As per IS 193.2000 and IS 1921: 2005 “Rosin based solder alloy”

Impurities

The supplier shall undertake that the solder alloy materials does not contain the following impurities in excess of the amount stated below:
Element Percent, Max
Arsenic 0.05
Iron 0.02
Copper 0.05
Antimony: 0.2

The solders shall not contain impurities, such as Aluminum, Zinc or Cadmium in amounts which injuriously affect the properties of the solder (limited to 0.08%). Solder proved to be faulty in use and found to contain these impurities shall be rejected.

Even though this standard has called for control of impurities it is lenient towards critical impurities like Aluminum, Zinc and Cadmium.

As per ESA standard “ECSS-Q-70-08C”2009 “Manual soldering of high-reliability electrical connections:

Solder bath contaminants levels

Solder bath 1 Solder bath 2
Use Gold dissolution Pre-tinning
Contamination limits Au<1 Cu<0.25, Au<0.2, (Au+Cu)<0.3, Zn, Al, Fe traces

Solder Alloy contaminants levels

Alloy composition Ag min & .max Bi max. Cu max. Fe max. Zn max. Al. max. As max. Cd. Max. Others max.
Tin 63 0.1 0.05 0.02 0.001 0.001 0.03 0.002 0.08
Tin 60 0.1 0.05 0.02 0.001 0.001 0.03 0.002 0.08
Tin 62 silver loaded 1.8 – 2.2 0.1 0.05 0.02 0.001 0.001 0.03 0.002 0.08

As per NASA standard “NASA-STD-8739.3: 1997 – Soldered Electrical Connections

Solder alloy contaminants levels

All solder used for tinning and solder connections shall conform to ANSI/J-STD-006. Flux-cored solder shall be either composition SN60 or SN63 containing flux types R or RMA, or equivalent. For all soldering applications where adequate subsequent cleaning is not practical, only solder containing flux type R shall be used. Solid solders (no flux) for use in solder pots shall be of the same composition.

Solder bath contaminant levels (maximum allowed)

Cu Au Cd Zn Al Sb Fe As Bi Ag Ni
0.25 0.20 0.005 0.005 0.006 0.5 0.02 0.03 0.25 0.10 0.01

Conclusion:Impurities/contaminants in solder alloys shall not be neglected and they may be potential reason for reliability of soldered joints.

 

Brief Bio of Author:Mr.R.V. Sudhakar is an M.Sc in Applied Electronics & MBA in Financial Management. He is a Certified Trainer as per IPC-A- 610 and IPC-J-Std 001 standards, Certified IPC PCB Designer (CID) and Qualified Supervisor as per European Space Agency for Soldering of Through hole & SMT Assemblies. He has conducted more than 1400 training programs for the last 28 years and trained around 35000 participants in the area of Electronics Interconnection Technology and Industrial Automation. He is presently the Director, Electronics Test & Development Centre, Government of India, Hyderabad.