A major contributor to component solderability problems is the inconsistency in testing for component solderability. Marginal components often go undetected and subsequently enter the manufacturing process.
There is a critical need for a standard test method to determine:
(1) As-received solderability, and
(2) Solderability after storage.
Component solderability problems can, in most cases, be traced to:
(1) Questionable surface plating, especially improper surface activations
(2) Solderability degradation (chemically or thermally induced)
(3) Metallurgical and/or oxidation reactions introduced during required military burn-in operations, and
(4) Natural aging reactions due to poor environments or prolonged storage periods.
The utilization of dry aging, cyclic temperature-humidity aging and other stress environments are giving way to the steam aging test.
In order to ensure adequate solderability of incoming components, it is important that consistent and proper solderability testing is conducted.
(1) Perform solderability test in accordance with MIL-STD-883, Method 2003.
(2) The following parameters should apply:
a. Solderability sample preparation: An AQL sample per applicable procurement document shall be exposed to 8.0 ± 0.5 hours of artificial steam aging per TM 2003 of MIL-STD-883C. Sample tem- perature during steam aging shall be maintained at a temperature range of 4șC to 10șC below the local boiling point of the water.
b. Type "R" flux: non-activated rosin, 25 to 35% solids content.
c. Insertion/withdrawal rate: 1 inch/sec ± 0.25 inch/sec, dwell time = 5.0 ± 0.5 sec.
d. Solder temperature: 245 ± 5șC, solder bath composition Sn60 or Sn63, static bath.
e. Static Sn60 or Sn63 solder bath maintained to the following contamination limitations:
Limits % Elements Limits %
0.05 Arsenic 0.005
0.003 Silver 0.01
Note: All Others--0.005%
f. Visual standards shall utilize defect distribution charts. When viewed at 10 X magnification, defects shall not exceed 5.0% in stated critical measuring area.
g. Sample size shall be based upon number of devices, not terminations.
(3) Product lots failing this solderability test should be returned to supplier.
4.4 Future Investigations
Recommended areas for investigation include:
(1) Determine a reliable quantitative test method. A long range goal would be an instrument to quantitatively determine solderability. The wetting balance test is a good step in this direction.
(2) Determine different test methods for unique component types. Chip devices require test methods consistent with their size and intended usage such as vapor phase soldering. Other special components require application-oriented testing and cannot utilize the kind of test used for leaded components.
(3) Wetting requirements (percent and distribution). Correlation should be established between component and assembly allowable dewetting/non-wetting. The 5% dewetting/non-wetting criterion is a problem depending upon distribution of the defect.