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Bluish color of tin plating over nickel after Reflow
2006
Q. Hi Guys,
We are plating pure Sn over Ni surface (originally it was pure Sn over Ni/ NiP surface).
The components (D2PAK transistors) undergo the following process:
1. 168 hours soak @ Moisture Chamber (85 °C / 85% RH).
2. Reflow 3 times in IR oven (total cycle time of reflow ~ 2mins); peak temp of 260 °C.
Observations:
1. Components with Sn plating over Ni surface turned bluish after reflow
2. Components with Sn plating over Ni / NiP surface did not turn bluish
3. All Components that did not undergo Moisture soak did not turn bluish
Questions:
1. What's the mechanism/principle behind the bluish color?
2. Is it safe to say that the P content on the Ni/NiP surface prevents this bluish phenomenon?
Appreciate your help.
Semiconductors - Laguna, Philippines
A. The bluish color after reflow is Sn oxide. Presumably, the oxide thickness is up to 400-500A judging from the color.
On Ni underplate, Sn is forming IMC slowly with Ni, which gives it plenty of time staying in its molten state. Serious oxidation/reflow discoloration is observed.
On NiP underplate, Sn is forming better IMC with Ni and NiP. The IMC is forming faster and the crystalline IMC so formed between Sn and NiP is helping Sn to recrystallized faster. Therefore, oxidation is not as serious.
- Hong Kong
2006
A. Just try running the freshly plated tin thru an alkaline cleaner and rinsing again before re-flowing.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
2006
Q. Dear Robert,
Do you notice that alkaline post-treatment, such as TSP [trisodium phosphate [on
eBay or
Amazon]], is not improving the Sn reflow discoloration problem? Instead, I think that acid post-treatment may be more effective in removing surface adsorbed organics (in a like-dissolve like manner?) which may contributed to reflow discoloration.
- Hong Kong
2006
Danny,
will supplying N2 gas during reflow eliminate the risk of oxidation/discoloration?
- Laguna, Philippines
2006
Dear Monty,
Reflow of Sn under N2 can certainty reduce the risk of reflow discoloration. However, it is adding up to the cost and the O2 content during reflow must be kept at very low conc. (say 50 ppm) or below to become effective.
- Hong Kong
2007
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