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topic 21840

Oxidation of immersion silver coating on PCB



A discussion started in 2003 & continuing through 2017

(2003)

Q. I have recently started working on the oxidation of immersion silver coating on lead free boards. The main objective of my study is to find out the shelf life and the oxidation trends of the silver coating for lead free PCBs. I am looking for the experimentation schemes and any literature available on the research/subject.

Mike Anderson
- Toronto, Ontario, Canada


(2006)

A. DEAR MIKE,
I HAVE CONDUCTED LOTS OF EXPERIMENTS FOR IMMERSION SILVER ON PCB SINCE YEAR 2001.
NORMALLY, THE IMMERSION COATED SILVER LAYER IS EASILY TARNISHED OR DISCOLORED BY HOT BAKING OR DIPPING IN SULPHUR-CONTAINING SOLUTION. THEREFORE, THE CHEMICAL PROVIDERS OF ImAg SUGGEST THE USER APPLY AN ADDITIONAL STEP AFTER IMMERSION SILVER COATING WHICH IS SO-CALLED SILVER ANTI-TARNISH AGENT. THIS CHEMICAL COULD PROTECT Ag LAYER TO AGAINST TARNISH AND DISCOLORATION IN THE AIR, AND MOST IMPORTANT, IT SHALL NOT HAVE WORSE EFFECT ON SOLDERING PERFORMANCE OF THE PCB.
TO STUDY THE PERFORMANCE OF IMMERSION SILVER COATING, YOU COULD DO IT BY THE FOLLOWING:
1. PLACE THE IMMERSION SILVER PCB SPECIMENS IN A DRY AND CLEAN OVEN @155 °C AT different TIMES, I.E., 1,2,3,4 HOURS AND CHECK THE APPEARANCE OF THE SILVER AFTER CERTAIN TIME BAKING;
2. IMMERSE THE ImAg PCB SPECIMENS IN A SOLUTION WHICH CONTAINS 5% WT Na2S WITH DIFFERENT TIMES, I.E., 1,2,3,4, AND 5 MINUTES AND CHECK THE APPEARANCE.
IN CONDITION 1, IF THE SPECIMEN COULD PASS 4H BAKING WITHOUT TARNISH (COLOR CHANGED TO YELLOW OR GRAY FROM WHITE) AND ITS SOLDERABILITY IS ACCEPTABLE, THIS COULD BE EQUIVALENT TO 12 MONTH SHELF-LIFE; SAME SHELF LIFE COULD BE CONCLUDED FOR 5 MINUTES IN CONDITION 2.
HOPE THE ABOVE COULD GIVE YOU ASSISTANCE.
BEST Regards,

James Chao
- Shenzhen, China



June 27, 2017

Q. Immersion Silver tarnish: Is it harmful when PCB is exposed to air and I can notice color change of the Immersion Silver from silver to bright gold? I'm looking for an article that explains if it's OK.

Jane Davydov
- Ofaqim, Israel


July 10, 2017

A. Hi Jane,

Silver tarnish may affect solderability and aluminum wire bondability of immersion silver deposit. You may optimize storage condition (e.g. vacuum packing) or apply silver anti-tarnish after immersion silver to prolong shelf life.

Regards,
David


David Shiu
- Singapore



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