Summary for immersion silver oxidation Discussion
immersion silver oxidation on PCB is discussed as a shelf-life and tarnish problem rather than a simple color-change issue. Mike Anderson asked for experimentation schemes and literature on oxidation trends for lead-free boards. James Chao replied that immersion silver is easily tarnished or discolored by hot baking or exposure to sulfur-containing solutions, and said suppliers often recommend a silver anti-tarnish step after immersion silver to protect the Ag layer without hurting solderability. He suggested two practical tests: bake specimens in a dry, clean oven at 155 °C for 1 to 4 hours, or immerse them in 5 wt% Na2S for 1 to 5 minutes, then inspect appearance. If a board survives 4 hours of baking or 5 minutes in Na2S without tarnish and still solders acceptably, he suggested that could indicate about 12 months of shelf life. In a later question, David added that tarnish may also affect aluminum wire bondability and recommended better storage, such as vacuum packing.
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
2003
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 sulfur-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
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
June 27, 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
July 10, 2017