Plating Integrity

Plating Integrity | 16 June, 2005

Good Morning Everyone,

We have had an issue with a specific board house on a job that has bright tin plating. At one point, a lot recieved proved to be unsolderable. The plating looked OK but no wetting would occur at reflow. We hagled with the board house and they ended up stipping the bright tin (or so I was told) and replating with HASL. We are currently running these boards again and we have two of the HASL boards that are not wetting (Out of 50) My question is if there is any way to tell in advance that there will be a problem or is it a scenario where we cannot tell until it goes into manufacture? Once again, it was only 2 of 50 so an AQL solderability test would not apply.

Any help would be greatly appreciated.

Thanks

Chris

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Doug

#35003

Plating Integrity | 16 June, 2005

Chris,

From your email I am assuming you are using an immersion tin process. Is that correct? If so, I would highly recommend you talk to your board house about the amount of tin that is being deposited on your boards. If the tin deposit becomes very thin you could end up with copper migrating up through the tin as the board goes through heat cycles. The immersion tin process is a self limiting process whereby over a certain length of time the tin will no longer be deposited on the board. Having said that I know that there is a recommended time by most of the immersion tin people for the amount of time that the boards must be in their "bath" but I would caution you that it may not be long enough to put enough tin on your boards to survive the heat cycles. The other issue you could be facing is one of contamination. The company doing the immersion tin process must be very aware of all the chemical issues and also have very strict guidelines to follow (and of course follow them).

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davef

#35011

Plating Integrity | 16 June, 2005

Doug: While your points are well taken, we've never heard of "bright tin" immersion plating. Electroplating is our bet.

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davef

#35012

Plating Integrity | 16 June, 2005

Chris

As you imply, testing board and component solderability prior to assembly is preferred. * ANSI/J-STD-002 - Solderability Tests for Component Leads, Terminations, Lugs, Terminals, and Wires * ANSI/J-STD-003 - Solderability Tests for Printed Boards

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Doug

#35020

Plating Integrity | 17 June, 2005

We have had some experience with the immersion tin. The appearance can be "bright" or "dull". Don't discount the fact that it is "bright" doesn't necessarily make it HASL. Furthermore, I personally have never heard of a board manufacturer "stripping tin" from a HASL board and reappling a HASL finish. That makes no sense to me. However, I know than can strip the tin from the immersion tin process and then reprocess the boards with any finish you desire. I would bet that if they stripped the tin they initially used a immersion tin process and when you haggelled with them they reproceesed the boards using HASL.

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davef

#35022

Plating Integrity | 17 June, 2005

Doug: We agree that imm tin can be easily stripped and reapplied.

Do you order imm tin as bright/ dull? Is bright/dull a function of who the fabricator lisences the process? Er wha?

Electroplate tin can be either matt or bright. It is different than HASL. which is tin/lead.

Board fabricators use tin as an etch resist, instead of tin/lead, all day long. The chemistries for stripping tin or tin/lead are: * Ammonium bifluoride/peroxide * Two step, nitric followed by ferric chloride * Nitric/ferric nitrate * Nitric/hydrofluoric acid

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Doug

#35024

Plating Integrity | 17 June, 2005

Dave,

The finish I'm most familiar with is the "white" tin by Omikron. Once the boards have been immersed the finish takes on much more of a white color however I have seen other immersion tin finishes where they are dull or bright in color. This is just speculation on my part but I'm thinking that it is just the chemical makeup of the particular tin bath that is installed by the manufacturer of the immersion tin.

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