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Lead Free finish

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#57106

Lead Free finish | 28 October, 2008

Hi

We have recently specified the following on a bareboard that has an ENIG finish. Does it sound about right or is it likely to give me problems ?

Nickel : 3uM � 7uM Gold : 0.05 uM � 0.1 uM

Thanks in advance

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#57109

Lead Free finish | 28 October, 2008

Your gold might be a bit thin after you consider fabricator's tolerance.

IPC-4552 ENIG specification: * Ni thickness: 3 - 6 microns [120 - 240 microinches]. * Au thickness: 0.075 - 0.125 microns [3 - 5 microinches]. * Recommended specification is 2 microinches minimum at -4 sigma from the mean for the gold with 3-5 being typical.

You can use this solderability protection with leaded solder if you choose.

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#57111

Lead Free finish | 29 October, 2008

Thanks for the feedback dave. What kind of problems am I liely to face if I was to stick it out with my current specification ?

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#57112

Lead Free finish | 29 October, 2008

During soldering, the gold dissolves into the solder connection. You solder to the nickel. The gold protects the nickel from oxidation. Corroded nickel is very difficult to solder. So if your gold is thin, it may not protect the nickel from corrosion well and you will back here asking questions about poor solderability. [On the other hand, if your gold is too thick, it could produce a weak solder connection. Search the fine SMTnet Archives for more.]

Test a couple boards for solderability without components. If they solder well, say a small prayer and run with it.

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#57150

Lead Free finish | 1 November, 2008

In addition to what Dave has said, you shouldn't worry much about having too much (too thick) of Au, as it's a self-limiting process. So Gold embrittlement isn't an issue.

Regards,

Vlad

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#57152

Lead Free finish | 3 November, 2008

Vlad,

That was a bad statement to make... Enig should never be thicker than 5 micro inches max. or it will cause embrittlement.

This process does self limit but it can put on as much as 10-14 Micro inches which would be a major cause of concern.

The average coating that most shops are applying is between 3-5 micro inches. All you are trying to do is get the minimal coverage that will protect the nickel.

As Dave stated above, to thin can cause tarnished nickel (black pad) and to much will cause joint embrittlement.

I know you work for a Lab but that comment was just wrong.

Boardhouse

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#57153

Lead Free finish | 3 November, 2008

One other note, NEVER have an ENIG board fabricated by a house that "farms out" the gold. This always ends in disaster! Make sure whoever you choose does the whole process.

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#57158

Lead Free finish | 3 November, 2008

We agree with Boardhouse that immersion gold inherently self-limits <10-12 uinch.

We disagree with Boardhouse that routine corrosion of nickel [nickel oxide] is "black pad." "Black pad" is hypercorroded nickel and is totally unsolderable. Surface corroded nickel can be soldered with a sufficiently active flux.

We agree with Doug's comment about the need to research your fabricator's capabilities.

Specifying a maximum gold thickness with the intent of preculuding embrittlement issues is a common rule of thumb, but it is not foolproof. Gold should be limited to 3 percent of the weight of the solder connection to preculude embrittlement issues.

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Reflow Oven

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