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April 2000

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Subject:
From:
Phil Crepeau <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 28 Apr 2000 07:36:38 -0700
Content-Type:
text/plain
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hi,

first, let me say that there is absolutely no intent to offend anyone in my reply.

now let me say that this message is very intriguing, but i'm missing most of the meaning due to some of the terminology that is used.  is there someone that might talk to thomas in his native language and clarify this?  for example, what are 'chimneys' and 'cigars'?

sincerely,

phil

-----Original Message-----
From: Thomi [mailto:[log in to unmask]]
Sent: Thursday, April 27, 2000 3:18 PM
To: [log in to unmask]
Subject: [TN] tin-iron bridging in selective soldering


Dear TechNetters,

who shares problems with us concerning a formation of tin-iron intermetallic needles during high temperature selective soldering? These needles are forming electrical shorts between neighboring pins; for more details, see the following paragraphs:

Driving the leadfree issue is automotive electronics due to drastic increase in use environment temperature. We now have a case where selective soldering of THT contacts is performed by chimneys which are loaded with liquid solder by immersion in a solder bath, then raised to heat and fill the joint. Using a lead free alloy (Sn-Ag-Cu), the bath temperature is set to ca. 320C, as the heat capacity of the chimney is not overwhelming (but simultaneous access is reducing cycle time considerably in comparison to a miniature wave system). The bath itself is kept under a nitrogen blanket to reduce drossing. The bath containment is made from stainless steel (Cr-Ni I suppose), while the chimneys are made from tool steel, then pretinned. (By the way, this method is used with tin-lead alloys just as well as with leadfree solder alloys.)

It turns out that two different types of unwanted needles are found after the soldering process. One type looks like cigar fish with its fin tails and is found in the bath, on the side rails. The other type is found on the solder resist, typically in between adjacent joints, forming an electrical short. The first type was analysed to consist of mainly the solder alloy, the second (and worse) type was analysed to consist of tin and iron. Further, the bath showed an iron content of 6% after a couple of days of operation.

Now there is a range of issues:
(1)    formation of tin base cigars (not the most critical at the moment)
(2)    formation of tin-iron needles (which is understandable if iron is freely available)
(3)    enrichment of iron in the solder bath - where is the acceptable limit?
(4)    iron leaching from the container or the chimney tool - how long until damage occurs?
(5)    how much iron may a fresh supply of leadfree solder alloy contain
(6)    how can the adhesion of the iron tin needles on the PCB surface be avoided?

Please give your comments, and if you encounter similar problems, I would be very much interested in a direct exchange of experience; you can access me directly at [log in to unmask] <mailto:[log in to unmask]> .

Thank you very much
Thomas Ahrens, Fraunhofer ISIT

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