Make sure the gold content will dissolve down to below 10% (or 5%, I can't
remember... look at the phase-diagram in a metallurgy handbook and make sure
you're below the smallest % of Au-Sn compounds) or you will have a problem.
But I have first hand experience that using a very thin layer of gold with
solder is not detrimental (and this recipe was implemented into
manufacturing.
carey
-----Original Message-----
From: Kasprzak, Bill (esd) US <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, September 30, 1999 12:12 PM
Subject: [TN]
>Steve:
>
>I was in the same ballpark except that my part is somewhat smaller.
>
>I can give you more details if you call me, if needed..
>
>A quick summary of events ........
>
>We're using an Actel CQ-132, 132 lead, 25mil pitch.
>Using this here 'Net I got a great lead for a suggested land pattern from
>Bob Vanech over at Northrup.
>Got our design folks to agree to the suggested land pattern.
>Sent land pattern to Fancort, who made me a die for forming this part in a
>pneumatic press (12K)
>(Die 9K, press 3K)
>The part is programmed with the tie bars on.
>When we get the part on the floor, we have to remove the corner metal
>section that joins the tie bars.
>Part is formed in the Fancort press and then transferred to a special
matrix
>tray that holds the part body without any pressure on the leads.
>We then use a standard solder pot, and with a little practice and steady
>operator hands (not mine !)
>We lightly (I mean lightly) brush RMA flux on the feet along one side of
the
>QFP while holding the part with a high quality vacuum pencil.
>We then dip the leads into the solder pot. The dip barely touches the top
of
>the molten solder with a quick withdrawl.
>With a little practice, no bridges, comes out quite nice. Remember, only
try
>to tin the "feet"
>Lots of articles in past periodicals and there is a "how to"document from
>EMPF called "manual tinning of fine-pitch"
>By the way, we have also done an analysis that shows that by leaving the
>gold on the part will not affect the solder joint due to the
>insignificant amount of gold remaining on the "feet" of the QFP. (Foot size
>~ 50 mils by 12 mils wide.)
>But we're going to tin the parts anyway our whole program will use about 50
>parts over a years time.
>I was lucky. I have only one part to deal with .
>
>Bill Kasprzak
>Moog Inc.
>716-652-2000 ext 2507
> ----------
>From: Stephen R. Gregory
>To: [log in to unmask]
>Subject: [TN] Pre-Tinning Fine Pitch...
>Date: Tuesday, September 28, 1999 7:34PM
>
>Hi ya'll!
>
>Boy, I may have quite a task ahead of me. There's this assembly that has
hit
>the floor that has just about every part (CQFP's and ceramic SOIC's) in the
>un-formed state, and we've got to form and trim, then Pre-tin the gold
>plated
>stuff. I've already got a call into Six Sigma to get a quote on doing this
>stuff for me, but it may turn-out that we won't have the time to deal with
>this build and sub-contracting the forming and trimming, and pre-tinning
out
>(what's new?).
>
>But in case I have to pre-tin the CQFP's myself, are there any tips that
>anyone can share to help me get through this as painlessly as possible?
>These
>parts are 320-pin 25-mil pitch and still have the tie-bars attached. I've
>got
>a bottle of nitrogen and plan to "Rube-Goldberg" me an enclosure to blanket
>the pot with nitrogen over the tinning pot that I'll be using...I think
>that'll help to cut down on the bridging I'll see. I'm going to use an RMA
>flux as well...any other suggestions? PLEASE?
>
>Thanks!
>
> -Steve Gregory-
>
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