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1995

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Subject:
From:
[log in to unmask] (Jerry Cupples)
Date:
Thu, 5 Oct 1995 19:17:08 -0500
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Darrell Drake asked:

>     I am looking for some technical information/ assistance regarding
>     component lead tinning.
>
>     Does anyone have any experience with a tinning process causing thermal
>     shock to integrated circuits that would cause them to separate at the
>     lead frame bond ? What is an acceptable thermal ramp rate ?

Maybe a little. It would depend on the package, and you would likely find
some recommendations on a good component data sheet. They are usually very
conservative; i.e. you can often go well beyond what the IC packaging folks
tell you is safe. But you might threaten to send some back to the
manufacturer and ask them for support on solving a problem with their
inability to solder without damage. They will probably tell you they
absolutely guarantee them up to 150 C for 0.5 seconds, as long as you
preheat at 1 degree per second max.

>     The solder pot is held at 313 deg. Celsius (high temp solder), the
>     component is suspended in type "RMA" flux for 5 seconds and then
>     suspended in the solder pot for another 5 seconds. This process is
>     repeated for two cycles. We are using an automated tinning machine
>     which controls emmersion speed and depth for both the solder and flux.
>
>     What is the typical thermal ramp rate that a component, such a DIP, is
>     able to withstand ? This includes the cool-down (second immersion into
>     flux prior to second tinning cycle is approximately 8 seconds).

Whew. I have seen some (weak dog ceramic) parts suffer package damage
hitting a 260 degree C solder wave. That is with a preheat cycle in a
typical wave solder, and for only about a 2.5 second solder exposure, once.
You have a very tough process. No data to give you, but I can asure you you
are asking for something pretty extreme in terms of thermal shock
resistance.

>     Any ideas ?

1) preheat the parts using hot air jets at one station of your robot
(that's what it sounds like you are using), say after fluxing to reduce the
thermal shock at the first solder immersion.

2) lower the dwell time in the solder to reduce the thermal transfer from
the external leads to the inside of the package. If you can't wet in 2-3
seconds, you probably won't in 5.

3) clamp some metal "heat sink" device or fixture on the lead shoulders to
reduce heat xfer.

4) switch to a lower temp alloy if all you are doing is pretinning for
military requirements. If tinning is only for solderability protection (I'm
often wrong) then perhaps the later assembly soldering could use
subsequently apply the high temp alloy.

Your mileage may vary. Suggestions invalid where any risk whatever is
involved. Don't quote me.


cheers,

Jerry Cupples
Interphase Corporation
Dallas, TX




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