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August 2001

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Subject:
From:
Steve Abrahamson <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 24 Aug 2001 16:25:21 -0600
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I am not an expert on this topic, but I do have a book from Jennie Hwang
that goes over some experiments in which eutectic tin lead was cooled down
at;
.1C/sec, 1C/sec, 50C/sec and 230C/sec.
From what I can gather, slowing down the cooling rate will increase the size
of the grain boundaries and thus reduce boundary slip.  Unfortunately the
large grain boundaries are a great place for cracks to start.
By cooling quickly, one will increase the occurances of irregularly branched
Pb-rich detrites and the two phase mixture results in inferior low cycle
fatigue resistance.

It appears that she goes on to say that the grain size does not get much
smaller than that of a cooling rate at 1C/sec.  Thus cooling at 3C/sec will
not result in smaller grain size, but may increase the chances of Pb rich
pockets.

Another big no no is to have a two step cooling rate.  It seems as though
most of the products I have dealt with seem to cool at about 1C per second.
You wouldn't want to start cooling at that rate, and then bring the product
into a cold chamber to excelerate cooling.  Uneven cooling may promote
cracking and dynamic recovery.

This is all out of the book "Modern Solder Technology for Competitive
Electronics Manufacturing" by Jennie S Hwang, PH.D
Chapter 14.3 entitled Microstructure Strengthening.

Hope this helps..

Steve A

> -----Original Message-----
> From: Ochenas, Tom [SMTP:[log in to unmask]]
> Sent: Friday,August 24,2001 10:52 AM
> To:   [log in to unmask]
> Subject:      [TN] Cooling rates for solder reflow processes
>
>
> Hi Technetters,
>
> My first post (although I do work with the estimable Glenn Pelkey so
> perhaps my question will garner a response or two) to Technet:
>
> There is debate within my group about the recommended cooling rate (to
> optimize grain structure and fatigue resistance of the solder joints) for
> eutectic solder (incidentally we are using a convection type reflow
> oven)... One camp holds that a fast cooling rate (~ 3C/sec) should be
> maintained to a temp. of liquidus MINUS 50C to optimize grain structure.
> The other camp contends that what happens below the liquidus temp. is of
> little consequence to the grain structure. If anyone can resolve this
> argument with a materials science based explanation I would be extremely
> grateful...
>
> Best Regards,
> Tom Ochenas
> Maxtek Components Corporation
> [log in to unmask]
>

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