Werner,

Please see my response to this question.

In our case we were doing SM assembly and either COB or FCOB as well (with
lots of continual constructional analysis and environmental test data going
back over many years). The flex circuits we made were fairly densely
populated. If we did the COB or FCOB operation first (die attach / globtop
cure) it meant we couldn't screen print the solder paste
afterwards.....simple as that. These were high reliability products and each
ran in the order of 25K per week.

Regards,

Martin


===== Original Message from [log in to unmask] (TechNet E-Mail Forum.) at
29/09/00 05:41
>In a message dated 09/28/00 22:11:12, [log in to unmask] writes:
>>My question is: are there any known problems on solder joint and PCB
>reliability
>>issues if the assembly is additionally cured for one hour at 175 degrees
>>C for the die attachment process?
>
>Hi Kuan,
>Your curing process is what I have termed 'extraordinary' in the e-mail
>subject: "Re: [TN] Solder Joints". At this temperature and duration, you
>will
>see a significant increase in the thickness of the intermetalic layers with
>a
>significant change in the alloy composition of the remainder of the solder
>joint volume, because the IMC formation consumes the available Sn. Also, you
>will see a tremendous growth in the grain structure of the SJ. None of this
>is particulalry good for the SJs, even though, I can not think of anything
>published on this.
>Whith the cure temperature this high, why on earth do you need to the
>soldering first?
>
>Werner Engelmaier
>Engelmaier Associates, L.C.
>Electronic Packaging, Interconnection and Reliability Consulting
>7 Jasmine Run
>Ormond Beach, FL  32174  USA
>Phone: 904-437-8747, Fax: 904-437-8737
>E-mail: [log in to unmask], Website: www.engelmaier.com
>
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