In a message dated 09/20/99 7:38:50, [log in to unmask] writes:
>No wonder that no maximum thickness is defined, since Werner states that
>the solder joints are more reliable the thicker the solder between pad and
>pin is. I can't fully agree with this statement however. We saw in various
>experiments that shear bands at the interfaces ( Tin/pad; Tin /lead) occur
>that are approx. 15um wide on either side. This results in a maximum
>thickness for reliability predictions of 30um ( I'm sure Werner and I could
>have endless discussions about this ) . But also this does not imply a
>maximum allowances for the solder thickness.
>Guenter
Hi Guenter,
We are not in disagreement; we've got a slight misunderstanding.
The increased reliability with increased solder joint height pertains
primarily to leadless solder joints, e.g. CCs, RCs, BGAs, CGAs, flip-chip,
etc. For SJs with compliant leads, lead compliancy obviates the need for SJ
height to reduce the strain levels.
Also one basic difference between leaded and leadless SJs is that most
leadless SJs see relatively uniform loading (not CCs and RCs with fillets)
leading to multiple crack initiations and cracks throughout most of the SJ,
whereas leaded SJs always have highly non-uniform loading producing almost
always a single main crack advancing from the heel to the toe. Thus, for
leaded SJs you get fractures along 'shear bands' at the interfaces (mostly at
Tin/lead; sometimes partially at Tin/pad) about 15 micro-m wide; that is, you
are just outside the intermetallic compound layer in the SJ-region depleted
of tin to form the IMCs.
Guenter, how are you doing in your new job? Is anything going on at ETH on
reliability?
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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