Improper rework heating can cause most solder balls to be left on the motherboard. This may also be the case where mother board bond pads are considerably larger than the pads on the BGA substrate. Two comments point to the likelihood that you have a black pad problem....your customer found the problem via thermal stress testing, and you report that the "dewet" pads are dark in color. Such an appearance would suggest electroless nickel-immersion gold (ENIG) plating problems on the BGA bond pads. It is now commonly thought this is due to deposition of an electroless Ni with a low phosphorous content (typically less than 6 - 8%). With such Ni, the aggressive immersion Au chemistry is capable of attacking the Ni, first at the electroless Ni nodule boundaries and ultimately the attack can spread radially inward from the nodule boundaries. This attack displaces Ni and result in a P - enriched region. Proper Ni3Sn4 intermetallic cannot form in these areas. If the attack is confined to nodule boundaries, solder can wet and form the intermetallic bond in the center of the nodule surface areas. This phenomenon has been associated with the electrical potential of the individual pads. Pads at the ends of long conductors are more likely to exhibit black pad than those pads closely connected to large copper planes in the PCB, so it is not uncommon to see this problem occur on the same set of pads within a given lot of substrates or assembled components. This could cause intermittent failures if a black pad contact is surrounded by solder ball joints that are reasonably well bonded to the PCB. If the BGA bond pad metal is electrolytic Ni the problem may need more analysis. It is possible for the solder near the interface to the Ni to be virtually depleted of Sn if the BGA has experienced a severe thermal history (excessively high temp-long dwell time above liquidus, or multiple reflows due to double side assembly, or rework of adjacent devices. This can lead to separation from the Pb-rich solder, but the pad may not look like a black pad.
Regards,
Leo Higgins
-----Original Message-----
From: Tore Jakobsen [mailto:[log in to unmask]]
Sent: Monday, September 23, 2002 2:53 AM
Subject: Re: BGA failure and Rework
Victor
We've put quite a bit of effort into this, but the fact remains.
After removing the suspect components (which fail during test of the
assembled product) we find that some pads on the component are gray/brown.
Further analysis of these darkened pads, show that they probably never
gained a proper intermetallic connection. This kind of fault is a real
problem, as it's one of the faults we cannot detect by any other means than
functional testing. And even then it's not guaranteed the product will
continue working.
We put focus on this problem, due to our customers finding products failing
after thermal stress testing. (The boards were fully functional as they
left our site)
Upon removal, we found several black pads beneath the failing components.
Rgds
Tore
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