We're moving into this technology within the next year or so, and
currently use OSP in our lead-free processing.

Are we going to have to change our board finish in order to accommodate
BGAs?

Leland Woodall

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, February 19, 2008 7:52 AM
To: [log in to unmask]
Subject: Re: [TN] Best surface finish for BGA

I do not know a lot about the differences between the application of
gold flashing over nickel, versus immersion gold over nickel. In either
case the soldering is done to the nickel underneath the gold, not to the
gold itself which will be absorbed into the solder during reflow. For
BGAs, soldering to the nickel requires a much hotter temperature profile
due to nickel's slower rate of dissolution into the solder joint.
Apparently your assembly house is discovering this also. 
You have two options. 
One is to suggest to the assembly house that they check their profile
with thermocouples placed up through the bottom of the board into the
solder ball of one of the BGAs near the center of the assembly. As you
can see by Werner Engelmaier's comments on this forum yesterday, the
temperature must be close to 255 deg. C if you are using a SAC 305 alloy
solder paste and SAC solder ball alloy. Make sure the BGAs are in fact
SAC alloy, and not 63/37, if you want a RoHS-compliant product. Mixing
SAC alloy and 63/37 balls, or vice versa, will lead to problems with
BGAs soldering properly.

The second option is to qualify the product using a different finish. I
would recommend trying an immersion silver finish. It usually performs
much better, especially when soldering many BGAs with high-density BGA
pads. However, only you can tell if this finish is appropriate for your
particular product. If it is, you gain the advantage to soldering to the
copper underneath that silver finish.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Greaves, Bernie
(IMP)
Sent: Tuesday, February 19, 2008 12:20 AM
To: [log in to unmask]
Subject: [TN] Best surface finish for BGA

The question has been raised here, what is the best gold finish for high
density BGA pads - Immersion gold or gold flashing?   Keep in mind it
needs to be an RoHS process.  Or is there another finish more suitable.
It has come up as a result of our pcb contractor having difficulty with
some BGA's taking.
 
Regards,
 
 
Bernie Greaves

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