Hi Gita,

Q:   What is the preferred surface finish for PWB's with PBGA packages?
A:   The critical process factors for BGA assembly are:
     * Surface Coplanarity
     * Solder Volume
     * Solderability
     * Moisture Sensitive (PBGA)
It 's okay to use Entek 106A, but you should consider the solderability of 
second reflow or BGA  repair if there is any. Gold is very solderable, and 
just as important, very coplanar.  It is increasingly becoming a popular 
choice for SMD fine pitch board assemblies.  With gold, there is no 
embrittlement concern as long as the thickness of  gold is 0.000003-5". You 
can consider gold as an alternative to HASL.

Q:   Using Entek boards, what kind of flux is better to use, NO CLEAN or 
 water soluble?
A:   No clean Flux is definitely a  better choice. According some technical 
studies I read, water soluble is more likely to form solder balls if the 
temperature profile and solder mask design are not optimum. In your case, 
finding solder balls beneath the BGA is the last thing you want to see. With 
no clean flux, the reduction in solid content will tightens the process 
window and nitrogen atmosphere reflow  has proven helpful in improving 
process yield when no clean flux is being used.

Thanks

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 ----------
From: TechNet-request
To: TechNet
Subject: FAB/ASSY: PREFERED PWB SURFACE FINISH FOR BGA
Date: Monday, May 06, 1996 2:08PM


     What is the preferred surface finish for PWB's with PBGA packages?

     I have been looking at some published industry data regarding PWA
     assemblies with plastic BGA's and so far all have had HASL surface
     finish and very little data on OSP (organic Solderability
     Preservatives such as Entek) coated boards.

     Has anyone had any OSP coated (Entek 106) mother board designs with
     BGA packages in production?
     What are the pro and con's with HASL Vs. ENTEK for BGA assembly?
     Using Entek boards, what kind of flux is better to use, NO CLEAN or
     water soluble?
     The preferred surface finish for assembling fine pitch components has
     become Entek (or Entek equivalent) for better coplanarity criteria,
     but is there a danger using Entek boards for BGA assembly?

     I would greatly appreciate any information you can share with me.

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     (800) 289-3355 EXT 85408
     or
     512-728-5408 direct line

     Thanks, Gita