I realize that minimal gold (3-5 millionths) is best for soldering, 
     but:
        a. Isn't that a pretty tight range?
        b. What if there is something on the other side, say, that will be  
           used for wire bonding?
     
     
     MB


______________________________ Reply Separator _________________________________
Subject: RE: BGA FInish
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    5/7/96 02:24 PM


     
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
     
[log in to unmask]
     
     
 ----------
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.
     
     [log in to unmask]
     (800) 289-3355 EXT 85408
     or
     512-728-5408 direct line
     
     Thanks, Gita