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From: Yuen, Mike
To: mbyrne
Subject: Re[3]: BGA FInish
Date: Thursday, May 09, 1996 10:20AM
MB,
Q: Isn't that a pretty tight range?
A: The number provided in my response is a conservative answer. You also
have to consider the solder joint volume vs the gold thickness. According
to a study done by an OEM, if the gold content is less than 3% of the
resultant solder joint volume, no appreciable solder embrittlement will
occur. Therefore, the volume of gold on all pads must be less than 3% of
the resultant solder joint by volume. Since gold is porous in nature, the
electro deposited gold of 1.5 um or less will be porous and will give poor
protection to the underlying metal. Frequently, immersion gold will leave a
thickness on top of the nickel of 2 to 8 micro inches. However, when using
electrolytic gold, the thickness is greater and much more difficult to keep.
Therefore, if your vendor uses electrolytic gold, you must have a note that
controls the maximum thickness to 15 micro inches. Over this thickness, you
must be concerned about embrittlement. On the other hand, with immersion
gold, you don't have to worry about the thickness getting greater than 8
micro inches.
Qb: What if there is something on the other side, say, that will be used
for wire bonding?
A: A thicker gold coating can be achieved with electroless gold if
there is a need for wire-bonding.
Thanks
Mike Yuen
[log in to unmask]
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From: mbyrne
To: TechNet
Cc: MYuen
Subject: Re[2]: BGA FInish
Date: Thursday, May 09, 1996 9:52AM
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]
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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.
[log in to unmask]
(800) 289-3355 EXT 85408
or
512-728-5408 direct line
Thanks, Gita
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