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July 2001

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Subject:
From:
"<Peter George Duncan>" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 12 Jul 2001 11:21:37 +0800
Content-Type:
text/plain
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text/plain (191 lines)
Thanks for your comment, James. I have heard of reports not recommending
ENIG for BGA applications, but haven't found any to read them myself. Can
anyone help?  Recommendations vary depending on application, I find, but
what thickness of Gold do you think would be appropriate? Our assembly
house was more concerned about the thickness of the Nickel than they were
about the Gold before we conducted solderability tests and found it was OK.

Our application is Military Avionics, subject to the usual thermal cycling
for such equipment and also some substantial, (relatively) low-frequency
vibration. Board finishes like HASL, even with very good tolerences on the
leveled solder height, do not seem to be as good as the Gold over Nickel in
terms of flatness. Flatness for BGA's equals most even joint heights, which
in turn means most even distribution of stresses (or minimising stress
focal points) and this in turn leads to greater joint reliability under
conditions of temperature extremes and vibration.

If not Gold over Nickel as a board finish for this application (recommended
by Cadence), then what do my learned TNagers suggest instead?

Pete Duncan




                    James-Kester
                    <jameswang@KEST        To:     [log in to unmask]
                    ER.COM.TW>             cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
                    Sent by:               Subject:     Re: [TN] PCB surface finishes
                    TechNet
                    <[log in to unmask]
                    G>


                    07/12/01 10:15
                    AM
                    Please respond
                    to "TechNet
                    E-Mail Forum.";
                    Please respond
                    to James-Kester






Dear Mr. Duncan ,

I think your gold is too think when applied in soldering .
Basically , what Zhangshoukai said is correct , there are some report ,
such
as Hp , has studied such problem and didn't recommend to use ENIG on BGA
application .


James Wang
Assistant Product Manager
Litton - Kester Solder , Taiwan

-----Original Message-----
From:   TechNet [mailto:[log in to unmask]] On Behalf Of <Peter George Duncan>
Sent:   Thursday, July 12, 2001 8:20 AM
To:     [log in to unmask]
Subject:        Re: [TN] PCB surface finishes

Hi, Shou Kai,

We had a CPU card designed and made for us in the States, and it carries
BGA's - 256 and 388 pin devices. This board has an ENIG finish to provide
adequate flatness for the BGA's - the finish is 4.5 microinches hard gold
over 150 microinches Nickel on a 12 layer FR4 board - quite a thick Nickel
layer, I think, but that's what the designers specified.

Soldering was a concern to me too, with visions of high melting point
solder with silver content and risks of frying dies trying to achieve a
good soldering temperature. But we tried a eutectic solder paste (63/37)
specially formulated for fibre optics work and it worked very well. Wetting
was good, solder joints were properly formed. Peak soldering temperature
was 218 deg C for about 16 seconds, bringing the centre pins of the BGA's
up to 205 deg C.

If your board is in good condition, you should have no problems.

Pete Duncan




                    Shoukai Zhang
                    <skzhang@HUAW        To:     [log in to unmask]
                    EI.COM>              cc:     (bcc: DUNCAN Peter/Asst
Prin Engr/ST Aero/ST Group)
                    Sent by:             Subject:     [TN] PCB surface
finishes
                    TechNet
                    <[log in to unmask]
                    ORG>


                    07/11/01
                    05:27 PM
                    Please
                    respond to
                    "TechNet
                    E-Mail
                    Forum.";
                    Please
                    respond to
                    Shoukai Zhang






Dear TechNetters:

As we know that the preferred metal surface finishes are HASL, EN/IG
(Electroless Nickel and Immersion Gold, and OSP etc.

It's reported that EN/IG(also called immersion gold) suface may lead
unreliable solder joint for BGA component,  and suggest do not use
immersion gold for designs that have BGAs.

Please give your advices.

Thanks with best regards,
Zhangshoukai, PE, Huawei Technologies.

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