I think the dominating benefit of choosing ENi/IAu over HASL is it can get flat pad surface(and lead free, one less time thermal shock...). Am I right? But does pad flatness matters so much?
My point is: 1, you can get flat enough pads for 1.27mm and 1.0mm pitch BGA with HASL, if you can't, ask your PCB vendor to improve. 2, I agree that HASL pad surface is not so flat like ENi/IAu but so what? If all the BGA pads are in the same littel convex profile, means there are same solder volume left on each pad, so what to worry? 3, "Flatness for BGA's equals most even joint heights"? The solder sprayed on pad when HASL, solder paste, BGA's ball, they all melt in the reflow oven. So the BGA's body should be floating over these melten solder, you will certainly get even joint height unless something warp. But if you have solder paste printing problem and have uneven solder paste distributed on each pad, that will cause uneven solder joint diameter, and that will envoke reliability problem.
BTW: you can download HP's report on ENi/IAu failure on this page: http://www.nukcg.org/downloadfiles/
Jiang Ping
-- Original Message -----
From: <Peter George Duncan> <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, July 12, 2001 11:21 AM
Subject: Re: [TN] PCB surface finishes
> 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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