I think the other ME should stick to mechanical engineering.  Yes you can have "Double Reflow Brittle Fractures at an IMC when you reflow a soldered BGA and pull it off but if you had an electrical open at those pads before doing the reflow an pulling the BGA off then there must have been something wrong to start with.  Joyce is correct a cross section would tell you a lot about the defect but if the assembly is worth $45 than there is going to be a real reluctance to destroying the assembly to find out.

George

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Thursday, December 21, 2017 4:31 PM
To: [log in to unmask]
Subject: Re: [TN] BGA pad non-wetting

Thanks for the thoughtful response. One of the other MEs here is thinking that this is a normal appearance, that the SAC305 solder uniformly separated from the intermetallic when we reflowed and pulled the BGA.

I wish this was the case but it seems unlikely. Isn't the affinity of the bulk solder to the intermetallic higher than its surface tension?

On Thu, Dec 21, 2017 at 4:26 PM, Yuan-chia Joyce Koo <[log in to unmask]>
wrote:

> Steve, your friend might need cross section and see if UBM is done 
> correctly, besides, the solder mask looks extra thick for a device...
> didn't look like std BT substrate... cross section of good vs after 
> reflow one might give you some clue what is wrong... the color of the  
> pad look like poor adhesion of the nickel layer... it might be poorly 
> done either possiviation layer prior to balling (either ImAg, ImAu or 
> ImTin), or missing etchant step (or pH adjustment not done properly) 
> prior to last immersion plating step...IMHO.
> On Dec 21, 2017, at 2:39 PM, Steve Gregory wrote:
>
> Hi all,
>>
>> I've been asked to post this to the Technet by a dear friend:
>>
>> *We installed this little processor. At test we identified opens on 
>> the device. When we removed it we saw a number of pads that were 
>> matte gray with no evidence of solder adhered to the pad. * *We 
>> attempted to bump the pads with solder, they did not wet easily. In 
>> some cases we had to scrape through the matte finish to a shiny metal.
>> But, we did get all the pads to appear wetted.*
>>
>> *We cleaned the site well and then fluxed the bumps with a tacky flux 
>> ROLO designed for POP, placed the BGA and reflowed it on a rework 
>> station.*
>>
>> *The assembly failed test with similar result, opens under the 
>> processor.*
>>
>> *The attached photo shows that the bump process only appeared to wet 
>> the pads. The condition returned when we removed the BGA  second 
>> time.*
>>
>> * http://stevezeva.homestead.com/2017-12-21_11.21.17.jpg
>> <http://stevezeva.homestead.com/2017-12-21_11.21.17.jpg>*
>>
>> *Have you seen this before? Is there any recovery? This assembly is 
>> worth about $45K. *
>>
>> Thanks,
>>
>> Steve
>>
>> --
>> Steve Gregory
>> Kimco Design and Manufacturing
>> Process Engineer
>> (208) 322-0500 Ext. -3133
>>
>> --
>>
>>
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