Inge,

The appearance of the PCB land and that of the BGA are the same.  A very
flat, uniform surface, dull grey in colour with, under 45x magnification, a
fine, grainy appearance.  If, under that magnification I touch the pad
surface with an x-acto knife, ever so gently, I can rub off the grainy layer
leaving a shiny silver surface that 'looks' like solder to me.

On hand I have chemistry that will strip tin/lead but apparently it leaves
nickel with a black finish.  I don't have any chemistry that would strip
nickel.

John

-----Original Message-----
From: Inge [mailto:[log in to unmask]] 
Sent: Friday, November 19, 2010 1:25 PM
To: [log in to unmask]; [log in to unmask]
Subject: Re: [TN] BGA Falling off PCA

John,

we have seen this a number of times,and I've been looking at some of these 
cases.You can start with some brief examinations:

1. are there any solder remnants on the board pads. If so, how does it look 
like? Randomly spread or ring formed along the pad's circumperifery?

2.apply a solder etchant that does not etch Nickel to get an opinion about 
the solder wetting

3. apply a nickel etchant that does not etch solder to get an opinion about 
the nickel covering, skip plating, IMC s

4. dispens a 50-100 micrometer solder paste on the failing board pads and 
run through your oven.Wetting, dewetting or non-wetting.

You need a experienced person to do the tests and to interprete the results.

In right hands, often better than pages and pages of EDAX analysis.


Inge


---- Original Message ----- 
From: "John Parsons" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, November 19, 2010 9:31 PM
Subject: [TN] BGA Falling off PCA


>I am not seeing a lot of the usual Friday quiz traffic so I know you
> guys-n-gals are just itching for some real work :o).
>
>
>
> We are a PCB fabricator and one of our customers has returned some
> assemblies to us for feedback.  What they noticed when someone 
> accidentally
> dropped an assembly is that the BGA (352 pin Cirrus Logic EP9312-CBZ) 
> popped
> off the board.  They then proceeded to drop a few more from a height of
> about a foot with the same results.  All the other components on this
> Pb-free assembly stay fast.
>
>
>
> We have received three PCA's and 5 BGA's.  On one of the BGA's a single 
> ball
> has remained on the PCB.  The rest of them all the balls have broken 
> cleanly
> from the PCB leaving a very flat (under x10 magnification) dull gray
> surface.  I sure wish that I could take photo's but lighting is critical 
> and
> I don't think that I have the resources to get photo's that would do 
> justice
> to what the naked eye is seeing.
>
>
>
> Also of note is that the BGA lands are mask defined with a 31mil circuit 
> pad
> and 26mil solder mask relief.
>
>
>
> Does anyone have any ideas as to how we can determine the root cause of 
> this
> failure.  This part has been built for some time but previously the 
> surface
> finish on the PCB was a solder finish.  This is the first run of the 
> boards
> that has an ENIG finish.  This doesn't look to me like a raw board issue 
> but
> that is where everyone looks first isn't it ;o).
>
>
>
> Regards
>
>
>
> John Parsons
>
>
>
>
>
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