Okay John,

even grayish gradient , then we can exclude a number of failure categories.
There is a large difference in solder joints's behaviour between shock and 
'slow' mechanical exposure.
Can you do following simple test:

1. Place a pencil under the cc line of the board and press down both edges 
simultanously.  If the board is larger than 10 inches, increase diameter of 
'pencil' . Did any BGA lift? If it happens, you really have a weak joint, 
possibly a brittle IMC ( I use this expression not to start a discussion).

2. If nothing happened under 1) , dip the same sample for a couple of 
minutes in liquid nitrogen and repeat 1). If nothing happens, your solder 
joints are not that bad.

Have you specified a drop test?

Inge










----- Original Message ----- 
From: "John Parsons" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, November 19, 2010 10:59 PM
Subject: Re: [TN] BGA Falling off PCA


> 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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