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July 2008

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Subject:
From:
"Croslin, Robert" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Croslin, Robert
Date:
Mon, 21 Jul 2008 14:21:39 -0400
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Ioan, we've had similar problems and eliminated them by routing a slot along the edge of the V-groove for some distance wider than the edge of the part itself.  Of course, that's assuming you have an adjacent piece of scrap or if the boards are only separated by the groove, you have enough real estate to intrude with a slot.  After adopting that design practice, the only time since we've experienced a problem was when the fab house, no longer used, did the scoring improperly.


Bob Croslin
The Nielsen Company

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Friday, July 18, 2008 3:18 PM
To: [log in to unmask]
Subject: Re: [TN] Mechanical damage to BGA

Vladimir,

You have some good points, just like Victor.

The process is leaded and I have no idea when the damage occurs. We've only built 20 pieces, inspected them under the microscope and only realized there is a problem at test.

Under limited investigative resources, I can only say that everything makes me believe the soldering recipe is good enough to create proper joints and that I suspect mechanical action to be the cause.

Therefore I can only rely on this forum's expertise and try to figure out whether this is really an over the edge design, or rather something normal.

If the design is too much, how can it be improved?

If the design is regular stuff, I will put more effort in controlling depanelizing and handling.

Thanks,

Ioan

-----Message d'origine-----
De : Igoshev, Vladimir [mailto:[log in to unmask]] Envoyé : July 18, 2008 3:01 PM À : TechNet E-Mail Forum; Ioan Tempea Objet : RE: [TN] Mechanical damage to BGA

Hi Ioan,

You didn't specify the type of the joints/process (Pb-free?) and when the joints fail? Is it right after the SMT and if that is the case, then what is the cooling rate for the assembly. 

Larger pads could help, of cause (almost any larger thing can carry a larger load - I'm curious are there going to be any Friday comments on that statement? :-))

Regards,

Vladimir

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Friday, July 18, 2008 2:48 PM
To: [log in to unmask]
Subject: [TN] Mechanical damage to BGA

Dear Technos,

 

I have this BGA that gives me a headache. In many instances the boards do not pass test, we press on the BGA and they start working; a classic... The problem solder joints are always on the same side of the component and on the outer row; needless to say, we have a V-groove line that is manually cracked. So the cause is most likely mechanical stress that causes the pads to detach from the PCB. After prying off some BGAs we see the soldering is good, all the pads come off the board and stay solidly attached to the balls.

 

The specifics of this board are a bit special and I would need your opinion on what to improve in the design in order to eliminate the propensity to damage.

 

PCB is 18.55x28.65mm, FR4, 0.55mm [0.021"] thick

The BGA is 17x17mm, 1mm pitch, 0.65mm balls assembled on 0.25mm circular pads. In the troubled zone the pads are connected to 0.050mm traces.

 

My opinion is that the board flexes very easily and even if we fix the depanelization (I will modify the panel) all the subsequent handling, including the integration of this module, puts the joints at high risk.

 

I think the pads are too small so they do not have sufficient adhesive force to stay put on the PCB. On top of it, the traces are so minute, so the slightest stress will be destructive.

 

Would increasing the pad size help? Is this construction (thin PCB + superfine traces) viable?

 

Could the CTE mismatch be strong enough to cause the damage, instead of the mechanical stress from bending the board?

 

Thank you,

 

Ioan

 

 


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