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March 2008

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Subject:
From:
Paul Reid <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paul Reid <[log in to unmask]>
Date:
Thu, 20 Mar 2008 16:28:01 -0400
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text/plain
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text/plain (135 lines)
Yes - Delamination protects the barrel of the PTH, stopping damage accumulation. That is our experience in about ~90% of the groups we test where coupons have delamination. 

For example we have repeatedly and frequently seen the following trend.

Coupons tested "as received" achieve 500 thermal cycles to failure (end of test).
Coupons precondition 3X230°C (tin/lead assembly simulation) achieve 400 thermal cycles. 
Coupons precondition 6X230°C (tin-lead assembly and rework) achieve 300 thermal cycles. 
Coupons precondition 3X260°C (lead free assembly) achieve 350 thermal cycles. 
Coupons precondition 6X260°C (lead free assembly and rework) achieve 500 thermal cycles. 

When ever we find increasing cycles to failure with increasingly aggressive assembly and rework, we have found delamination, 

We have tested maybe 40 different "lead-free" materials. Many have demonstrated that more aggressive lead-free assembly and rework extend thermal cycles to failure by delaminating.

Because of this finding we have been forced to develop methods to identify delamination in test coupons and develop unique material tests (using TMA) to better understand the role of materials in reliability testing. 

We now know you have to test for delamination in all lead-free applications. And, surprisingly, grid size (hole spacing) is a major influence.

Once in awhile delamination in a group of coupons will reduce the cycles to failure by ~50%, but most of the time delamination produces a false positive result by extending cycles to failure. We differentiate delamination as either stress relieving or stress focusing in a give PWB build.

Paul Reid
PWB Inc.
613 596 4244 ext. 229


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Franklin D Asbell
Sent: Thursday, March 20, 2008 8:44 AM
To: [log in to unmask]
Subject: Re: [TN] How much stress is needed?

Paul,

Interesting comments. If I read that correctly you state that delamination can actually protect the barrel?

What if there was no visible (at 400X) delamination in the samples? Yet there is visible fracture of the barrel and fill material?

Franklin


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Reid
Sent: Thursday, March 20, 2008 7:52 AM
To: [log in to unmask]
Subject: Re: [TN] How much stress is needed?

Hi Franklin,

We perform reliability testing by thermal cycle testing coupons and one of my roles in the company is to analyze cracks in the PTH, dielectric and fill if it is present. I microscopically examine hundreds of thermal cycle induced PTH failures a year.

We frequently see cracks that propagate from a glass fiber, across the copper of the PTH and through the PTH fill. Not in every case, but often enough that it is not surprising. 

We frequently find delamination in the dielectric is not continuous with cracks in the barrel of the PTH. Most often delamination in the dielectric protects the barrel of the PTH and the fill. Delamination usually significantly extends thermal cycles to failure. Dielectric delamination and extended cycles to failure is becoming a common occurrence with advent of RoHS imposed lead-free assembly and rework.

On occasion delamination in the material will align with cracks in the PTH and fill. Usually when that happens thermal cycles to failure are significantly reduced.

I am of the opinion that it is the expansion of the dielectric that causes cracks in copper and via fill.

Paul Reid

Program Coordinator
PWB Inc.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Franklin D Asbell
Sent: Wednesday, March 19, 2008 6:33 PM
To: [log in to unmask]
Subject: Re: [TN] How much stress is needed?

Werner,

Would this also fracture the via fill material? I've seen fractured vias before, caused by thermal expansion of the copper, but never one where the via fill material is also fractured and yet the PCB is not affected at all.

Franklin

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
Sent: Tuesday, March 18, 2008 9:45 PM
To: [log in to unmask]
Subject: Re: [TN] How much stress is needed?

Hi Franklin,
You describe a classic via barrel crack-comes from the thermal expansion mismatch between the ED copper and the PCB resin in the z-direction during the soldering processes [the hotter the worse-read lead-free] [you should be taking my workshop on the subject at IPC EXPO/APEX on the morniong of March 31.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com



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