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October 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:
Sat, 4 Oct 2008 19:28:17 -0400
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Hi Ido,

I am responding from a reliability, rather than a QC/QA, point of view. Therefore I will not be commenting on whether this passes or fails to meet a requirement.

I have reviewed the photos and what I noticed is that the cracks in the barrel are more or less horizontal and proceed through copper crystals (as opposed to going around copper crystals). I also noticed that the cracks are open at ambient. There is no obvious smoking gun here; the copper looks well plated with good distribution and the material is not significantly degraded (i.e. no delamination, crazing) based on visual examination. It does not appear that the electrolytic copper has a columnar crystal structure, burning or nodulation all of which would suggest a copper plating problem.

I would say that this is a typical failure mode created by high temperature thermal excursions. The barrel cracks associated with temperatures greater than 230°C frequently present in this manor. We see this failure mode frequently in lead/free assembly and rework typically at 260°C.

Since you floated the specimen at 288°C this is the expected failure mode if the barrel is going to fail. My visceral reaction is that the material is playing a role in this failure. Notice that there is pad rotation exhibited by some of the inner layers. The material has undergone Z axis expansion and has begun to degrade as demonstrated by the plastic deformation visible in the section (open crack). If the dielectric were not adversely affected by the thermal excursion we would expect the material to retain its' elasticity and the crack to be closed at ambient (self healing). What we see in this section is a reluctance to return to the original shape (physical hysteresis). Electrically the circuit would have exhibited an irreversible increase in resistance (electrical hysteresis). Also there appears to be some degree of copper thinning or deformation at the facture sites that suggests significant strain to the PTH. Most would say that the CTE is the major factor for this deformation but I am beginning to believe it is function of CTE and viscosity (material stiffness) that work in concert to increase the stress in the copper barrel. 

I would say that this is a high temperature barrel crack type failure and the material is playing a significant role. 

Reliability at higher temperatures is a synergistic relation between copper quality, material robustness, and PWB design (layout and build). I believe this failure is a sum total of the influence of those three factors.

Sincerely,

Paul Reid

Program Coordinator

PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1

613 596 4244 ext. 229
Skype paul_reid_pwb
[log in to unmask] <mailto:[log in to unmask]>

 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ido Mashall
Sent: Friday, October 03, 2008 2:13 PM
To: [log in to unmask]
Subject: [TN] Barrel and Corner cracks in copper elctroplating

 
Hello technet

We are suffering a serious problem of barrel and corner cracks in the pattern copper plate bath.
Please look at the following pic. (all after 288C solder thermal stress)

http://www.drivehq.com/file/df.aspx/shareID4765976/fileID233954134/eo-d3497-
1.jpg
 
http://www.drivehq.com/file/df.aspx/shareID4765976/fileID233954135/es-c9633-
1.jpg
 
http://www.drivehq.com/file/df.aspx/shareID4765976/fileID233954138/as-d3715.
jpg

This very strange phenomenon occurs only in the second copper, and does not occur in the panel plated copper.

After testing all bath components, they seem to be ok. CVS is oK, hull cell is OK. It seems the copper lost its mechanical properties.

We are looking for possible contamination and changed the bath for a new fresh bath, and it happens again after a few days..

Does somebody have some idea why this cracks can occur? What other reasons..?


Thanks.


Ido Mashall


Fax: +972-4-6376666, Tel: +972-77-2127172, 

Mobile +972-52-5901114,  http://www.beyond-materials.com/index-en.htm

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