Journal of SMT Article

Mechanical Failures in Pb-Free Processing: Evaluating The Effect of Pad Crater Defects on Process Strain Limits for BGA Devices

Authors: John McMahon and Brian Gray
Company: Celestica Inc.
Date Published: 4/30/2010   Volume: 23-2

Abstract: The increased temperatures associated with Pb-free processes have produced significant challenges for PWB laminates. Newly developed laminates have different curing processes, are commonly filled with ceramic particles or micro-clays and can have higher Tg values. These changes which are aimed at improving the materials resistance to thermal excursions and maintaining electrical integrity through primary attach and rework operations have also had the effect of producing harder resin systems with lower fracture toughness.

Industry guidelines for mechanical stress limits were developed for materials processed using eutectic SnPb solders. A series of design and material implementations have gained wide acceptance by the industry to address mechanical failures at the corners of area array packages. Current accepted levels of process strain were established when the dominant and limiting failure mode was interfacial fracture (IFF) in complex intermetallic compound (IMC) layers at the solder / package interface. Changes in packaging processes, conversion to Lead (Pb) free solders and the subsequent compensation by laminate suppliers have produced significant shifts in failure mode occurrence and the “Pad-crater” failure mode has become far more common than IFF. By conducting a testing program that focuses on materials and geometries consistent with high complexity “Enterprise Computing and Telecomm” electronic assemblies, and evaluating the results against the current industry guidelines it is possible to determine whether the dominance of the “Pad-crater” defect mode will require revision of process strain guidelines. Test methods, test results, failure analysis and likely mitigation techniques are discussed.

Key words: Pad crater, spherical bend test, mechanical failure mode, process strain.

Members download articles for free:

Not a member yet?

What else do you get when you join SMTA? Read about all of the benefits that go along with membership.

Notice: Sharing of articles is restricted to just your immediate work group. Downloaded papers should not be stored on an external network or shared on the internet.


SMTA Headquarters
6600 City West Parkway, Suite 300
Eden Prairie, MN 55344 USA

Phone +1 952.920.7682
Fax +1 952.926.1819