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Pad Cratering

Idea/Definition Information
Summary: 

Lead-free (LF) solder joints are stiffer than tin-lead solder joints, and LF compatible (Phenolic-cured) PCB dielectric materials are more brittle than the FR4 (dicy-cured) equivalent. These two factors, coupled with the higher peak reflow temperatures used for lead-free assemblies, could transfer more strain to the PCB dielectric structure, causing a cohesive failure underneath BGA corner pads. This project will examine the phenomenon with the goal of determining test and screening methods.

If you are interested in this project and would like to subscribe to the project mailing list, please click here (members only) or email the project facilitator, Jack Fisher - fish5er@hdpug.org.
 

Background: 

“Pad cratering”. is a bit of a misnomer, since the “crater” is the space left vacant when a BGA ball pad lifts off the board.

Simply put, pad cratering is a failure of the resin system as the weakest point (rather than the solder, intermetallic, or pad bond) when mechanically or thermomechanically stressed.  This has been exacerbated by RoHS the non-dicy cured FR4 materials required to survive the higher temperatures associated with Pb-free soldering are more brittle than the historically used dicy cured FR4 materials and the stiffness of Pb-free SnAgCu (SAC) solders compared to SnPb solders is greater, transferring the stresses to the interfaces of solder balls and the PCBs.. 

The ultimate root cause of pad cratering failures is overstress of the material. The driving forces are thermo-mechanical or mechanical stresses. These can be due to stresses generated during assembly reflow and cooling or stress on the joints caused by mechanical processes (bending) or other mechanical shocks, such as drop shock. 

The failures normally begin at the outside corners of the BGA package and may not be apparent at first if the connecting traces to the pads don’t immediately fracture.  The most common electrical failure point is at the perimeter of the ball pad where a routing trace connects to the pad.

A mechanically induced fracture in the resin between copper foil and outermost layer of fiberglass. May be within the resin or at the resin to glass interface.

The pad remains connected to the component (usually BGA) and leaves a “Crater” on the surface of the printed circuit board (PCB).

Problem: 

A method is needed to rank order materials that is directly related to actual pad cratering.

Approach: 

Create a relatively simple test vehicle with a single large BGAassembled in the middle

– 6 or 8 layers, thickness .093

• Fabricate the bare boards from multiple different materials

– Include a significant variety of materials, filled and unfilled

– Include materials that have also done “well” in HDPUG Pb-free materials

projects

– Other materials as suggested by members

• Perform bend to break testing to rank order the materials

– Design TV to virtually ensure trace breaks simultaneous with laminate.

            Won’t break identically, but data supports a “soon after” break that should be sufficient to rank order materials.

– Spherical bend testing preferred or 4-point bend

– Will need to include some amount of strain gage measurements

– Both Single Bend to Break and Repeated Load to Break

  • Possibly include drop testing also (team/resources dependent)