Pad Cratering: The Basics

Pad cratering, what is it?  Pad cratering is defined as a separation of the pad from the PCB resin/weave composite or within the composite immediately adjacent to the pad.  It triggers failures only if the crack propagates into a copper trace or conduction pad makes the circuit open or intermittent.

Pad Cratering Drivers:

• Finer pitch components
• More brittle laminates (lead-free compatible)
• Stiffer solders (SAC vs. Sn Pb)
• Presence of large heat sinks
• Location
• PCB Thickness
• Temperatures and cooling rates

Issues with Detecting Pad Cratering

• Companies are frequently unaware until failure occurs
  • Recalls can be frequent and painful 
• Potential Warning Signs
  • Excessive BGA repair rates
  • High percentage of “defective” BGA’s
  • High rate of “retest to pass” at in circuit test (ICT)
• Difficult to detect using standard procedures of X-ray, generally must use destructive testing such as dye n pry, ball shear, and ball pull
• Expensive advanced testing methods now available to screen for pad cratering

“Band Aid” Solutions for Pad Cratering

Solutions to prevent costly damage are labor intensive, costly, or have marginal impact on the problem.

• Expoxy fillets at BGA corners
  • Ineffective and do not prevent board flexing
• Increase board thickness to eliminate flexing
  • Expensive
• Changing BGA and PCB pad design
  • Limited effectiveness, limits design and sources of components, can be expensive
• Underfill of BGA components
  • Labor intensive, prevents rework of BGA parts after assembly

Zeta Cap as a solution

 

How does it work?

• Zeta Cap requires no special processing or equipment.  It simply replaces the outer layer foil in the PCB construction
• When used as a cap layer, it becomes the interface between the copper pad and the rest of the PCB
• The more pliant cap will prevent or block fractures and protect copper connections (traces) the the pad

Why does it work?

It serves as a barrier to block pad cratering with a unique combination of properties:

• High strength without being brittle
• Modulus allows for some flexibility
• Very high Tg and Td
• Very low CTE (close to copper and solder)
• Halogen free
• Lead free compatible
• Used on PCB surface with convention materials

If you are interested in learning more, please contact us for additional information.  Material spec sheets and test and reliability data are readily available! 

 

Purchasing and designing printed circuit boards does not have to be difficult! 

 

A special thank you to Insulectro for working with us and compiling this information! 

 

  

 www.omnipcb.com

PCB Material Data Sheets

For all of our friends that feel just a little uncertain when material data sheets are discussed and words like dielectric constant, Tg and  coefficient of thermal expansion are thrown into the discussion, we have compiled a short cheat sheet just for you.

Dk – Dielectric Constant – The way the electrical signal moves through the material from front to back.

Dissipation Factor – Loss Tangent – How much the signal dissipates or “leaks” out of the substrate.  The higher the speed, the higher the losses.

Coefficient of Thermal Expansion – Thermal fractional change in material for unit change in temperature.

Resistivity:  How strongly the material resists a current.

Tensile Strength:  Relates to the rigidity of the material as well as the peel strength.  How far can the material be stretched before permanent damage occurs.

Dimensional Stability:  The tendency of material  to change in the x and y dimensions when copper is etched away from the dielectric material or when exposed to temperature extremes.

Tg – Glass Transition Temperature – Temperature that material exhibits a change in physical characteristics. The resin changes from rigid or hard to a soft, rubber like material.

Td  – Thermal Decomposition Temperature – Temperature at which the material weight changes by 5%.  This parameter determines the thermal survivability of the resin material.

 

As always, if you have questions or would like additional information, please contact us!
Purchasing and designing printed circuit boards does not have to be difficult!