Pad cratering, is a microcrack or fracture underneath the pad of a surface mount component. Most commonly occur on high-frequency and high-speed PCB which are used in the telecom industry. In general, the characteristic of high-frequency and high-speed material has weaker bonding strength with Cu in comparison with normal dielectrics material.

In addition, in order to satisfy lead free assembly and halogen free compliance, the material will be prone to stiffer and more brittle which would result in higher propensity of PCB pad cratering during PCB assembly and in service life, especially when assembling with larger size package like BGA and QFN.

However, PCB pad cratering defect is hard to be found through electrical test and visual inspection. The internal microcrack sometimes would not induce electrical failure. Most ODM/EMS manufacturers cannot know whether there is microcrack generated underneath the pad in the PCB before product shipping.

“If the end product with pad cratering is adopted in the market as normal product, it could not show any problem in the short term. However, in the long term, it could cause significant effects on the reliability of product operation, especially when those products are designed for high reliability with stringent requirements as well as long term service life more than 7 to 10 years, such as telcom, server and cloud computing product.” Jeffrey Lee, Assistant Vice President of Global Engineering Development Division of iST, said.

In order to find out PCB pad cratering in early phase, iST cooperate with international telecom firm and IPC (Association Connecting Electronics Industries) to develop Acoustic Emission testing method. In the end of 2013, IPC published Standard IPC 9709 for AE test guide. iST formally introduce AE test to our customer for product quality validation.

Acoustic Emission testing method is used in earthquake monitoring and material strength testing in architecture/ aeronautic field. However, iST apply AE testing method in PCB validation field. Use four sensors to detect the acoustic emission events generated during board level bending test.

It can plot out all position generating pad crater in whole PCB and quantify corresponding failure energy level. Hence, PCB manufacturers and PCB assemblers can differentiate the performance withstanding pad cratering among various PCB materials by adopting AE/ bending test method.

The left figure shows Dye and Pry analysis; the right figure shows cross-section analysis. These samples do not perform fail in electrical test, however, AE test can detect there is a crack generated underneath the pad. It can be evidenced by D&P analysis and X-section analysis to show there is a crack occurred in actual.