Pad Cratering Evaluation of PCB

Pad cratering in the PCB is a new failure mode encountered in electronic assemblies, particularly in lead-free products. The failure mechanisms and root causes are not yet fully understood, and lack appropriate industry standard tests for PCB qualification with regard to pad cratering. This paper reviews major publications and research reports on various PCB materials from industry studies in this field. Various PCB tests, such as flexural strength test and pad strength test, have been studied. It is recommended that the qualification of the PCB can be done in two stages: PCB board level and PCBA product level. From those results, a new qualification method is suggested for screening out PCB pad cratering failures. Introduction Pad cratering in the PCB (printed circuit board) is one of the major failure modes encountered in electronic assemblies and has become more prevalent as the industry moves to lead-free soldering. It impacts the production yield as well as long term reliability. 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 and makes the circuit open or intermittent. Pad cratering starts from cracking of the laminate. The crack may initiate from the intersection of the solder, copper pad and laminate as shown in Fig. 1, as this is a stress concentration point for crack initiation. The crack may propagate from the interface of the epoxy to the glass bundle under certain stresses. In this case, as the stress may concentrate on the other side of the perimeter of the pad, the crack may propagate through the pad and create a total pad cratering. According to Roggerman et al, the propagation of the crack through the glass reinforcement and along the glass-resin interface takes a significant amount of time and stress to become a complete failure [1]. Fig. 1 Pad cratering occurred in the BGA assembly during functional test in the production line. Root Causes of Pad Cratering Pad cratering has been widely seen in drop and shock tests as one of the major failure modes, giving an impression that pad cratering only occurred in drop and shock tests or other mechanical tests. In reality, pad cracking could actually occur under thermal stress or thermomechanical stress, especially for large BGAs (body size >30mm). Fig. 1 is an example taken from production with no mechanical stress test. Testing for Pad Cratering There are three levels of tests related to pad cratering: PCB materials, soldered PCB (pad-solder level), and system level. The PCB material is associated with the raw materials (such as epoxy resin, filler and fiber), and the layer structure and As originally published in the IPC APEX EXPO Conference Proceedings.