Polymeric reinforcement approaches and materials selection to improve board-level drop reliability of SnAgCu soldered area array packages

Abstract

The board-level drop performance of area array package (AAP) assemblies is becoming increasingly critical due to the shift from desktop to mobile computing. Furthermore, challenges have arisen from the introduction of lead-free solders and miniaturization of solder joint dimensions. Polymeric reinforcement of AAPs offers a solution for drop reliability concerns. However, polymeric reinforcement increases the unit manufacturing cost of materials, capital equipment, cycle time, and rework. All of the polymeric reinforcement approaches, such as full capillary flow underfill (FCFU), partial capillary flow underfill (PCFU), edge bond adhesive (EBA), and corner bond adhesive (CBA), improve the drop reliability of lead-free fine-pitch AAP assemblies. However, the use of a polymeric reinforcement strategy with improper implementation and/or material properties may cause an unnecessary rise in manufacturing costs and/or cause the assemblies to fail to meet the drop performance requirements of a specific application. This study compares the different polymeric reinforcement approaches (FCFU, PCFU, EBA, and CBA) and material properties for AAPs using a vertical free drop test. One set of AAP assemblies with no polymeric reinforcement was tested as the control. The test results indicated that the drop performance of reinforced CSP assemblies increased with the use of better polymeric reinforcement material volume and modulus and higher adhesive strength of the materials. The components closer to outer edges of the PCB were more prone to failure compared to the components at the center of the PCB. In addition to the failure criteria based on daisy-chain resistance, the drop impact life based on the CSPs that fell of the PCBs can also be used simply to compare the performance of different polymeric reinforcement strategies. Failure analysis demonstrated that the dominant failure mode was brittle fracture at the CSP IMC/solder interface for all the test groups except the underfilled samples. However, the percentage of the PCB pad cratering failure mode significantly increased with the application of polymeric reinforcement materials.