Impact of Ag plated surface roughness towards die bond and wire bond

Abstract

Lead delamination is one of the major challenges faced in semiconductor manufacturing. It happens when there is a separation between silver plated surface (Ag) and encapsulation molding compound (EMC) inside a package. One of the most common approach that were often used is through roughening of Ag surface to improve the mechanical interlocking between Ag surface and EMC [1]. However, the characteristic of Ag surface roughness towards die bond and wire bond performance is not studied in depth. In this paper, evaluation will be done to check the characteristic of Ag surface roughness towards die bond and wire bond process. Relative average roughness (Sa) of the Ag surface will be measured prior to the evaluation. Leadframes with different Sa will be used for each evaluation leg. During die bond process, the condition of surface prior bonding has significant implications towards the quality of the bonds formed. The peak to valley texture of Ag surface is able to affect the mechanical bonding between leadframe and adhesive epoxy. Therefore, die shear test will be done to check the bonding quality of the interface bond. In addition, correlation between Sa towards bond line thickness (BLT) and epoxy bleed out (EBO) will be assessed due to different surface roughness will influence the conformity of the epoxy to the surface. Xray will be done at 0hour and after Temperature Cycle (TC) to check for potential voids caused by different surface roughness. In wire bond, due to the characteristic of Ag, the bonding between Au wire and Ag plated surface is reliable and does not form intermetallic compounds (IMC) at high temperature. [2]. Despite the reliability of the bonding, the impact of surface roughness towards 2nd bond is not fully understood. The ultrasonic power applied on different Ag roughness during wire bond process will induce different friction power density at the bonding interface. It is believed that different friction power will initiate different mechanical interlocking between Au and Ag. Therefore, stitch pull test and Non stick on Lead's (NSOL) ppm will be collected during the study to check the correlation between 2nd bond quality and Ag roughness. The stitch integrity after stress was investigated through two temperature extreme of Temperature cycle (TC) test. Focused ion beam (FIB) analysis will be done to correlate the adhesion between Au and Ag for different surface roughness. Besides, it is predicted that 2nd bond cutting will be less consistent when the surface is rougher. This will lead to inconsistent Free Air Ball (FAB) size and thus impact 1st bond. Therefore, FAB size will be collected through scanning electron microscope (SEM). This paper will present as a fundamental guideline to control Ag surface roughness without compensating the quality at die bond and wire bond processes.