No abstract

Saw singulation is an indispensable process to sever the Mold Array Package (MAP) strips into individual units in assembling Quad Flat No-lead (QFN) packages. Mechanical saw, cutting the MAP strip along saw streets using dicing blade, is a mainstream mode in the saw singulation process. Accordingly, the dicing blade, a continuously consumed part, takes a significant portion of the assembly cost. The saw blade consumption is generally determined by saw blade type, MAP strip structure and process parameter. In this study, the effects of MAP strip structure, specifically, the saw street configuration in lead frame, on the saw blade saving have been experimentally investigated. The saw street configuration was first investigated by comparing the wear amount of saw blade for cutting discrete and continuous saw streets of equal length and same configuration in 3×3 mm, 6×6 mm and 9×9 mm QFN packages. It is found that (i) the wear amount of the discrete saw streets may be significantly less than its counterpart of the continuous saw streets and (ii) the saw blade saving in the discrete saw streets goes more as the package size goes smaller. The saw blade saving is attributed to the additional saw process improvement contributed by the discrete saw streets. The saw street configuration was also investigated by comparing the wear amount of saw blade for cutting saw streets of 5 mil and 8 mil lead frames in the 6×6 mm QFN package. It is proved that less metal amount in the saw street consumes less saw blade. The merit of the discrete saw street and metal amount reduction in saw street were then implemented in the revised lead frame of the 6×6 mm package, and the saw blade consumption per MAP strip dropped to 15.8 μm in revised lead frame from 113.0 μm in original lead frame in statistical sense.

Finite Element (FE) simulation is an effective approach to investigate the thermal stress status and the reliability trend when qualifying a new packaging material going through the reliability test. However, simulation of thermal stresses in gold wires was rarely reported in previous days due to the hardness in FE model building. This study performs 3D modeling to evaluate the thermal stresses in 2N gold wires with the full package model to identify which wires to peel for process control setup. The results from this modeling will be used to verify the second bond to file the process specification of wire peel test. The FE model in this paper describes a PBGA package with 2N gold wires surrounded by mold compound and other parts of the package. Stress evaluation is done through discussing the Von Mises stress, the equivalent plastic strain and the peeling stress. TC (temperature cycle) condition is applied in the modeling. The simulation results indicate that shorter wire(s) has higher stress than longer wire(s) and shorter corner wire(s) has the highest propensity to fail in TC. Different from the other wires, the highest peeling stress on bonding interface for the shorter center wire(s) is at the wire heel location instead of the tail end of the stitch bond. The wire heel location is usually regarded as the sensitive region related with the initial crack leading to the stitch bond failure. Thus, the shorter corner and center wire(s) should be regarded as high priority to be wire peeled and filed into wire peel specification. Besides, the relationship between stress status (reliability propensity) of stitch bond and the stitch bond shape is discussed. According to the measurement data of various stitch shapes, three typical stitch bond shapes (marked as A, B and C) are proposed and modeled. The modeling data show that the shortest stitch length (shape A) has the highest stress and plastic strain in the stitch bond comparing with the other shapes, and produces the highest peeling stress at bond interface. It might imply that the shortest stitch length could cause the higher propensity leading to bond lift in stress test. It could become a guideline for wire bond process that engineers should avoid forming the short stitch length when bonding 2N gold wires. Introduction2N gold wires with specific dopants are applied in many types of package for stringent reliability required field. With containing the special additives, 2N gold wires are provided with the enhanced bond integrity at the first bond (on the die) by improving the IMC performance. At the second bond (also called as stitch bond) on the lead frame, 2N gold wires also show good bond capability comparable to 3N and 4N gold wires. As a new material introduction, the packages containing 2N gold wires should pass the qualification of reliability. It's inarguable that the first bond (ball bond) of 2N gold wire has better reliability behavior during reliability test