Characterization of copper etching process on micro leadless land grid array (ゼLLGA) via design of experiments approach

Abstract

In the fast-paced semiconductor industry, the need for innovative package solutions arises in order to cope with emerging miniaturization trend. Micro leadless land grid array (μLLGA) surpasses other plastic encapsulated packages with remarkable profile of 0.38mm. As one of the thinnest packages available in the market up to date, the unique structure of μLLGA is illustrated in this paper. As we know, there are many limitations in thin die advancement from the point of wafer manufacturing, machine capability and assembly challenges. In view of that, copper etching process has come to play an essential role in μLLGA assembly. This report discusses the mandatory Copper etching process using alkaline etchant that contributes to the thin package profile. Prior to the characterization of machine parameters, it is crucial to understand the chemical equilibrium of echant chemical. A full factorial DOE was conducted to characterize the factors that affect Copper etching efficiency. The four main factors included in this study are Bath Temperature, Conveyor Speed, Nozzle Pressure and Bath Specific Gravity. The etching efficiency is determined by many observations after etching process. Compound discoloration is being observed to ensure no cosmetic defect due to chemical spraying. This report also addresses the package robustness through SAT observation. Special lead frame design contributes to the passing of reliability test at MSL 1. Solderability test for this fine pitch package is also being studied in this paper, comparing the typical dip-and-look and surface mount solderability test method. From the experiment, key parameters that affect etching efficiency have been identified to be Bath Temperature and Conveyor Speed. With optimized parameters and good chemical maintenance, an effective copper etching process can be realized.