QFN assembly dilemma - solutions

Abstract

In the competition of market share of Quad Flat No-lead (QFN) package, there were several improvement methods being utilized, such as leadframe design for higher density, cheaper tape, and more functionalities per chip (with smaller package size, larger die, and smaller pad size). Most of these improvement methods created extremely weak die attach and wire bonding structures, causing significant amount of issues in production of QFN packages. Some of the new die attach requirements had caused the ratio of die size to flag size reaching almost one for epoxy dispensing process. This allowed larger die to go inside the QFN package, but epoxy quality control became very stringent. Meanwhile, half etched flag and lead structures caused further amplification of resonance effect, generating more stress inside bonded wires. Eventually, wire weak points, neck or heel started showing stress effects, until failing at actual applications. By utilizing special statistical analysis, each significant factor was screened out, and then combined into optimum interaction. With acid testing method, optimum interaction was proven to be robust and stable in production.