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Prevention of Au Wire Fatigue Break in Gel Filled Package

IP.com Disclosure Number: IPCOM000234840D
Publication Date: 2014-Feb-10
Document File: 5 page(s) / 309K

Publishing Venue

The IP.com Prior Art Database

Abstract

A typical gel filled Tire Pressure Monitor Sensor (TPMS) device includes a pressure sensor, an accelerometer and a micro-controller chip. It is a multi-die package, and to protect the pressure sensitive die, encapsulation with low modulus, high CTE (Coefficient of Thermal Expansion) gel is needed instead of conventional plastic molding material. To meet stringent automotive requirements, TPMS devices must pass extensive temperature cycling stress tests. This has posed challenges to wire bonds where thermal mechanical stress in extensive temperature cycling aggravates weakness within the wires, which increases the risk of wire fatigue crack at wire bending areas such as neck, kink or second bond heel. In this paper, an in-depth analysis of the fatigue failure mode, failure mechanism hypothesis, wire materials comparison and wire bond process DOE (Design of Experiment) are presented. From the reliability stress result, we consistently observed that wire fatigue damage always occurred at the first kink of corner longest wire of the MCU to lead, and did not occur on other inter-chip or shorter side MCU wires. DOE was performed to investigate and understand this observation, which includes wire looping, wire material, and gel properties. For wire looping profiles, a number of different looping shapes were evaluated with the objective of reducing wire tightness and to provide a more rounded kink. DOE results showed that smaller kink angle, more loose, and higher looping profile are critical parameters for reducing wire fatigue damage. Further, in terms of wire properties, wire breaking load and HAZ length were found to be important. Based on the DOE result, a solution combining looping optimization and wire selection was validated. For gel material change, it was found that lower modulus gel provides better protection of wire bond during temperature cycling stress.

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Document Title 

Prevention of Au Wire Fatigue Break in Gel Filled Package

Abstract 

A typical gel filled Tire Pressure Monitor Sensor (TPMS) device includes a pressure sensor, an accelerometer and a micro-controller chip.  It is a multi-die package, and to protect the pressure sensitive die, encapsulation with low modulus, high CTE (Coefficient of Thermal Expansion) gel is needed instead of conventional plastic molding material.  To meet stringent automotive requirements, TPMS devices must pass extensive temperature cycling stress tests.  This has posed challenges to wire bonds where thermal mechanical stress in extensive temperature cycling aggravates weakness within the wires, which increases the risk of wire fatigue crack at wire bending areas such as neck, kink or second bond heel.

In this paper, an in-depth analysis of the fatigue failure mode, failure mechanism hypothesis, wire materials comparison and wire bond process DOE (Design of Experiment) are presented.  From the reliability stress result, we consistently observed that wire fatigue damage always occurred at the first kink of corner longest wire of the MCU to lead, and did not occur on other inter-chip or shorter side MCU wires.  DOE was performed to investigate and understand this observation, which includes wire looping, wire material, and gel properties.  For wire looping profiles, a number of different looping shapes were evaluated with the objective of reducing wire tightness and to provide a more rounded kink.  DOE results showed that smaller kink angle, more loose, and higher looping profile are critical parameters for reducing wire fatigue damage.

Further, in terms of wire properties, wire breaking load and HAZ length were found to be important.  Based on the DOE result, a solution combining looping optimization and wire selection was validated.  For gel material change, it was found that lower modulus gel provides better protection of wire bond during temperature cycling stress.

Body 

In qualifying a TPMS device at a subcontractor assembly site, it was found that about 20% of the units in a qualification lot failed electrical test at one specific pin after 650 cycles temperature cycling stress (TC650), with temperature range of -40ºC to 125ºC.  Upon FA, it was found that the root cause for test open failure was due to broken wire as illustrated in Figure 1. The broken wire is one of the longest corner wires of MCU to lead, and the breaking point is at wire first kink area.

Extensive inspection using Scanning Electron Microscope (SEM) was performed to find the root cause.  In order to verify if the wire damage developed through prolonged temperature cycling stress or inherent defect caused by wire bond, an interval base SEM study at T0 (time=0), TC400, TC650 and finally TC1300 was carried out.  At T0, there was no wire defect observed in any wires, including the most critical long corner wires.  However, as the temperature cycling stress took place, it w...