The Simulation of Thermomechanically Induced Stress in Plastic Encapsulated Ic Packages

by Kelly, Gerard

• ISBN13: 9780792384854
• ISBN10: 0792384857
• Format: Hardcover
• Copyright: 1999-06-01
• Publisher: Kluwer Academic Pub

Summary

This book is motivated by the need to understand and predict the complex stress distributions, transfer mechanisms, warpage, and potential failures arising from the encapsulation of devices in plastic. Failures like delaminations, package cracking, and metal shift occur due to the build-up of residual stress and warpage in the packages because of the TCE mismatch between the package materials as the package cools from its molding temperature to room temperature. The correct use of finite element tools for these problems is emphasised. F.E. techniques are used to predict the internal package stress distribution and help explain the stress transfer mechanism between the die, die paddle, and plastic after molding. Out-of-plane shear stress components are shown to be responsible for experimentally observed metal shift patterns on the die surface. Delaminations dramatically alter the internal stress state within a package, increasing the tensile stress in the plastic and so the likelihood of plastic cracks, the stress on wire bonds, and the incidence of wire bond failure. The application of F.E. techniques to predict the post-mold warpage of both thermally enhanced PQFPs and TQFPs is described. Simulations of a thermally enhanced PQFP warpage based on standard modelling assumptions alone fail to predict either the magnitude or its direction correctly. The modelling assumptions need to be modified to include the chemical shrinkage of the molding compound to enable accurate predictions of package warpage to be made, particularly when the packages are asymmetric in structure. Microsystem packaging in both plastic and 3D package body styles is reviewed. Although microsystem packaging is derived from IC packaging, additional requirements for microsystems, not common to IC packaging are highlighted. The assembly stresses on a novel microsystem, incorporating a micromachined silicon membrane pump integrated into a 3D plastic encapsulated vertical multichip module package (MCM-V), are analysed.

Table of Contents

List of Figures	ix
List of Tables	xiii
Preface	xv
Acknowledgments	xix
An Introduction to Plastic Packaging	1	(12)
Manufacturing sequence for a PQFP	1	(2)
Properties of packaging materials	3	(2)
Silicon	3	(1)
Die attach	4	(1)
Leadframe materials	4	(1)
Molding compound	5	(1)
Stress induced failures of plastic packages	5	(4)
Die related failures	6	(3)
Plastic related failures	9	(1)
Chapter References	9	(4)
A Review of Package Stress Modelling	13	(10)
Introduction	13	(1)
Analytical approaches to package modelling	14	(1)
Finite element methods	15	(4)
2D structural approximations to a plastic package	16	(1)
2D finite element design studies	16	(1)
Die surface shear stress	17	(1)
Stress at the interfaces of the package	18	(1)
Warpage prediction	18	(1)
Chapter References	19	(4)
Thermomechanical Stress in a PQFP	23	(24)
Introduction	23	(1)
Origin of thermomechanical stress in TCE mismatched materials	24	(1)
Finite element analysis of a PQFP	25	(1)
2D representation of a 3D structure	25	(5)
Plane stress	26	(1)
Plane strain	27	(1)
2D plane stress approximation to a PQFP	27	(1)
2D plane strain approximation to a PQFP	27	(1)
Plane stress vs. plane strain	28	(2)
Interpretation of die encapsulation stress	30	(4)
Adhesion models	31	(2)
Factors influencing die stress	33	(1)
Mechanism of stress transfer	34	(2)
Deformation of the package structure	36	(1)
Die surface compressive stress distribution	36	(4)
Reduction of die encapsulation stress	37	(3)
Material and geometric factors which influence encapsulation stress	40	(4)
TCE and elastic modulus of the plastic	40	(2)
Plastic thickness	42	(1)
Die paddle TCE	43	(1)
Die size	43	(1)
Other factors	43	(1)
Chapter References	44	(3)
The Correlation of Modelling with Measurements and Failure Modes	47	(22)
Introduction	47	(1)
Measurement of die stress with stress sensors	48	(1)
Die surface stress	48	(1)
Simulated die surface stress	49	(3)
Comparison between measured and modelled encapsulation stress	52	(1)
Analytical model	53	(1)
The correlation of simulations with failure modes	54	(5)
Shear stress and metal shift	54	(4)
Correlation between shear stress and metal shift	58	(1)
Influence of delamination on stress	59	(2)
Analysis of stress in a wire bond	61	(5)
Description of 2D wire bond stress models	61	(1)
Direct stress distribution in a wire	62	(3)
Shear stress distribution in a wire	65	(1)
Chapter References	66	(3)
Accurate Prediction of PQFP Warpage	69	(18)
Introduction	69	(2)
Warpage of a 208 lead power PQFP package	71	(4)
Measurement	71	(1)
Simulation	72	(1)
Influence of adhesion	73	(1)
Influence of differential cooling	74	(1)
Influence of molding compound shrinkage	74	(1)
Variation of power PQFP warpage with temperature	75	(3)
Improved F.E. model incorporating chemical shrinkage	76	(2)
Significance of chemical shrinkage for asymmetric packages	78	(1)
Warpage of a small body size PQFP	78	(4)
Measurement	78	(1)
Simulation	79	(3)
Warpage of a large body size PQFP	82	(1)
Warpage sensitivity of both large and small body size PQFPs	82	(1)
Asymmetric structure of BGA packages	83	(1)
Chapter References	83	(4)
Microsystem Packaging in Plastic and in 3D	87	(20)
Introduction	87	(1)
Microsystem packaging - Lessons from IC packaging	88	(2)
Additional requirements for microsystem packaging	89	(1)
3D packaging methodologies	90	(2)
3D microsystem packaging - a European example	92	(13)
Assembly of the micropump module	93	(2)
Simulation of the assembly stress	95	(4)
Simulation of the membrane stress	99	(3)
Low stress packaging options	102	(3)
Chapter References	105	(2)
Concluding Remarks	107	(13)
Problems remaining to be solved	107	(1)
A comment on the numerical tools	108	(1)
For the Future	109	(1)
Chapter References	109	(2)
References	111	(9)
Appendices	120	(11)
A-- Analytical model of encapsulation stress	121	(4)
A.1 Force equilibrium	122	(1)
A.2 Strain compatibility	122	(3)
B-- Fundamentals of stress and strain	125	(4)
B.1 Direct and shear stress conventions	125	(1)
B.2 Longitudinal strain and Poisson's ratio	126	(3)
C-- Axial stress and bending stress	129	(2)
Index	131