STAYCHIPTM Capillary Underfill Encapsulants
SCOPE
This document is intended to assist package and product assemblers with handling, dispensing, curing and troubleshooting capillary-type underfill encapsulants. An introduction is included to acquaint the reader with the history and nature of these materials. Please contact your Cookson representative for assistance with underfill selection or reliability issues.
Introduction to Underfill Encapsulants
BACKGROUND
Underfilling encapsulants were originally developed to improve reliability of early flip chip assemblies employing alumina (Al2O3) substrates. Solder fatigue of the outermost solder bump interconnections caused the failures in these assemblies. The relatively small thermal expansion mismatch between the silicon die and the substrate (typically a few ppm/°C) was responsible for the onset of fatigue during thermal cycling. The temperature range of the cycling and the size of the solder bump array (closely correlated with the die size) were the main factors determining the lifetime of the assembly. Inserting a curable liquid epoxy resin between the die and substrate was shown to dramatically improve assembly reliability by spreading stress from the CTE mismatch over the entire area of the die, instead of just over the solder joints.
Underfill development accelerated with the introduction of epoxy-based laminates as substrates for flipchip assemblies. A large CTE (coefficient of thermal expansion) mismatch between the laminate and silicon (typically 12ppm/°C or greater) made underfill a necessity to prevent very rapid solder fatigue. The underfill spreads the stresses caused by the CTE mismatch over the entire area of the die and forces much of the stress into the substrate. Interfacial delamination between the underfill and the die, not solder fatigue, is the dominant cause of failure in underfilled flip chip assemblies. In verydemanding applications Ball Grid Arrays (BGA’s), themselves packages, are underfilled to reinforce the relatively large solder joints connecting them to their motherboard. These joints, typically 1.0mm or more apart, normally withstand thousands of thermal cycles before fatiguing. Underfills significantly extend joint life. In addition to imparting thermal cycling reliability, underfills protect chips and interconnections from environmental factors and increase the mechanical strength of the package attachment. The latter is particularly important for chip scale packages (CSP’s), which typically pass thermal cycling and environmental tests without underfill but fail when subjected to mechanical shock or impact (i.e. drop testing). These forces cause circuit board flexure that can lead to cracking at the substrate-to-ball interface or at the ball-to-package interface. Underfills protect CSP solder ball connections by locally stiffening the assembly, thus reducing the strain related to board flexure.
Underfill Classifications
DOWNLOAD PDF FILE:
capillary underfill application guidelines (317.7 KB, 2 次)
您没有权限下载此文件。
