Advances in Liquid Encapsulated Chip-on-Board / MCM-L Technology
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ABSTRACT: Chip-on-Board technology (COB), or the currently more often used MCM-L acronym (MultiChip Module - Laminate), has long been touted as the low cost, high density electronics packaging choice of the future. Unfortunately, poor reliability performance in comparison to traditional plastic packaging, has been a trademark of COB/MCM-L. Recent advances in processes and materials for COB/MCM-L are providing a paradigm shift in reliability, and providing cost effective high reliability packaging solutions for COB/MCM-L. This paper examines the history of recent advances in liquid encapsulated COB/MCM-L and provides guidelines for selecting appropriate materials and processes for their successful implementation into cost effective manufacturing.
INTRODUCTION: Traditional IC (integrated circuit) "packages" have progressed over time (see Figure 1) from Dual Inline Packages (DIP) to Plastic Quad Flat Packages (PQFP) to Pin Grid Arrays (PGA) and more recently, to Plastic Ball Grid Array Packages (PBGA) or Ceramic Ball Grid Array Packages (CBGA). These traditional packages effect the "first" level of electronic interconnect between the IC device and the IC package. The IC package then effects the "second" level of interconnect through connection, usually via soldering, to the printed circuit board.
Figur e 1: Progression of packaging sophistication, from left to right, Dual Inline Packages (DIP), Plastic Quad Flat Package (PQFP), Leadless Chip Carrier (LCC), Pin Grid Array (PGA), and Plastic Ball Grid Array Packages (PBGA). 61 Mat. Res. Soc. Symp. Proc. Vol. 390 01995 Materials Research Society
There are significant reasons for the electronics industry to investigate IC packaging solutions which may be "non"-traditional, such as MCM-L or COB. Chip on Board, or COB, refers to the packaging technology wherein the interconnection from the IC device(s) is made directly to the end application printed circuit board, often referred to as a "PCB". The term "COB" has recently become associated with the term "Multichip Module-Laminate" or "MCML" acronym. MCM-L refers to the packaging technology wherein the interconnection from a multitude of IC devices is made to an intermediate laminate, or printed circuit board, material which then takes the form of a component much like a traditional IC package. COB and MCML are essentially identical in method of construction differing only in their configuration of either being a component or and end application PCB. A typical cross section of COB/MCM-L construction is shown in Figure 2. COB and MCM-L represent opportunities for cost and size reduction and performance improvement through the elimination of one level of interconnect or the increase in density which results in a smaller, higher performance component or end application circuit board.
Wire Bond
IC "Die"
Liquid Encapsulant ("Glob Top")
Die Attach Material
Multilayer Organic P inted Circuit Board
Via thru PCB
Figure 2: Typical COB and MCM-L Construction Applications which are size driven, such as pagers and portable cell
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