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TWO STEP ENCAPSULATION PROCESS FOR IMPROVING YIELD ON PLASTIC PACKAGES

IP.com Disclosure Number: IPCOM000008829D
Original Publication Date: 1998-Sep-01
Included in the Prior Art Database: 2002-Jul-17
Document File: 2 page(s) / 124K

Publishing Venue

Motorola

Related People

E. W. Mace: AUTHOR [+2]

Abstract

The shrinking of integrated circuit semiconductor devices has resulted in decreasing wire bond pitch and greater numbers of wire bonds. This general trend has made molding of plastic packages increasingly difficult and in some cases economically unfeasible. The prob- lem is typically one of wire shorting or movement which is unacceptable from an electrical or quality viewpoint. The end result is a poor assembly yield.

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MOTOROLA Technical Developments

TWO STEP ENCAPSULATION PROCESS FOR IMPROVING YIELD ON PLASTIC PACKAGES

by E. W. Mace and R. A. Munroe

   The shrinking of integrated circuit semiconductor devices has resulted in decreasing wire bond pitch and greater numbers of wire bonds. This general trend has made molding of plastic packages increasingly difficult and in some cases economically unfeasible. The prob- lem is typically one of wire shorting or movement which is unacceptable from an electrical or quality viewpoint. The end result is a poor assembly yield. BACKGROUND

  Plastic molding compounds of the thermoset and thermoplastic class have been used for several years to encapsulate semiconductor devices to form packages of many types. In integrated circuits the most common are dual inline plastic packages (DIP), and variations of peripheral leaded packages called flatpacks. The leads may exit the package on two sides or four sides and the latter is more common for devices with a lot of input and output (I/O) connections. The four sided devices are called quad flatpacks (QFP) or variation such as thin quad flatpacks (TQFP). In the last several years array packages have become increasingly popu- lar in use to handle the higher number of I/O and wiring density. Plastic ball grid arrays (PBGA) are an example of this type of package which typically con- sists of a miniature printed circuit with the device bonded to it, wire bonded for interconnection of the I/O and subsequently molded.

tant to the overall yield and cost. As semiconductor devices have become more integrated and reduced in size while the I/O count has risen the problem of mold- ing plastic packages has been made more difficult. In many high density cases the advance of molding com- pounds and processes have not kept pace.

  Although liquid encapsulants by themselves can be used alone without subsequent molding this is not sat- isfactory where planar surfaces are necessary and pre- cision dimensions are required. The liquid encapsu- lants tend to form curved surfaces and to wet the sur- faces in a pattern not as precise as a molded part. The two step process uses the low stress and flexible fea- ture of the liquid encapsulants with the precision prop- erties of the molded encapsulant% Another advantage is that most liquid encapsulants are not flame retardant by industry standards and the molding compounds gen- erally are flame retardant.

  An example of a method to immobilize wire bonds to reduce wire sweep and displacement in molding is described in U.S. Patent 5,331,205 for peripheral device...