Warpage and Wire Sweep Analysis of Qfn Molded Array Strip Using Modeling and Experimental Methods

In this paper, both experimental and modeling works were resorted to analyze the warpage and wire sweep of QFN molded strip. The effect of QFN package size was investigated to provide the relation between warpage and metal to mold compound ratio. Design guideline for optimum metal to mold compound ratio has been obtained. Nonlinear large deformation of finite element analysis has been performed to investigate the effect of die size and mold compound material properties on the warpage and stress induced. Thermal loading was applied to simulate the cooling process after molding stage. For wire sweep analysis, full factorial design is performed by using three factors, i.e. transfer time, transfer force, and two types of mold compound. The detailed result was shown that the new mold compound induced lower die stress but slightly higher wire sweep than current material. INTRODUCTION The Quad Flat No-lead (QFN) technology produces a near chip size leadframe based molded package in a land grid array format. It is known as a small package, good production yield and high in thermal and electrical performance. However, due to large matrix of molded strip of these packages as illustrated in Fig. 1, the excessive warpage and over stress by thermal mismatch of different materials of package occur during manufacturing process. The package warpage is primarily from internal stress of the package that cooled from the molding temperature to the room temperature. A large coefficient of thermal expansion (CTE) mismatch is associated will result in strength with the of interfacial shear and peeling stresses occur at near the free edges and corners, will act as stress concentration points. When the local stress level in the packages of these sites exceeds the bond strength of the interface, an interfacial crack will propagate [1]. Therefore, it is importance practically to be modeled stated of stress model through analytical equipment, so susceptibility to mechanical failure which predicted without trial and error of expensive test. There are several factors that affect warpage, but underlying package geometries and molding compound properties are top among the causes. While a set of molding compound properties may produce very little warpage in one array package, they Solid State and Technology, Vol. 16, No 2 (2008) 153-163 ISSN 0128-7389 Corresponding Author: [email protected] may generate completely unacceptable warpage in another due to the variations in package geometries. In order to ensure the minimum warpage in these types of array packages, regardless of varying geometries, molding compound properties have to be adjusted and customized for each package. Because of the package geometry is generally predefined in the design process and rather cumbersome and costly to alter; modification of the materials properties of the package components is the only logical and cost-effective means of adjusting package warpage. Figure 1: Crossbow warpage on QFN molded array strip. EXPERIMENTAL METHOD In this paper, both experimental and modeling were used for analyzing warpage and wire sweep on QFN molded strip. For experimental work, QFN package size 3 mm x 3 mm, 4 mm x 4 mm, 5 mm x 4 mm, 5 mm x 5 mm, and 7 mm x 7 mm in units sizes were used. This provides the relation between warpage and metal to mold compound ratio. The performance of the new epoxy molding compound (EMC) with low stress properties is compared against current production EMC in the wire sweep study. The materials properties of these two EMC are shown compared in Table 1. The new EMC has lower coefficient of thermal expansion (CTE). Table 1: Comparison of properties of two EMC. Property Unit Old EMC New EMC Spiral flow cm 115 120 Gel Time @ 175C sec 35 40 CTE 1 e-6/C 8 7 CTE 2 e-6/C 39 34 Tg deg.C 140 135 Flex.modulus @ RT N/mm2 2800