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have-to choice making more challenges in terms of weak bond Many discussion and activity were focusing on the 1st and heel crack concern. bond (ball bond) on fine pitch for IC wire bonding in the past The case is as shown in the Fig. 1. and Fig. 2., where the few years. However, the industry has been getting more and units are formed by 4x4 matrix in one block (index). The units more challenges on the 2nd bond (stitch bond) to seek the in the Row#1 were found with heel crack while the ones in the robust interconnect solutions. The weak bond and heel crack other three rows were without. are two major issues in terms of the manufacturability and Row #1 reliability especially on the metal leadframe with tape __ underneath, i.e., copper leadframe with NiPdAu plating for I_l_l_I_I_I_I_I_I_I_l_I_I packaging of MAP QFN, Power QFN, MLP, etc. TheI contamination from the reflow process of solder die attach material will make the bondability of 2nd bond much worse. For automotive applications, the crescent quality of 2nd bond is _ very critical to pass the stringent reliability requirements. Fig. 1. Block Patternfor 4 x 4 PQFN Leadframe Some paper [1] discussed the heel crack at stitch bond during molding process. Some paper [2] discussed the heel crack failure during temperature cycling test because of the low adhesion strength between the noble metal finish of the PPF leadframe and epoxy molding compound. This article will show that heel crack can be induced by the improper structure of frame because of the sympathetic vibration of the frame at local area when the ultrasonic power applied during bonding process at TO. It can significantly reduce the sympathetic vibration thus avoiding the over-bond of the crescent (heel (a) Crescent shape on (b) Crescent shape crack) by modifying the frame structure at local area. Besides, Row#2,3,4 on Row#1 possible ways to solve the weak bond and heel crack and how to improve the crescent quality at TO were discussed including Fig. 2. Bond Shape Comparison bonding tool selection, parameter optimization (using security Leadframe Structure Comparison bump and tail scrub to reduce ultrasonic power) by a caseIt is strongly suspected that the sympathetic vibration has study on PQFN package (PPF leadframe using solder paste die caused such failure mode of heel crack. Then the leadframe is attach). By these efforts, the adhesion strength of the stitch modified as shown in Fig. 3. in an attempt to mitigate the bond on the PPF surface with poor bondability has been able vibration by changing local feature with the minimum cost. to be improved significantly without the risk of heel crack The tie bar connecting the Row#1 units to the strip edge is even after the 500 cycles of Temperature Cycling test (-65C to modified from full thickness feature to half thickness feature 150C). The failure mechanism of heel crack caused by (half-etched at bottom side). sympathetic vibration has been also tried to be explained by Two kinds of leadframe are tested by actual bonding with FEA modeling in this paper....

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