Thin electroless Cu/OSP on electroless Ni as a novel surface finish for flip chip solder joints

The reliability of flip chip solder joint is significantly affected by the property of surface finish. Various surface finishes such as ENIG, OSP, DIG, immersion Sn, and ENEPIG have been developed and widely used in the electronic packaging industry. Each surface finish has own advantages and disadvantages in terms of reliability and processing cost. ENIG has good diffusion barrier of Ni against solder and good wettability due to Au finish. However, brittle failure known as "black pad" has been issued as a potential reliability problem. In order to eliminate "black pad" failure, alternative Cu-based surface finishes such as OSP, DIG, and immersion Sn were suggested. However, these surface finishes also have reliability issues of rapid consumption of Cu and rapid growth of IMC. In this study, a surface finish was suggested with a concept of good diffusion barrier, which is advantage of ENIG, and good reliability without "black pad", which is advantage of Cu-based finishes. This surface finish, ENEC/OSP, is consisted of thick electroless Ni (~ 6 mum), thin electroless Cu (~ 1 mum), and OSP layer (~ 0.3 mum). In order to prevent the "black pad" failure caused by formation of P-rich Ni layer, thin electroless Cu layer was used as a sacrificial layer. After fabricating ENIG, ENEPIG, and ENEC/OSP finishes, interfacial reactions with Sn3.0Ag0.5Cu solder were investigated using SEM, EDX, EPMA, and TEM. In ENIG and ENEPIG, P-rich layer formed by Ni-Sn interfacial reaction was observed after solder reflow. Both ENIG and ENEPIG have potential risk of the "black pad" failure due to excessive P-rich layer growth. However, in ENEC/OSP, formation of P-rich layer was restricted by thin electroless Cu layer on electroless Ni-P layer. The sacrificial electroless Cu layer was not completely consumed after 1times reflow. In addition, even after consumption of Cu layer in further reflow, thick Cu
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Sn
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IMC layer also prevented diffusion of Sn and Ni, and therefore, the growth of P-rich layer was extremely limited. Thin electroless Cu layer which can restrict growth of P-rich layer should enhance the long-term reliability of solder joint without "black pad". Because of rapid Cu-Sn reaction rate, Cu
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Sn
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IMC formed in ENEC/OSP finish had almost 3 times higher thickness than (Cu,Ni)
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Sn
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IMC formed in ENIG and ENEPIG. (Cu,Ni)
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Sn
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IMC observed in ENIG and ENEPIG was gradually growing with heat treatment, however, Cu
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Sn
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IMC in ENEC/OSP was rapidly growing during initial reflow, then the growth rate of the IMC significantly reduced after consumption of the Cu layer. It means IMC thickness can be controlled by thickness of electroless Cu layer