Degradation of Electroless Ni(P) Under-Bump Metallization in Sn3.5Ag and Sn37Pb Solders during High-Temperature Storage

Journal of Electronic Materials, Aug 2004 by Chen, W-M, McCloskey, P, Byrne, P, Cheasty, P, Et al

The interfacial reaction between electroless Ni(P) under-bump metallization (UBM) and solders is studied. A P-rich layer forms in the UBM along the solder side after reflow and thermal aging. Crack formation inside the P-rich layer can sometimes penetrate throughout the entire UBM layer structure. The Ni(P) UBM degradation occurs earlier in the Sn3.5Ag solder than in Sn37Pb because of its higher reflow temperature. Despite the formation of a P-rich layer and cracks inside the UBM, it still keeps its original function within the high-temperature storage period in this study. Explanations for the formation of the P-rich layer and cracks in the UBM are outlined along with explanations for the Ni(P) UBM degradation process.

Key words: Electronic packaging, reliability, lead free, electroless Ni(P), under-bump metallization (UBM), intermetallic compound (IMC)

INTRODUCTION

Electronic packaging provides the mechanical support and electrical connection to outer circuits for devices. Improvements in silicon processing and fabrication techniques have resulted in more efficient silicon devices with significantly lower RDs(on) values in the region of

Flip-chip packaging adopts direct interconnects from die to substrate through solder bumps, which differs from the conventional wire-bonding interconnection method. It offers small, light, high input/ output (I/O) pin counts, low profile packages, etc., thus, enabling high-density packaging and performance improvement of devices.

For power modules, high I/O counts are usually not required; however, wire-bonding interconnections cause power loss (which contributes approximately a half of the total resistance for the power devices) and circuit speed delays.3 Replacing wire bonds with solder bumps in power modules enables higher frequency performance, better noise control, and higher power dissipation. Thus, recently, metal-oxide semiconductor field-effect transistor (MOSFET) manufacturers have started to provide wire-bonding-free, chip-scale packaging solutions for low voltage (20-30 V) power devices in packages, such as the FlipFET(TM) and DirectFET(TM) from International Rectifier (El Segundo, CA) and the MOSFET ball grid array from Fairchild Semiconductor (South Portland, ME).

The top metal layer of most integrated-circuit bond pads is aluminum, which is suitable for conventional wire-bonding interconnection. Because of its tendency to form native oxides quickly, the aluminum bond pad is not solderable,2 so an under-bump metallization (UBM) transition layer is necessary for successful flip-chip and solder-bump interconnected power packaging.

Before UBM deposition, this native oxide should be removed through either sputtering or a chemical etching method.2,4,5 The UBM also acts as the barrier layer between the solder and the underlying aluminum pad protecting the fragile bond pads from fluxes and solder dissolution during reflowing, thus improving the interconnection reliability.

The UBM can be produced with a variety of techniques, such as high-temperature evaporation, electroplating, or electroless deposition. A wide range of materials and structures have been used as UBMs for flip-chip packaging. Some typical UBM structures are NiV/Cu,6 Cr/Cr-Cu/Cu,2,7 electroplated Ni,8 TiW/NiV,9 etc.

Recently, electroless nickel has been used as UBM for flip-chip packaging.4 In general, acidic hypophosphite baths were used for nickel deposition, and consequently, phosphorus is always incorporated in the electroless nickel deposition.10 By controlling the solution pH value and the deposition temperature, the phosphorus content in Ni(P) deposition can be adjusted.11 The crystallization rate and crystallinity of Ni(P) decreases with increasing phosphorus content.11 If the phosphorus content is high (>9.5 at.%12,13) in the nickel plating, electroless Ni(P) will be in the amorphous state; the fast grain-boundary diffusion is, therefore, suppressed.

Electroless Ni(P) bumping of Al bond pads followed by solder paste printing is one of the cheapest, high-volume solution routes for wafer bumping prior to flip-chip assembly.5

Usually intermetallic compounds (IMCs) will form between the UBM and the solder during the assembly process and device operation. If the solderable UBM layer is consumed entirely, lower adhesion or even delamination between the solder bump and substrate will be induced.

The Sn/Pb-based solders have been used in the electronics industry for a long time. However, because of the toxic nature of the Pb element, there is an increased environmental and legislative imperative to use more environmentally friendly, Pb-free alternatives. Despite the fact that no economically compatible replacements are available for Sn/Pb solders, there are many Pb-free solder candidates, such as Sn3.5Ag, Sn3.5AgO.7Cu, and SnO.TCu. Among them, the eutectic Sn3.5Ag appears to be a high strength solder with good wetting performance for most applications.14 It has been used by some industries, such as the automotive manufacturer, Ford (Dearborn, MI), and the telecommunication manufacturer, Siemens (Munich).

With the continuing trend of increased integration and the increased use of Pb-free solders in electronic packaging, it is becoming necessary to obtain an increased understanding of the reliability of lead-free solders with electroless Ni(P) UBM. Actually, some studies have already been done on the interfacial reaction between electroless Ni(P) UBM and liquid solders during prolonged reflow.13,15-19 In this study, the interfacial reaction between the eutectic Sn3.5Ag solder and the electroless Ni(P) UBM is studied after one standard reflow process and different periods of high-temperature storage at 150�C. For comparison, the interfacial reaction between eutectic Sn37Pb solder and electroless Ni(P) UBM is also studied after the same treatments.