The Challenges of High-Temperature Lead-Free Solder Paste for Power Discrete Applications

High-lead (Hi-Pb) solders (Pb>85wt%) remain exempt in die-attach and clip-bond usage for power discrete semiconductor applications until 2024. However, it is possible the exemption will be extended, since the current technology supporting lead-free materials is not yet sufficient to replace the commonly used Hi-Pb options. To address the need for a Hi-Pb replacement, tin-antimony- (SnSb)- based solder pastes have been developed with Durafuse
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technology, combining the merits of the two constituent powders. The SnSb-based solder pastes contain the controlled quantities of the Sn-rich phase due to the Sn-rich constituent powder, and exhibit partial melting starting at 228°C. However, the mechanical strength of the reflowed joints around temperatures of 260°C or higher, inherited from the Sb-bearing powder, was still comparable to or stronger than high-lead solders. The successful attempt of replacing Hi-Pb solder pastes with SnSb-based solder pastes in a diode assembly (Si die 2.5mmx1.6mmx0.1mm) validates their potential as one of the drop-in high-temperature lead-free (HTLF) solutions for power discrete applications. The current article will use a TO-leadless (TOLL) Power metal-oxide-semiconductor field-effect transistor (MOSFET) (Si die size: 6.8mmx4.7mmx0.1mm), built with HTLF-1 paste through the Hi-Pb process, demonstrating the drop-in processing compatibility. The TOLL MOSFET components passed the moisture sensitivity level 1 (MSL-1) rating. HTLF-1 solder pastes and Pb92.5/Sn5/Ag2.5 had comparable drain-source on resistance (RDS(on)) before and after 1,000 cycles in a thermal-cycling test (TCT) (-65/150°C). Although HTLF-1 has been verified as a potential alternative solder to replace Hi-Pb, challenges with the practical usability of lead-free solders persist. Sufficient solderable metallization layer thickness on the Si die surface is required for the lead-free solders to maintain the continuity of the interfacial intermetallic compound (IMC) formation. Lead-free solders are also stiffer than their Hi-Pb counterparts. While this increase in stiffness improves the joint fatigue resistance, it also increases the risk of die cracking and interfacial delamination. Thin Si dice (around 100μm and below) with the Ni or Ni/V solderable layer thickness above 350nm would favor the usage of HTLF-1 solder as a viable alternative, lead-free option to Hi-Pb. Additionally, there is still ongoing optimization of flux chemistry relative to the SnSb-base solders for high-temperature reflow.