Abstract
A fluxless process of bonding silicon (Si) chips to iron (Fe) substrates was successfully developed. Iron substrates are plated with Ni, followed by tin (Sn). Si chips are deposited with thin chromium (Cr) and gold (Au) using E-beam evaporation in high vacuum. The Si chip is placed over the Fe substrate and bonded at 240 °C. The resulting joint is very uniform and contains few voids as examined by scanning electron microscopy. The reactions between Fe/Ni and Sn and between Si/Cr/Au and Sn during the soldering process are studied. The Si/Cr-Sn interface is free from intermetallic compound (IMC), i.e., IMC-free. Three-dimensional Ni3Sn4 microstructure is revealed by etching Sn away from the samples. The Fe/Ni–Sn interface forms rod-shaped or polygonal-shaped Ni3Sn4 microstructure. This new fluxless bonding process should be valuable in expanding the use of iron to electronic packaging applications such as substrates, base plates, thermal blocks, and chip-attachable casings.
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