Formation of solder cap on Cu pillar bump using formic acid reduction

In this paper, we report a reflow process that uses formic acid to remove the native oxide of a solder on a Cu pillar instead of using flux. To study the effect of the solder on the Cu pillar, we compare the shape and the crystal structure of a wetted solder on a Cu pillar after a reflow process using formic acid and flux. To conduct the experiment, we also had to confirm the effect of the intermetallic compound layer between a solder and a Cu pillar. Therefore, we prepared the sample with a Ni layer between a solder and a C u pillar to prevent the formation of an intermetallic compound layer. Both samples with/without Ni layer were investigated simultaneously. The results showed that the solder which was fabricated by a reflow process using formic acid crystalized even at a peak temperature of 228°C, which is ciose to the solder melting temperature. There was no difference in the reduction abilities between formic acid an d flux in the wettability test o f the solder. However, it was found that the amount of formic acid are proportional to the reflow temperature. Atmosphere of formic acid is main factor of raising peak temperature of reflow.