Elimination Of Cu Layer Beyond Phasing Region Module Structure
Original Publication Date: 1979-Dec-01
Included in the Prior Art Database: 2005-Feb-20
A typical module structure contains a Cu layer about 1 micron thick adjacent a Cr layer. During a first reflow for soldering, about 80% of the Cu reacts with In or Sn (depending on whether the solder is Pb-In or Pb-Sn) to form Cu In and Cu Sn intermetallics. After two or three reflows, residual free Cu is reduced to few percent, forming a discontinuous layer mostly attributable to the Cu phased with Cr. The mechanical stability of the joint is mostly dependent on the phasing integrity and thickness rather than on the amount of Cu deposited. Reduction of the Cu layer to less than 0.1 micron is beneficial in terms of cost, mechanical stability, and corrosion resistance.