High-Temperature solder on Flip Chips and Low-Temperature Solder Paste on Laminate Substrates

Flip-chip interconnection technology has been widely used in high-performance and consumer electronics. High-performance packaging has achieved steady development, realizing the interconnection with more than 10,000 I/Os and a pitch of less than 200um, realizing the transition from ceramic substrates to low-cost organic substrates, and achieving the popularization of lead-free solder pastes. The transition to organic substrates has prompted low-temperature solder pastes to replace high-temperature solder pastes.

As we all know, organic materials are usually epoxy resins, which are unstable at temperatures above 250°C for a long time. One way to avoid this problem is to deposit a low-temperature solder paste on the high melting point bump or the laminate pad. As shown in the figure below, the high melting point solder bumps are interconnected with the low-temperature solder pastes on the pads. This combination allows for the assembly of chips and laminate substrates at temperatures compatible with organic laminate substrates. To ensure that the solder joints can withstand the large strains caused by the thermal mismatch between the laminate substrate and the chip, underfill can be performed. When the chip, underfill, and laminated substrate are bonded, the deformation occurs at the same time, reducing the relative movement between the chip and the laminated substrate, thereby reducing the strain of the solder bumps. The package components interconnected by the high melting point solder bumps on the chip and the low melting point solder on the laminate substrate need to be cleaned to remove the flux residue and underfill after interconnection. 

Structural diagram & cross-sectional diagram of actual solder joints

If the UBM is composed of Cr/Cu and Cu/Cu, the Sn content in the solder matrix close to the UBM increases with the number of reflows when the double solder layer undergoes multiple reflows, which accelerates the reaction with the UBM and eventually leads to Sn-Cu IMC lift-off from the UBM substrate. To solve this problem, a more stable reaction barrier layer, such as Ni, can be used, or the thickness of the Cu layer can be increased. Besides, a low-temperature solder with targeted formulation optimization, such as Fitech’s FL-170 low-temperature solder, can be used.

After the flip chip is attached to the organic laminate substrate, flux residues should be thoroughly cleaned. The assembled module is underfilled and cured to overcome the thermal mismatch between the chip and the substrate. In this way, even large chips of 15mm can pass the thermal cycle test (-45~100℃, 1cycle/h).