Optimization of flip-chip laser soldering for low temperature stability substrate

Interconnect technology is becoming increasingly more complex due to miniaturization of surface mount devices. These trends in electronics industry have led to the demand for new, highly controllable selective soldering technologies. Laser soldering methods can be an adequate answer for the mentioned demands. Optimization of laser soldering process is extremely important especially when there are more than 100degC difference between the temperature limit of the substrate and the melting point of the solder eg. PMMA substrate (T_g~105degC) and SnAgCu solder (MP=217degC). Temperature distribution of the laser soldered structure can be simulated by our model which also considers reflection, absorption and transmission as well as the Gaussian energy distribution of the beam, beyond the thermal properties of the sample. Simulations and experiments were carried out at frequency trippled Nd:YAG laser wavelength (355 nm) by direct heating of the flip-chip. Soldering process parameters (pulse energy, average power, soldering time, beam intensity) were optimized based on both the simulation and experimental results. The solder joints were qualified by resistance measurements, X-ray micrographs, micro-sections and shear tests.