Thermal Process
Bjorn Dahle
KIC - Embedded Designs Inc.

KIC
15950 Bernardo Center Dr. #E
Sand Diego, CA 92127
tel:858-673-6050
fax:858-763-0085

What is an ideal reflow profile for forward (Lead free paste and leaded component finish) and backward (Tin-Lead paste and lead free component finish) compatibility?

With some sectors exempted from the European Union’s RoHS directive, some products are having a mix of either Sn-Pb paste with lead free component finish (Backward) or lead free paste with leaded component finish (Forward). The issues related to such a mix arise because of the difference of 34°C between the melting point of Sn-Pb paste (183°C) and Pb-free paste (217°C). The compatibility issues worsen in case of area array packages.

Backward Compatibility: In backward compatibility, the major concern is soldering of lead-free bumps as the contribution of solder to the joint formation is more (70-80%) from the bumps. With a lesser peak temperature (210°C for Sn-Pb) and time above liquidus (TAL: 45-60 sec.), the bumps will not be able to melt completely resulting in an unacceptable joint. The bumps need to be exposed to high temperature for long enough time for solder to melt and for the mixing to occur. Reliability of the joint becomes an issue when the portions of solder do not melt completely. For proper melting and homogeneous microstructure distribution the peak temperature should be greater than 217°C. Experimental studies show that a peak temperature of 222°C and a TAL between 90-120 seconds result in a more uniform and reliable joint.

Forward Compatibility: The major issue with the use of Sn-Pb bumps with lead free paste is short stand off of bumps leading to poor compliance. Furthermore, assemblies in forward compatibility are more prone to voiding. The reason for the above-mentioned defects can be accounted to the Sn-Pb bumps (with a melting point of 183°C) being exposed to a much higher temperature for quite a long time resulting in drying out of flux. However, the peak temperature (235-245°C) and TAL (60-90sec.) for SAC are good enough for the homogenization of the joint.

What parameters should be controlled during reflow to increase the performance reliability of a lead free assembly?
Some of the important parameters to be controlled during reflow are:
1) ΔT across the components
2) ΔT across the entire assembly
3) Ramp up rates
4) Reflow cycle time
5) Time above liquidus
6) Peak Temperature
7) Cooling rates

The temperature difference across the entire assembly should be within a range of 5-20°C in order to prevent any board or component warpage. Slow ramp up rates in the order of 0.8-1.7°C are desired for proper flux activation to minimize bridging, skewing, opens, solder beading etc. Soak zone should be minimized as exposure to higher temperatures at a high ramp up rate would result in drying out of flux completely which may result in solder balling, poor wetting and voiding. Time above liquidus should be within a range of 60-90 sec. The grain size of the joint can be controlled with the cooling rate. Rapid cooling is desired to prevent the formation of undesired intermetallics and to get a finer grain structure. However, the cooling rate should not exceed 4°C/sec.