The EMPF Helpline fields a number of questions regarding the compatibility of lead-fee components with an existing tin-lead process. A customer contacted the EMPF helpline in order to test the effect of a tin-lead (Sn-Pb) reflow profile on Pb-free BGAs. The EMPF staff analyzed the BGAs and evaluated their structural integrity. The BGA solder joints were composed of tin, silver, and copper (SAC) alloys and were soldered using SN 63 solder paste.

The boards were soldered with a normal tin-lead profile where the peak reflow temperatures reached 220ºC.

Transmission X-ray analysis was conducted in order to determine the void density of the BGA solder joints. Void densities were measured against the IPC-A-610 Rev D standard that states void densities should not exceed 25% of the total solder ball area. A high density of voids was observed, as shown in the pictures below. However, none of the void densities exceeded 25%, indicating the Pb-free reflow profile did not create voids that interfere with the structural integrity of the BGAs.

The main concern when soldering with lead-free solder balls using a tin-lead profile and tin-lead solder paste is the compatibility of the two solder alloys. To investigate the solder alloys, the BGA solder joints were cross-sectioned and examined using polarized light microscopy. Of significant importance was the final microstructure of the SAC BGAs after soldering. The SAC solder has a melting temperature of 218 ºC as opposed to the tin-lead eutectic melting temperature of 181ºC.

AC solder typically does not melt when reflowed with the lower tin-lead profile. Investigation of the microstucture showed that the solder joint experienced partial reflow. The tin-lead solder paste melted and formed with the SAC solder ball. This created two very distinct regions within the solder ball. The solder balls maintained the original SAC solder microstructure near the interface of the solder ball and the component pad.

However, a different microstructure was observed near the solder ball center and at the interface of the solder ball with the printed wiring board. There were clear indications of dendrites at the bottom half of the solder ball. These dendrites appear as dark regions in the optical image below. Much of the lead from the tin-lead paste is contained within the inter-dendritic regions. The non-mixing of the tin-lead paste and the SAC solder is a direct result of the low temperature reflow profile.

Non-mixing in the solder balls degr ade s the structural integrity of the balls and inc r e a s e s the probability of cracks and failures. Thermal cycling of solder balls containing similar microstructures showed that these balls are 5 times more likely to fail than thoroughly mixed or conventional leaded solder balls.

Array components containing Pb-free solder alloy should not be reflowed using a Sn-Pb reflow profile. ACI made recommendations to the customer to utilize a reflow profile optimized for lead-free solder paste when soldering lead-free BGAs with tin-lead paste. The leadfree profile has a peak reflow temperature of 240 ºC or greater. These higher temperatures promote better mixing of the tin-lead paste and the SAC solder. A completely mixed SAC-tin-lead solder joint, like the one pictured above, is considered reliable.