Am I in risk if I use tin-lead finished components in a lead-free process?
Yes, there is a potential concern on joint performance. I have seen cases of unintended reflow that have affected the solder joint formation. As an example, we had a case study where a tin-lead QFP was assembled using a lead-free reflow process (SAC alloy). During the wave soldering process, using SnAgCu, this component experienced an unintended second reflow. The consequence was a 50 percent reduction in the QFP lead joint strength as characterized by a pull test.
In Figure 1, a cross section of the QFP solder joint lead is illustrated. A gap was observed between the lead and the solder joint. Analysis of the cross section revealed Pb migration (Figure 2) from the component lead into the solder joint. As a consequence, the Pb formed with the Sn a low melting point alloy and the lead base material was left exposed.
Figure 1. Cross Section of a Solder Joint of QFP Lead
Figure 2. Mapping of the Solder Joint
Yellow is Fe; Pink is Sn; Dark Blue is Cu; Light Blue is lead
During the wave soldering process, the area surrounding the QFP (topside of the board) reached 200°C. At this temperature, Sn/Pb/Ag alloy remelted and resulted in the joint pulling further away from the lead, leaving the base material exposed. The iron base on the lead was unsolderable resulting in this gap and the poor pull force.
In a second investigation on the impact of lead contamination, DSC analysis was performed to quantify the effect of lead contamination. The DSC analysis was done on Sn/Bi (m.p. 138°C) solder paste with 30 percent lead contamination. The results (Figure 3) shows the formation of a lower melting point alloy (Sn/Pb/Bi) which melts at 97.5°C, a lower melting point of the original lead-free alloy (from 138 to 119C) and an increase of its pasty range (around 14°C).
Figure 3. DSC Analysis of Sn/Bi Paste with 30% Lead Contamination
Similar results were also observed with only 5 percent Pb contamination. As a conclusion, mixing tin-lead and lead-free alloy reduces solder joint performance as well as affecting the metallurgical behavior of the pure lead-free alloys.
Do cooling rates on a reflow process affect the microstructure of the solder joints?
Yes, faster cooling inhibits the formation of intermetallic and forms a homogenous microstructure. A four-month project took place to investigate the impact on different cooling rates. A DSC was used to simulate the reflow oven behavior; cooling rates of -0.5°C/sec and -2.5°C/sec, which are found on the production floor, were tested.
Figure 1 shows a SEM image of lead-free solder joints that was cooled at -0.5C/sec. The microstructure shows large Ag3Sn needles formation as well as Cu6Sn5 intermetallic on the interface between the PCB pad and solder joint.
Figure 1. SAC Alloy - Imm Sn Surface Finished Board, Cooling rate -0.5C/sec
Figure 2 shows the microstructure of a lead-free joint that was cooled at -2.5°C/sec. Cu6Sn5 was formed between the PCB pad and the solder joint. However, few cases of Ag3Sn needles were observed and if so, in smaller dimension. Also, a more uniform matrix (Sn/Ag/Cu) was observed.
Intermetallics are needed to form a robust solder joint. However, new intermetallics such as Ag3Sn are unknown. The impact of these large intermetallics was characterized by shear testing. Results showed that joints formed on CuOSP surface finished pads with faster cooling rates have slightly stronger joints that their counterparts. An increase of 8 percent on the maximum shear value was recorded.
It also has been observed that these large intermetallics might help cracks to propagate and in other cases they might stop them. This effect will depend of the orientation of these large plates. Due to the fact that their orientation is impossible to control, it is preferred to reduce their formation.
Figure 2. SAC Alloy - Imm Sn Surface Finished Board,
Cooling rate -2.5°C/sec
Beside the cooling rates, the formation of the intermetallics depends on the alloy composition and the amount of time that the alloy remains in its liquid state.
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