A lecture on Soldering
Ⅱ. Pb-free Soldering
4. Advantages and disadvantages of Pb-free tin solder
You will not understand the essence of Pb-free solder without knowing its advantages and disadvantages.
◇ No Pb contained, good for environmental protection
◇ High mechanic strength (compared with eutectic tin solder)
◇ Low in electric resistance (compared with eutectic tin solder)
◇ Low in specific gravity (compared with eutectic tin solder)
◇ Its fusing point is relatively higher, so in soldering operation, electronic parts may be damaged if the operator is not cautious enough.
◇ Its wettability and fluidity are not ideal (compared with eutectic tin solder).
◇ Higher in material costs (compared with eutectic tin solder).
◇ Operation by using Pb-free tin solder may be more difficult (compared with eutectic tin solder).
◇Iron heads are worn out more quickly, so it gives rise to the operation cost.
All these above mentioned differences, both advantageous and disadvantageous, come up because of the absence of Pb. If the differences which have been repeatedly mentioned can be understood correctly, a lot of trouble can be saved in soldering operations.
4.1 Consumption of iron heads
Iron heads of using Pb-free tin solder are worn out 3 times as quickly as those of using eutectic tin solder. It is because of the copper-corrosion phenomenon. Furthermore, Pb-free solder can corrode not only iron and copper, but also stainless steel.
An iron head is made of highly conductive copper bar tinplated with several-hundred-um-thick cladding. During pb-free soldering operation, the tinplate cladding itself may be fused a little bit in soldering, and then, some copper molecules may also find their way into fused tin solder. As a result, an iron head may get slimmer and slimmer after long-time use. Sometimes, some breaches will be seen or some holes will be formed inside the head.
This kind of worn-out is very remarkable, especially when iron heads are subject to high temperatures. Sometimes an iron head lasts only a few hours. As was stated in the section “Difference between Pb-free solder and eutectic Sn-Pb solder”, the fusing point of Pb-free tin solder is higher than that of eutectic tin solder. It is the scenario: the size of mother metal remains unchanged, and an iron will be found under thermal-capacity if Pb-free soldering is employed. Then the only way out is to raise the temperature at the iron head. Oxidized residuals attached to the head builds up if it is always subject to a high temperature. And when the iron head is repeatedly cleaned with wet sponge, it will repeatedly undergo drastic temperature reduction, which speeds up its aging. It is one of the reasons contributing to iron head worn-out
During Pb-free soldering, not only an iron head, even a Cu pattern on a PCB will be fused if long time under a high temperature. So the time to fuse tin solder on a Cu pattern should be as short as possible, especially during repairing operation.
Even stainless steel can be corroded by Pb-free solder, as is shown in the photo on the right. The stainless steel trough has been seen to have holes because of corrosion. Recently, titanium is used as major substitute in Pb-free solder to be used for solder layer and bolts.