A lecture on Soldering
Ⅱ. Pb-free Soldering
5. Complete separation of Pb-free soldering from eutectic soldering (in line with RoHS Mandate)
It is necessary to be aware of the “RoHS Mandate” which is part of European WEEE MANDATE. Its principle is to ban the side-production of harmful stuff discharged to environment. It has named six such chemicals: Pb, Hg, Cd, Cr6+ PBB and PBDE. The so called “RoHS Mandate” which imposes very strict regulations upon trades concerned is meant to limit such by-production of chemicals to a certain acceptable scope. Its measurement is in ppm.
During the transition period from using eutectic solder to using Pb-free solder, the use of the both can still be seen. Even if the transition period completely ends, eutectic solder may also be seen used occasionally. If both Pb-free soldering and eutectic soldering operations share one soldering iron, Pb may find its way to Pb-free soldering. It is recommended that not only the irons, but also the working benches, the iron stands, the head thermometers and even the clamps and tweezers should be separated completely for different soldering purposes. Although the Pb amount getting across may be trivial, it’s harmful.
If Pb has been detected from the products exported to Europe subject to “RoHS Mandate”, then the export business is at the risk of being suspended and products recalled. In order to make the products and operation fully to the standards, all associates with the enterprise should make joint efforts and attach great importance to identification marks on tools and materials as well as employee education.
It also needs to be cautious not to blend the products because some PCBs and electronic parts are not subject to “RoHS Mandate” and some eutectic solder may be allowed for binding electrical wires.
Firstly, the separation of Pb-free soldering from eutectic soldering should be done not only for implementing the “RoHS Mandate”, but also for the sake of reliability.
When conductive wires and parts on a PCB to be worked are undertaken dip soldering (in dip soldering trough), the phenomenon of “soldering tin peel-off” may still occur even if a tin soldering alloy layer has fully been formed. The “soldering peel-off” will not occur unless all the following three scenarios are seen at the same time:
- Ⅰ Eutectic solder containing Pb on the electric wires
- Ⅱ The temperature at the joint part of electronic parts and electric wires installed on a PCB is above 175℃ while in dip soldering.
- Ⅲ Big electronic parts on a PCB
The reasons for above mentioned problem are complex. However, they still can be summarized as follows:
- ① In the case of dip soldering for electronic parts onto a PCB, the Pb on the soldering surface will be dissolved and tends to gather at the joint between conductive wires and electronic parts. (It is called “segregation”)
- ② Then the Pb contributes to form alloy with Pb-free solder (Sn-Ag-Cu). The fusing point of the three-element alloy is 178℃
- ③ In the case of dip soldering, part of the alloy layer will be melted down when the temperature exceeds 175℃.
- ④ The thermal expansion will cause PCB deformation which in turn produces stress, under which the fused segregated part will peel off.
Usually the iron head which has been used for eutectic solder will be attached with Pb, though in trivial quantity. If such iron head is directly used in pb-free soldering, the attached pb will be fused into Pb-free soldering tin, which may cause some consequence. If the tin solder contains no Pb, there will be no worry about the consequence like that. In a word, the iron head having been used with eutectic solder should not be used for Pb-free soldering.
Secondly, if any parts with eutectic (Sn-Pb) solder on electric wires undergo dip soldering on a through-PCB, the fillet “peel-off” may occur.
It is the same scenario with the above-mentioned Sn-Ag-Pb 3-element alloy joint between electronic parts and the PCB. Because the fusing point at the joint is only 178℃, the fused solder will remain liquid for quite a while and finally peel off due to the thermal expansion stress produced by PCB.
It happens only to through-PCBs. There is no such a problem with single-side PCBs.
It can be seen from the above statements that one iron head should be used exclusively as per the solder material for the sake of product reliability. If not for the above scenarios, one iron head can be used with different solders without causing quality problems.
However, with regard to the production site, it is suggested to make a thorough seperation between eutectic (Sn-Pb) soldering and pb-free soldering, for the purpose to comply with RoHS Mandate and avoid confusion.