A lecture on Soldering
Ⅱ. Pb-free Soldering
7. Improving the performance
7.1 Thermal capacity study
The factors contributing to difficulty in Pb-free soldering operation are as follows:
① High fusing point temperature
② Less good in fluidity (wettability)
③ The fused solder turning into a small ball instead of a Mount Fuji-shape fillet
④ hard to mend
Why is the case like that? It may have a lot to do with the Pb-free solder material characteristics, but the main reason lies in the thermal capacity difference between the fusing points of Pb-free solder and eutectic solder.
When a soldering iron is employed for soldering operation, the heat with the iron head passes to the mother metal and wire solder through heat conducting, thus the temperature of iron head must be higher than the fusing points of mother metal and wire solder. At the moment the iron head contacts the mother metal, the temperature with the head goes down drastically as the heat has been absorbed by the mother metal. Here is the formula for calculating the thermal capacity difference:
Hcd=(Vmm +Vws +Vih) × dfp
“Hcd” means “thermal capacity difference”;
“V “means volume;
“V mm” means “mother metal volume”;
“V ws” means “wire solder volume”;
“V ih” means “iron head volume”;
“dfp” means “difference of fusing points”. Here “dfp” is 34℃ (217℃-183℃)
As we can learn from the above formula, if the temperature of iron head is raised by 34 (difference of fusing points), the action only expands the thermal capacity of iron head while the thermal capacities of mother materials and soldering wire remain insufficient. So that is why the soldering tin can not be fused easily even if the temperature of iron head has been raised.
If the temperature with an iron head is raised casually, the oxidization at the head will be quickened, with the oxidization film covering the head in seconds. An iron head clad with oxidization film will not only lead to cold soldering, but also cause flux evaporated instantly, which simply adds insult to injury.
7.2 Wire solder and iron head
What should be done to improve our performance so as to conduct quality Pb-free soldering?
The key to solving this problem lies in making up the shortage of thermal capacity! As has been stated that raising temperature with the iron head may give rise to difficulty in soldering operation, this way should be ruled out.
Although the difference between the fusing points of eutectic solder and Pb-free solder is 34 theoretically，the proper temperature for the formation of Sn and Cu compound is about 250. So the temperature with the iron head is not supposed to be raised considerably.
One way out is to reduce the diameter of wire solder so as to reduce the thermal capacity, which proves to be very effective in soldering operation. The tin solder fusing can be surprisingly easy by this small change. In addition, the wire solder in smaller diameter easily facilitates regulating fused tin amount. Excessive fused tin amount contributes to the formation of a dome due to its stickiness and less good fluidity.
Another way out is to replace the iron head with a thicker one of same profile. The result is good due to the increased head volume that results in more heat stored.
Wires of φ0.8mm and φ0.3mm
The volume change with either an iron head or wire solder doesn’t look outstanding, but it works without increasing the temperature with the iron head. Should you still be troubled by this issue, have a try.
7.3 A pre-heater
It also works by using a pre-heater to make up the shortage of thermal capacity with an iron. A pre-heating-plate or a pre-heating-blower can be used to heat the mother metal (PCBs and electronic parts) to be worked on. The soldering job turns to be very easy because the shortage in heat has been made up. It is a tendency nowadays that electronic parts are getting smaller and smaller, a PCB is getting more and more crowded with parts and furthermore, the space between two terminals is getting narrower and narrower; PCBs have been developed into multi-board from single-board. In this case, a big iron is not able to work though it is needed. So a pre-heater may be a good solution.
In addition, using a pre-heater for soldering has another advantage: temperature must be restricted within 260 for soldering of some electronic parts that are not heat-resistant. In this case, a pre-heater can be used to preheat the whole PCB to the temperature of about 180, then even if the temperature with the iron head is merely below 260, it still works.
If the issue of thermal capacity can be solved properly, all other problems regarding soldering can also be handled smoothly.
7.4 Clamping soldering
Clamping soldering, which is different from “shadow soldering” regarding basic steps, is a very remarkable technique after Pb-free soldering was invented.
☆ Solder flux can be 100% used.
☆ No heat waste at the iron head
☆ Little damage caused due to overheat to electronic parts
The clamping soldering steps are as follows:
① Make solder wire contact the mother metal first.
② Soldering is conducted by making the solder wire clamped between the iron head and the mother metal.
This technique is extensively used in soldering electronic parts
onto PCB surface, but not universally applicable. Anyway, it may
Lay wire solder on the Cu pattern
be worth trying.