Introduction
The dissolution of metallic parts in the solder is related to the tin content and to the solder temperature. If both are high, more intermetallic will be formed. These intermetallic that collect in the solderpot near edges and the solder pump area must be removed regularly, so that they will not affect the solder quality. The formation can not be avoided and will take place in every soldering system. Especially in lead-free solder systems iron containing parts can partly dissolve in the solder and create tin-iron crystals. Due to the relative small solder consumption in selective soldering, the saturation level of copper or iron in the solder will be rapidly reached. Over this saturation level these metals will form unsoluble crystals in the solder. In common wave soldering the refresh rate of the solder is often so large that these saturation levels are seldom reached. That is why these crystals are normally not found in wave solder pots when tin-lead solders are used. This is also due to the fact that the lead in the solder prevents the direct dissolution of iron parts. The needle shaped crystals can give short circuits between joints, which can be difficult to find due to their small dimensions. There are methods to remove these crystals from the solder bath that will be described.
Methods
1. Scooping with a ladle
At low temperatures, just above the melting point of the solder, the intermetallic iron-tin contamination in the solder bath becomes sluggish. By scooping out this sluggish solder with a perforated ladle these intermetallic can be removed. Adjust the bath level by topping up with new solder. Note: Often sluggish lumps with a high concentration of these intermetallic can be found in the 'corners' of the solder bath near the pump.
2. Sieving method
The iron-tin crystals can to a great extent be removed by a sieving operation. One has to form a basket out of stainless steel gauze with a grid of 20 MESH. The rims of this basket must be reinforced with some steel strips, so that the basket keeps its shape. Also a handle must be fixed to the basket, so that the basket can be held against the front of the solder nozzle in the solder machine. When the solder cools down iron-tin crystals will precipitate out from the liquid solder. By pouring the solder through the sieve at a temperature of 240 -230°C, the solid particles will remain in the sieve, while the liquid solder falls back, through the sieve, into the solderpot.
How to do it
1. First clean the solder bath by removing residues and dross.
2. Let the solder bath cool down to 240°C.
3. Bridge the thermal safety switch, which will normally protect the solder pump from running at too low temperature.
4. Place the sieve basket against the front of the nozzle and start the solder pump.
5. Let the solder flow back to the solderpot via the sieve. If the sieve clogs up with intermetallics remove these and proceed.
6. Stop, if the bath temperature comes under 230°C.
7. Top up the solder bath to the desired level with solder and remove the bridge wire from the thermal safety switch.
By repeating the sieving operation at regular intervals e.g. once in two/three months iron-tin crystals bridge formation can be prevented.