Hand Soldering Guidelines & Guidelines for Lead Free Hand Soldering Audit
Go to Hand Soldering Guidelines
Go to Lead Free Hand Soldering Audit
Hand Soldering Guidelines
Hand soldering remains an important element of electronics manufacturing. First-piece soldering, rework, and repair operations are performed using manual soldering irons. With the introduction of lead free solder alloys, operators performing hand soldering will have to become familiar with the differences between Tin Lead (SnPb) and lead free solders. The lead free soldering process requires more care and patience by the operator (Figure 1).
Figure 1. Example of Hand Soldering
Lead free solder alloys generally have higher melting points. Therefore, the solder tip must be set to a higher temperature. For example, when using the lead free solder alloy, tin silver copper (SAC-305), it was determined that the solder tip had to be set to 343°C / 650°F, as opposed to 315°C / 600°F for tin Lead (SnPb).
Due to the higher soldering temperatures, the iron must be kept clean and fully coated with the solder alloy; otherwise, oxidation of the soldering iron tip can occur. Lead free solders are more sensitive to the effects of a dirty soldering iron.
The soldering iron must be removed more quickly. Lead free solders and Tin Lead (SnPb) solders have different cooling rates and cooling characteristics. Icicles will be created if the soldering iron is removed too slowly. The size and frequency of solder icicles is dependent upon the alloy used and the soldering iron temperature setting. Quick removal of the solder iron also prevents disturbance of the solder joint and pads on the board. Lifted pads and fillets are more prominent with lead free solders.
Figure 2. Examples of Lead Free Hand Solder Joints. Note the icicle formed from an improper hand soldering operation.
To avoid contamination of solder joints, solder tips which are used with lead free alloys must be kept separate from those used with Tin Lead (SnPb). There is some evidence that lead free solders joints contaminated with Lead (Pb) are not as reliable as uncontaminated lead free solder joints.
To ensure adequate heat transfer, operators must select an appropriate solder tip and dwell time. A solder tip which is too small will not provide sufficient heat transfer. In addition, a longer dwell time (the time the soldering iron is in contact with the hardware) is required.
Increasing the solder tip temperature may damage the hardware if done indiscriminately.
Lead free solders do not wet as well as Tin Lead (SnPb) solders. To improve solderability, an operator may increase the solder tip temperature or use a more active solder flux.
If a more active flux is used, more aggressive cleaning processes will be required. Active fluxes leave residues on hardware, which will promote dendritic growth and poor adhesion of conformal coating.
It is recommended that Printed Wiring Assemblies (PWA) be preheated between 100oC to 120oC (Figure 3). These temperatures will be dependent upon the board material, board thickness, and configuration. Preheating the boards before soldering offers several advantages:
- Prevent board and component thermal shock
- Reduces the amount of thermal gradients in the board
- Prevent pad lifting by reducing the solder tip contact time
- Reduces soldering dwell time
- Improve wetting during the soldering process by having the board at an elevated temperature during the soldering process
- Reduces the effect of Coefficient of Thermal Expansion (CTE) mismatch between components and boards
There are 3 types of thermal preheating processes:
- Conduction – Used on ceramic and single sided boards
- Convection – Hot air heats the board
- Radiant / Infrared (IR) – The most expensive preheating process
Figure 3. Example of PWA being preheated.
The resulting lead free solder joints are dull and exhibit a grainy surface, as depicted in Figure. The J-STD-001 and IPC-A-610 standards allow for solder joints that have dull, matte, or grainy appearances, provided that such appearance is normal for the materials and processes involved. Operators will have to adjust their visual criteria for an acceptable solder joint to allow for these differences.
Lead Free Hand Soldering Audit
Record solder tip temperature. Determine if the solder tip temperature is sufficiently high enough to perform soldering operations.
Record what solder fluxes are used in the hand soldering process? Is it the same as with Tin Lead (SnPb)?
Record type of Lead Free solder used in the hand soldering process. Does the core solder contain flux?
Are Lead Free solder tips and Tin Lead (SnPb) solder tips separated to avoid Lead (Pb) contamination?
Is there a method to identify Lead Free solder tips and Tin Lead (SnPb) solder tips?
Are the operators trained to have their soldering iron tips clean and fully coated with Lead Free solders?
Have the operators been trained to know the differences in soldering and wetting between Tin Lead (SnPb) and Lead Free solders?
Is there a sufficient quantity of solder tip sizes available for operators to use when soldering with Lead Free solders?
With hardware representing the product line, perform hand soldering operations with Lead Free solders with an operator and document the results.
Are the boards and components baked prior to soldering operations to reduce the possibility of moisture damage?
Are hot plates available to the manufacturing shop and operators to perform hand soldering operations?
Have the manufacturing personal been trained to know the differences between Tin Lead (SnPb) and Lead Free solders, per IPC J-STD-001D and IPC-A-610D?
Has manufacturing documentation been updated to support Lead Free soldering?
Are the component finishes and board finishes identified?
Hand solder an assembly, using Lead Free solders. Inspect per IPC-A-610, Class 3 and document findings.
Document how the operators clean their hardware? With hand cleaning operations, can operators remove flux residues from Lead Free hand soldered operations?
