PCB Tech - General Principles of Head Soldering for PCBA Assembly

After a large number of PCBA automatic machine welding, when a small number of local bad solder joints, or high-temperature sensitive components, etc., will still be used to remedy the old-fashioned hand soldering process. In addition to tin soldering, hand soldering in a broad sense also includes silver soldering and welding. In the early days, the U.S. Navy was very particular about this kind of manual work. There were many standard operating procedures (SOPs) and rigorous systems for examinations, certifications, and licenses. His respect for practical craftsmanship is no less than his respect for theory and academics.

1. Hand welding with Soldering Gun

This is the most basic welding method, and its primary tool, the welding gun, is also commonly known as a soldering iron. The heating element and the tip of the soldering iron can provide enough heat for the soldering wire and the workpiece to enable high-temperature welding. Since the transformer of the welding torch will also produce electromagnetic waves that generate extraneous branches during the heating process, the welding torch must also have a good isolation (Isolation) function to avoid causing "Electric Overstress" (Electric Overstress; EOS) to the sensitive PCB components on the PCB surface.

Or "Electrostatic Discharge" (Electrostatic Discharge; ESD) and other injuries.

There are many items that should be paid attention to when selecting a welding gun, such as the shape of the soldering iron tip suitable for the type of processing, the sensitivity of temperature control (±5 degree Celsius), the rapidity of heat conduction, and the recovery temperature before operation in the standby temperature (Idle Temp.) Temp.) is fast enough, high enough, and stable enough. The convenience of operation and the ease of maintenance are all reference items.

2. Solder Wire

It is an alloy composed of various tin-lead weight ratios, and the solid flux core in the sandwich is additionally added, and the drawn metal wire-shaped solder can be welded and filled to become mechanically strong Solder Joint is called it. Pay attention to whether the flux is corrosive, and whether the insulation resistance of the residue after soldering (Insulation Resistance, which is an incorrect statement by ordinary people) is high enough to avoid poor electrical insulation of subsequent PCBA assembly boards. The problem. Even in the future, "No Clean" flux will be required. The evaluation method can be selected from the "Moisture and Insulation Resistance" section 2.6.3 of IPC-TM-650. Sometimes when the effectiveness of the flux in the welding wire is found to be insufficient, additional liquid flux can also be added to help its effect, but be careful of the subsequent ion contamination of such liquid flux.

3. Hand welding process and main points of the torch

(1) Clean the rust-free chromium iron tip and welding wire while contacting the position to be soldered, so that the molten tin can quickly adhere and fill. After that, the excess tin beads of the soldering iron tip must be broken, etc., using water wet Remove the sponge.

(2) Melt in an appropriate amount of tin wire solder and disperse it evenly, and not too much. Among them, the flux can provide the dual functions of cleaning and heat transfer.

(3) The tip of the soldering iron must be in continuous contact with the soldering position to provide sufficient heat until the solder has been evenly distributed.

(4) After completion, be careful when removing the welding torch to avoid disturbance of the solder joints before solidification caused by improper actions, which may damage the strength of the solder joints.

(5) When the PCB to be processed is assembled with single-sided parts, and the area to be soldered is large and large, the other panel without parts can be attached to some kind of hot plate for preheating; Speed up the work speed and reduce the damage of local board surface overheating. This kind of preheating can also be carried out with a special small hot air blower.

(6) The tip is a tool for heat transfer and tin filling. The area to be processed should have the largest contact area to reduce the time loss of heat transfer. In order to enhance the efficiency of conveying solder materials, it is necessary to maintain good solderability on the surface and not cause accumulation of various residues. Once the soldering iron tip is oxidized or excessively contaminated, it must be replaced.

(7) For manual welding of small PCB parts or thin legs, in order to avoid overheating damage, additional temporary heat dissipation accessories, such as metal alligator clips, can be added.