The drive towards fine pitch technology also affects the soldering processes. Selective soldering is a reliable soldering process for THT (through hole) connectors and offers a wide process window for designers. THT connectors can be soldered on the top and bottom side of boards, board in board, PCB’s to metal shields or housing out of plastic or aluminum are today’s state of the art.
The materials that are used to make the solder connections require higher temperatures. Due to the introduction of lead-free alloys, the boards need more heat to get the barrels filled with solder. This not only affects the properties of the flux and components, but the operation temperatures of solder machines become higher. A nitrogen tunnel wave solder machine requires a temperature control in the tunnel to prevent overheating. Advanced systems are available that insert cold nitrogen. The closed tunnel wave soldering process has a wide process window and is not sensitive to small changes in environmental conditions. The same counts for wave solder machines that have nitrogen blanket systems over the wave. Improved preheaters will bring sufficient heat in the assembly and exhaust systems are adequate enough to maintain required process conditions. The nitrogen will improve the soldering and minimize dross amounts at these elevated solder temperatures.
Selective soldering is a different process. Compared to wave soldering there are additional process parameters that are affected by the higher temperatures. Solder joints have to be made close to SMD pads or components. An off-set of 0.5 mm may result in solder skips or re-melting SMD components. The higher temperature may cause warpage of the board, which also affects the position accuracy of the solder nozzle. All materials will expand at higher temperatures, but not all expansion coefficients of the materials used are equal. This not only introduces stress, but also may create off-sets.