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The drive towards fine-pitch technology also affects the soldering processes. Selective soldering is a reliable soldering process for THT (through-hole technology) 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, PCBs to metal shields or housing out of plastic or aluminum and are today’s state of the art.
The materials used to make solder connections require higher temperatures. Due to the introduction of lead-free alloys, boards need more heat to get the barrels filled with solder, which 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, and 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 goes 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. Additionally, 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.
Introduction
First, the impact of temperature will be discussed for the separate process steps and for machine tooling. In the experimental part measurements are done to verify the accuracy that can be achieved using today’s selective soldering machines. Dedicated tooling is designed to achieve special requirements with respect to component position accuracy.
Flux Unit
A selective soldering process has three process steps: fluxing, preheating and soldering. During the first step, the fluxing of the solder side of the assembly, the board and machine parts have an ambient temperature. A high-frequency drop-jet device is able to apply very fine droplets. The device is mounted on a robot that is moving in an x, y direction to shoot fine droplets on those spots where the printed circuit will meet the liquid solder. The position of the flux is critical. Not only should the flux be on the soldering area to clean the board and support the wetting, but there should not be flux anywhere else on the board.
It can be critical if non-activated flux is mixed with solder paste flux residues close to the soldering spot. Any non-activated flux on the assembly may cause electro-migration in the field when it is exposed to humidity and a bias. Flux that is not applied correctly may affect reliability during the lifetime of the product.
Editor's Note: This article originally appeared in the April issue of SMT Magazine.
