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Solder technique boosts PCB productivity

03 June 2010

Surface mount technology has greatly improved PCB production speeds and component densities, but it does have its limitations. In particular the method is not suitable for the mounting of mechanically stressed components like PCB connectors, and manufacturers have to resort to traditional through hole techniques for these components. Now, Wieland has come up with a hybrid technique called ‘Through Hole Reflow’ that solves this problem at a stroke

Early printed circuit board (PCB) manufacture involved the 'through hole technology' (THT) process, the component connecting wires being inserted through holes pre-drilled into the PCB. The various components to be inserted - capacitors, transistors, resistors, integrated circuits - all required different preparatory procedures during which the connection wires had to be bent and cut to size to fit the holes - a labour intensive task. Once the PCB was configured, the components were subsequently wave and/or selectively soldered on the underside of the PCB.

When surface mount technology (SMT) came along, the connection wires were no longer inserted through holes in the PCB, but instead, placed on soldering pads on its surface. For the first time it was possible to populate both sides of the PCB. SMT heralded the reflow soldering process, which begins with the application of soldering paste using screen printing or stencilling followed by component population using automatic pick-and-place machines. The PCBs are then passed through an oven where the board is heated to within a specified temperature range. The solder is heated to
above its melting point and cooled off, securing the components to the PCB.

Mechanically stressed components, such as connectors, require a good connection to the PCB and the integrity of the joint is not easily achieved using SMT. As a result, these components are often mounted using the through hole technique. But in order to maintain the efficiencies of production achieved by the SMT process, it is desirable to combine both the THT and SMT techniques in one process. Now, thanks to work conducted by Wieland, this is possible using a technique called Through Hole Reflow (THR).

During THR, components are secured, plated through and reflow soldered - a process also also known as ‘Pin-in-Paste’ (PIP). Holes are drilled as in THT and the PCBs passed through the normal reflow process. Firstly, solder is applied to the solder pads and the holes filled with solder, then, the PCB connectors are automatically placed. The pins press the solder through the hole, forming drops on the tip of the pin.

During the subsequent reflow soldering operation, the solder melts and is drawn into the hole by capillary action, providing a robust joint for the connector pins. The whole process is conducted in one step, with the same equipment and under the same conditions.

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