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Reflow Soldering in SMT Assembly

7th July 2015

The majority of components used by the electronics industry today are provided as Surface Mount Technology, or SMT, assembly components. But other components such as connectors that need through-hole leads for mechanical strength still have to be soldered to a printed circuit board.

Wave soldering has been the standard large-scale industrial method for such electronic assembly. The name comes from the waves of solder required to glue the component to the circuit board.

An alternative soldering method is to apply the solder paste on the top side of the printed circuit board and around an annular ring. This forms a temporary attachment of the component to the circuit board. The assembly is then heated under controlled conditions to melt the solder and to form a permanent connection. This process is commonly known as reflow soldering or Pin-in-Paste (PiP) or Pin-in-Hole (PiH) soldering.

The PiP process lowers both SMT assembly production costs and the degradation of the circuit board because, in the latter case, the board is subject to fewer thermal impacts. In addition, the PiP process ensures that the solder melts uniformly and the board remains at an even temperature throughout its thickness.

In traditional wave soldering, the bottom of the circuit board acts as a heat sink and remains at a higher temperature than the top. This produces a temperature gradient where the solder migrates to the bottom of the board rather than filling in the joint at the top. Operators can never completely eliminate this temperature gradient, even with side heating. So the joint is always only partly filled.

Although the PiP process has been used in the SMT assembly ministry for over a decade, it still faces a number of challenges. The most important one is that standard components are designed for the wave soldering process and cannot withstand the higher reflow soldering temperatures.

The PiP process also requires an adequate component stand-off prior to assembly. The stand-off can be defined as the height of the component above the circuit board after reflow soldering.

The operator has to be able to calculate the stand-off prior to soldering to ensure that all of the components remain at the same height above the board after soldering. Sometimes this may involve the use of other heat sinks or thermal pads to counter the difference in final component heights.

For more information on Reflow Soldering in SMT Assembly then contact a team member at Trizo or visit their website.