Go with the flow: connector assembly made simple

ESUK Jul13 p18 Hitaltech1Printed circuit boards have long been moving to surface mount production, which is why Hitaltech offers connectors and terminal blocks designed for surface mount through-hole assembly

More often than not, electronics components are mounted and soldered onto a printed circuit board (PCB) using automated systems, with assembly technologies moving from traditional through hole to surface mounting techniques. Traditional components, such as terminal blocks and connectors can be a challenge, as they are often incompatible with with robotic system requirements and high temperature soldering.

As a result, Hitaltech has been increasing its range of terminal blocks and connectors that are suitable for surface mount device (SMD) through-hole (STH) assembly techniques.

The STH range has been specifically developed to withstand the temperatures of reflow soldering. Plastic housings are manufactured using a UL94 V0 self extinguishing halogen free material, which can withstand temperatures up to 270°C.

With this technology, the entire PCB is exposed to a thermic cycle that peaks around 270°C. This enables, firstly the casting of the soldering paste, then its solidification, with a characteristic reflow that is visible on solder beads.

Unlike traditional SMD components, connectors and terminal block pins are inserted into metalised PCB holes, already filled with solder paste. The pin insertion offers high mechanical resistance and balances the stresses caused when screwing down the wires.

Tips and tricks

To use STH components, the PCB needs to meet certain requirements. Holes must be metallised and feature soldering eyelets on both sides of the board. The hole diameter depends on the pick and place machine, but the aim is to keep the hole diameter as small as possible, therefore minimising paste consumption.

When calculating the paste quantity, take into account the PCB thickness. Thicker PCBs require a larger quantity of paste and the optimum thickness is around 1.6mm.

Stencil hole thickness also makes a difference. A larger stencil hole thickness delivers a larger volume of paste, however stencils with different thickness and a double print process may be required. The first print uses a thin stencil, while the second print uses a thicker stencil, positioned over the STH areas. If a double print is not possible then paste can be dispensed.

When it comes to solder paste, a no-clean paste is recommended, as it eliminates residual soldering material and avoids a cleaning process involving chemical products. Print speed must be calculated accurately regarding paste type.

Clued up on components

Looking at the components themselves, pin section can make a difference to this process. A square pin shape is ideal and to improve soldering quality, the pin length should be designed to almost touch the PCB’s lower surface. This avoids paste being pushed out of the holes. Optimum pin length also allows component mounting on both sides of the PCB.

Hitaltech’s STH range can be used with all reflow soldering processes including infra-red (IR), hot air convection and nitrogen. The last two options are recommended, as the plastic material absorbs IR radiation and can potentially lead to localised plastic overheating and insufficient heat reaching the soldering. Hot-air convection ovens are free from possible screening effects of the plastic material, allowing uniform heat distribution. As the air temperature/speed determines the heat quantity, both high temperature/low speed and high speed/low temperature processes are options. Comparing the two, the latter is recommended, due to lower thermal load.

Finally, STH components are compatible with pick-and-place robots, with packaging options including tube, tape-on-reel and trays.