Inductive sensors keep their cool on solder reflow oven

Printed circuit boards (PCBs) are assembled by placing electronic components onto a PCB along with a printed paste of solder. The PCB, with the components placed upon it, moves through a solder reflow oven, which heats the board to melt the solder paste so that the solder flows around and secures the components to the board and electrically connects them to its printed circuits.

To integrate a solder reflow oven into a new manufacturing cell, a customer wanted to reliably detect each PCB leaving the oven to trigger a transportation system. Because of the variety of colours, surface textures and differing degrees of reflectiveness of the different PCBs leaving the oven, a photoelectric sensor was unreliable.

The soldering reflow process secures components placed onto PCBs, by conveying an assembly of the PCB with a printed mask of solder paste and electronic components through a heated oven. A predetermined temperature profile heats the board up to 250degC to melt the solder, so it flows around the components, forming a strong metallurgical bond between the component leads and the PCB pads.

The ambient temperature immediately adjacent to the exit of the oven can reach 70degC, and so the customer had experienced failures or inconsistent results when using other manufacturer's sensors mounted at the exit of the reflow oven.

When considering using an inductive sensor, the challenge is that a populated PCB only contains very small amounts of metal, because the electrical tracks within the PCB and components are both narrow and thin. Additionally, it is desirable to place the inductive sensor as far above the PCB as possible both to allow taller components to pass beneath it and to reduce its exposure to the temperature of the hot PCB as it leaves the oven.

Contrinex 500 Series Extended Sensing Distance Inductive sensors were found to be ideal for this demanding application. Their Condist technology delivers long sensing distances and excellent temperature stability, plus they have a continuous operating temperature rating of 70degC. Trials confirmed their stable performance even in the elevated ambient temperatures around the reflow oven.

The sensor was mounted vertically on a steel bracket attached to the reflow oven's exterior, and each PCB's presence was detected as it passed beneath. The operating distance is set at 10mm, eliminating the risk of collision with components on the board, and an M12-threaded metal housing allows easy positional adjustment when necessary.

Integrating the sensor into the manufacturing cell was straightforward using the sensor's industry-standard NPN normally-open connection, and its flexible 2m long PVC sheathed cable. No additional electrical or mechanical protection is required despite the adverse operating conditions.

Previously, sensors from other manufacturers did not detect boards reliably and had unacceptably short service lives, frequently interrupting the manufacturing cell's production. Contrinex inductive sensors have been reliable and long-lived, providing a robust sensing solution with an excellent total cost of ownership.