Feature: PCBs


Contrinex’s Inductive Sensor’s keep


their cool on Solder Reflow Oven Mark Weymouth, managing director, PLUS Automation


P


rinted Circuit Boards (PCBs) are assembled by placing electronic components onto a PCB along with a printed paste of solder. Te 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, but could the traditional alternative of an inductive sensor work? Te high performance in terms of long


sensing distance and temperature stability of a Contrinex 500 Series inductive sensor mounted above the exit of the reflow oven were essential in providing reliable detection. Te long sensing distance of the


Contrinex 500 sensor enabled it to be mounted above the exit conveyor of the reflow oven.


Customer application Te soldering reflow process secures components placed onto PCBs, by


24 October 2024 www.electronicsworld.co.uk


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 250ºC to melt the solder, so it flows around the components, forming a strong metallurgical bond between the component leads and the PCB pads. Te ambient temperature immediately


adjacent to the exit of the oven can reach 70°C, 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 it’s exposure to the temperature of the hot PCB as it leaves the oven. An inductive sensor is immune to


changes in colour, texture or reflectivity, which would make a photoelectric sensor unreliable.


Customer solution Contrinex 500 Series Extended Sensing Distance Inductive sensors were found to be ideal for this demanding application. Teir Condist technology delivers best-in- class sensing distances and temperature


stability, plus they have a continuous operating temperature rating of 70°C. Trials confirmed their stable performance even in the elevated ambient temperatures around the reflow oven. Te 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. Te 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. With over 2,000 sensors in stock in the


UK, supported by stock held in Europe and short manufacturing lead times, PLUS Automation welcomes the opportunity to discuss improving automation and machine performance or reducing costs.


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