Glueing things together is not messy and uncontrolled anymore!

  • Polyoxymethylene (POM) to PCB solder mask
  • POM — Polypropylene (PP)

Problem 1: The Surface Tension

The main task to accomplish in glueing is to achieve is a chemical reaction between the top surface layer of one material and the top surface layer of the other. Glue in between serves as a binder. It is a chemical substance that has capabilities of forming a good adhesive bond to both materials. What one wants to achieve is the adhesive bond between the two materials being stronger than the cohesive bond, which is the one holding molecules of glue together. In essence, when breaking the bond by pulling the two glued pieces apart, we want to see a small patch of glue being left over on both pieces. This means the cohesive bond within the glue had broken sooner than the adhesive bonds between the glue and the material. This is the best we can do.

  • The PCB solder mask has a surface tension ~50 mN/m, which is higher than the boundary. This means no plasma treatment of PCBs is required!
  • The POM has surface tension ~40 mN/m. In theory, this should work but since it’s very close to the boundary value we don’t want to risk it!
  • the PP has a surface tension ~30 mN/m, which definitely needs to be treated.

Problem 2: The Squeeze-out and misalignment

Once we’ve figured out how to make the materials compatible with the adhesive, we need to ensure an accurate positioning of the two glued pieces. We also need to prevent extensive squeeze-out so we can make sure we have a thick enough layer of glue in between the two glued pieces. This is important for more than one reason:

  • first, to mechanically compensate for the difference in expansion of the two materials when the environment temperature is changed
  • to ensure an even distribution of adhesive around the glueing surface so we can have it evenly sealed

The Solution

Both of the problems mentioned above have to be handled in the design in order to avoid having to manually correct each and every manufactured piece. Let us illustrate a solution on an example of glueing a PCB onto an end of a cylinder milled out of POM.

Automation of glueing operation

The steps of the process were now defined:

  1. pre-process the surface with plasma
  2. dispense glue
  3. put the PCB in place
  4. squeeze together for alignment and let cure
  1. Turn on plasma probe
  2. make a slow small circle across the glueing surface on POM cylinder
  3. switch plasma probe off
  4. apply tool offset (which is the displacement vector between the plasma probe and the syringe tip)
  5. Begin silicone extrusion and follow the pre-defined pattern (which was developed and tested on its own beforehand)

Conclusion

Once we’ve successfully dispensed adhesive on a single sample we were able to synchronise that with the grid stencil dimensions and automate the process to manufacture approx. 120 samples/hour, repeatably and robustly. Because we’ve placed SMD components onto the PCB as spacers to ensure parallelism and perfect alignment, the PCBs could now be manually placed with ease on all samples straight after they came off of the CNC.

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Institute IRNAS

Institute IRNAS

We are applying today’s knowledge to create systems for an open future.