Five Things to Consider When Dispensing Form-In-Place EMI Gaskets
Automated form-in-place (FIP) dispensing of EMI shielding gaskets can be ideal for complicated patterns on electronics housings because automation allows for control over the size and shape of the bead. In addition to form-in-place EMI gaskets, thermally conductive gels can also be automatically dispensed, and often span oddly shaped gaps and conform to complex geometries.
The ability of dispensed thermally conductive gels to conform effectively makes them convenient solutions for reducing temperature and increasing the efficiency of electronics applications. Some automated systems can dispense both form-in-place EMI gaskets and thermally conductive gel on the same machine, allowing them to be dispensed simultaneously in the same program to easily integrate both materials into one housing.
Top five things to keep in mind when dispensing both form-in-place EMI gaskets and thermal interface materials on die cast aluminum heat sinks:
1. Be sure to use the correct tip diameter for the target bead dimensions and the particle size of the form-in-place material being dispensed. As a rule of thumb, the inner diameter of the tip should be six times the largest particle size of the material. A shorter tip with a larger tip diameter maximises material flow and produces less back pressure.
Tapered tips also produce less back pressure than straight walled stainless-steel tips, however they are more flexible which can cause variations in dispense paths.
2. The height of the form-in-place bead should be 85% of the width, and the desired compression is 20 – 30%. We recommend staying below 40% compression. If there are limits on the width of a bead but a taller bead is necessary, a double bead can be used to increase the ratio of height to width. Allow for higher tolerances in start and stop zones compared to straight runs and minimize the number of starts and stops in a bead profile.
3. To maximise thermal gel performance, choose a dispense pattern that will contact the entire target area on both the heat sink and component surfaces without air in between. Thermally conductive gel can be dispensed as a dot, a serpentine, a spiral, an X, or in various other shapes. The more simple the profile, the less likely that air will be introduced into the bead. A shot-size calibration process can help ensure dispense rates are consistent for a repeatable dispense volume.
4. Choose the proper valve type for your material and application. The viscosity of the material, the amount of material to be dispensed, and the abrasive nature of the material are the most important variables to consider when choosing a valve. Pneumatic valves with time-pressure dispensing systems are popular in the industry, where an air pulse pushes a piston that allows material to flow out of a disposable dispensing needle.
Auger valves displace precise amounts of material from cavities in a dispensing chamber with an auger screw, however filled materials are often too abrasive to achieve successful dispensing since the screw grinds down the particles. Positive-displacement valves are a good option for abrasive materials, where a motor-driven piston forces material out of a disposable barrel, because the system is not affected by external factors such as temperature and humidity.
Spool valves have very tight shot size control and a long life span when constructed with tungsten carbide, making them ideal for repeatable, consistent shots.
5. Keep in mind z-height obstructions with both form-in-place and thermally conductive gel materials. Tall walls next to the dispense path require a longer dispense tip, and the length of the dispense tip has an impact on the dispense speed. The slower the dispense speed, the longer the cycle time.
It is important to be familiar with key properties of the dispensing material to ensure the correct tip type, valve type, and bead path can be determined. These factors all have a direct impact on the quality of the bead produced. Making the correct decisions before dispensing will help maximize dispensing efficiency and minimise bead variations.
