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76 DESIGN007 MAGAZINE I JANUARY 2020 PCB and components under the resin leading to failure. These contaminants can be chemicals, moisture, and/or corrosive atmospheres. In the case of silicone resins, contaminants can pre- vent the resin from curing properly, as the cata- lysts used in silicone resins are very sensitive. 2. Why do some applications involve a no-clean process? Is this common within production cycles? The no-clean process is very common for medium- to high-volume PCB manufacturing, as it takes out the cleaning stages and hence saves time and money. However, it should be noted that no-clean solder and solder pastes usually mean that there is a low amount of flux in the formulation that is left behind. Paradoxi- cally, the flux residues left behind are harder to remove than the traditional systems and ad- here well to the PCB substrate. 3. What are the potential causes and consequences of a lack of adhesion between the resin/PCB/outer casing? Encapsulation resins are designed to have excellent adhesion to a range of substrates varying from FR-4 laminate, copper and tinned tracks, the plastics used for components, and the wide range of plastics and metals used for the enclosures/housings. The biggest cause of poor adhesion between a resin and the sub- strate is dust and/or grease. To gain optimal adhesion, the surface of the substrate should be cleaned to remove any loose debris and to degrease the surface. In the case of plastic housings, if they are extrud- ed mouldings, the plastic might have a layer of mould release agent on the surface which needs to be removed. If the surface is high- ly polished, then light abrasion of the surface is often sufficient to improve the adhesion. There are certain plastics that are very hard to get resins to adhere to, including polypropyl- ene, in particular. It is possible to get surface- treated materials or use a silane or plasma pre- treatment, which can help in these cases. 4. How important is the mixing and dispensing process in the performance of an encapsulation resin? With all two-component systems, mixing is absolutely critical to ensure that the resultant resin performs as described in the TDS. The aim of mixing is to obtain a homogeneous mix with parts A and B distributed uniformly. In- complete or poor mixing will result at best in a poor performing resin, and, at worst, an un- cured mixture. Selecting the correct sized stat- ic mixer for the machine and resin will enable the resin to be mixed consistently with each shot of resin dispensed. Next, the mixed resin has to be placed in the right location to allow it to flow across the PCB, displacing air from any voids and to coat the components to the required thickness. Air trapped in the resin is another source of poten- tial weakness within the resin as this can lead to a physical weakness due to thermal cycling, physical shock, chemical ingress, or in the case of high voltages, act as a concentration point where corona can build up. 5. What are your top tips for the manual mixing process of a resin pack? Firstly, select the right size resin pack for the job. If a lot of small volume units are to be potted, it might be easier to use a couple of small resin packs than a single large one to allow more time to dispense the material accurately, and the useable life/pot life of a small volume of resin is usually longer than with a large volume. In the case of polyurethane and silicone res- ins, make sure that the foil pack is sealed be- fore use. Do not open the outer packs until ready to mix and dispense. Remove the resin pack from the outer packaging and remove the middle clip separating the two halves of the pack. Use the clip to push the resin from one half of the pack into the other. Take the resin pack between two hands and mix vigorously for a couple of minutes. Lay the pack on a flat surface and, using the clip, push the material into the centre of the

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