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66 DESIGN007 MAGAZINE I OCTOBER 2019 to be safe. Examine how dense the component layout is on the board and consider how well the resin will flow across the board. It is worth noting that lower viscosity resins flow better; however, a thixotropic resin might afford more control to obtain the desired coverage. Does the resin need to flow under components, and, conversely, should it not flow in certain areas? Also, consider whether the resin needs to be flame retardant; if so, to what approval level? The end-use environment is another key fac- tor not to be overlooked. Consider what envi- ronment the finished unit will be exposed to as this may be harsh, which will also affect that resin that is most suitable. In turn, this will de- fine whether the resin needs to provide prim- ary or secondary protection against it. You might also need to protect electrical or electronic components that are likely to come in contact with chemicals, including acids, al- kalis, solvents, and a whole raft of other sub- stances that pose a threat to delicate circuits and components. Chemical resistance is very much the province of epoxy resins, though some of the tougher polyurethane products as well as some silicone-based formulations will provide a degree of protection, particular- ly against moisture/water ingress. Epoxy res- in products are available to protect electrical/ electronic units that undergo frequent or per- manent immersion in solvents, such as diesel fuel, leaded and unleaded petrol, and cellulose thinners. 2. Can Resins Be Dug Out/Reworked If Necessary? The simple answer is yes; there are special- ised dig-outable resins available for use. These are primarily aimed at development and pro- totyping projects where easy access to com- ponents is required. Generally, they have poor chemical resistance and physical properties, particularly in terms of strength and tough- ness. However, in the case of more general res- ins, it is possible to remove cured resin from around components and areas on a PCB, but these require either the use of a solvent or cut- ting away the resin from the area of interest. The hole can then be refilled with some more freshly mixed resin, but this leads to a poten- tial weak spot in the resin coverage as chemi- cals and water could penetrate through the res- in interface. Similarly, thermal cycling/shock could cause the interface to weaken and be- come exposed. Both polyurethane and silicone resins are more easily removed for rework pur- poses, and special solvents are available to as- sist with this process. 3. What Are the Possible Implications of a Resin Being Unable to Maintain Flexibility at Low Temperatures? Normally, the main reason a resin to loses its flexibility is due to brittleness. This can be due to a number of factors, but at low temper- atures, the most likely reason is that the res- in approaches or passes through its glass tran- sition temperature (Tg). This is the temper- ature at which a resin goes from a brittle or glassy state to a rubbery state. The effect upon the components/PCB is that they will experi- ence increased levels of physical stress, which might lead to components being broken and/ or legs being snapped. In worst-case scenarios, even the PCB itself could be broken. 4. What Are the Consequences of Leaving Contaminants on the PCB Before Encapsulating? Prepare for contaminant warfare! If a resin is applied to a PCB that is still covered by con- taminants, then the resin will adhere to the surface of the contaminant rather than the PCB substrate or the surface of the components. Therefore, if there is a failure of adhesion be- tween either the resin and the contaminant, or the substrate and the contaminant, then this will introduce a point of weakness in the res- in-substrate interface, which could allow other contaminants to attack the PCB/components. The contaminants may directly attack the PCB/components or induce an attack. For ex- ample, if a piece of solder is left, bridges across the copper tracks or across two component legs, and is then covered in resin, this increas- es the severity of the attack and prevents re-

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