Issue link: https://iconnect007.uberflip.com/i/1348195
MARCH 2021 I DESIGN007 MAGAZINE 57 the corner of the pack off and then tilt the pack to dispense the resin, applying slight pressure as required to maintain the flow. If the material from the corners of the pack is not pushed into the centre of the pack then unmixed material can be dispensed. If the resin is not mixed long enough, then the resin might not cure, or have a patchy cure. In the case of filled resin systems, some sedimenta- tion might have taken place over time, so it might take a little more mixing to ensure that the fillers are correctly distributed throughout the resin. With the optically clear resins, when first mixed, the resin may appear hazy. is is perfectly normal, and the haziness will disap- pear as the material cures. 4. What happens when air bubbles get trapped in encapsulation resins and how can this affect performance? ere are numerous effects that air bubbles can have on the performance of the cured resin. Depending upon the number and distribu- tion of the bubbles, the actual thickness of the polymer layer applied will be decreased, hence the level of protection will also be reduced, particularly against chemical attack. If the air bubbles are next to components, wiring or tracks, and bridge conductors, then the insula- tion between those points is compromised and design rules around spacing and clearance may be broken. Ultimately, this can result in prema- ture failures by creepage and clearance break- downs. Voids also act as a weak point under thermal and physical shock, which can lead to the resin cracking in service. 5. What are the differences between encapsulation resins and potting compounds? In terms of the resin chemistry used, there's lit- tle difference, but there is a difference in how the resin is applied and the performance expected from it. An encapsulation resin will totally cover the PCB and the components and can act as the protective support structure, while a potting compound is used to fill a housing or enclosure containing the PCB and components. So, an encapsulation resin will adhere to the PCB and the components, and its outer faces will act as the primary barrier to protect the unit. e encapsulation process may or may not be performed with the assistance of a mould. Without a mould, the flow characteristics of the material will be required to control the deposition of material, and a dam and fill pro- cess using two different resin chemistries may be used to ensure correct material deposition. A potting compound must adhere not just to the PCB and components, but to all materi- als within the device, including the housing. In this case, the differences in CTE (coefficient of thermal expansion) between all the materials used are oen a critical factor, as all the materi- als within the housing will be subjected to dif- ferent rates of expansion and contraction due to the differences in the materials' CTE values. is can put the resin, components, circuit board, and even the housing under extreme stress, and lead to failure over time. If in doubt, it is always advisable to dis- cuss with your suppliers which material is best suited to your application. e technical support teams of reputable suppliers have a wealth of experience to call upon and, should it become necessary, they have the expertise to modify chemical formulations to meet your application needs. I hope the points covered this month have been informative. Please do look out for my next column, where I'll be cov- ering more issues on getting the most out of conformal coatings. DESIGN007 Phil Kinner is the global business and technical director of confor- mal coatings at Electrolube. To read past columns or contact Kin- ner, click here. Download your free copy of Electrolube's book, The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments, and watch the micro webinar series "Coatings Uncoated!"