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The following is a brief excerpt from: The Printed Circuit Assembler’s Guide to... Encapsulating Sustainability for Electronics
By Beth Turner, MacDermid Alpha Electronics Solutions
Chapter 2: Application Overview
There are many important things to consider when applying an encapsulant. This chapter is an overview of the application process, we will cover many of the terms and definitions found on a technical data sheet and discuss the primary considerations for each of the process stages. It is worthwhile considering some of the terms and definitions found on a technical data sheet to understand how the different liquid, mixed system, and cured property values can dictate the method chosen to apply materials.
Storage
It is important to store liquid encapsulants according to the manufacturer’s specification to ensure shelf lives are achieved. Single component materials require external energy sources to initiate the curing reaction; heat-cure systems should be stored in cool, often sub-ambient temperatures; UV-cure systems should be stored in blackout containers to prevent premature curing. Partial curing can increase the viscosity and cause small gelatinous clumps to form in the liquid system; this can cause problems during the application, for example, blocking valves and lines in dispensing equipment. Some liquid encapsulants are sensitive to moisture; it is important to keep such encapsulants stored in airtight containers. In addition to the problems above, moisture contamination can also cause unwanted side reactions during the cure process that can lead to undesirable bubbles and voids.
The Liquid Properties
When asked, “What is the viscosity,” the short answer is, “It depends.” Most encapsulants in their liquid state exhibit non-Newtonian flow and thus viscosity is not a single number. Viscosity is dependent on a variety of factors, including temperature, shear rate, time, and volume. Rheology profiling could be a more useful tool to understand liquid flow properties.
It is also important to understand the density of the material to convert between the weight and volume mix ratio, but also to understand the weight contribution an encapsulant will have, to a PCBA or unit.
