Overview of Potting
Potting (embedment) is the covering of an electronic or electrical device to protect it from the surrounding environment. Most of the time it is for protection from water or moisture and /or to electrically insulate it so that it will operate as designed. This can be done by several methods: potting, casting and encapsulation. Many names have been used interchangeably so there is some confusion of the terms. The most commonly used term is encapsulation or potting. We will use the term "potting" in all of our discussions to mean all of the methods below.
The potting method uses a "pot" or case or shell to put the device in and then pour the liquid potting compound to the top of the case covering the device and completely encasing it. The case becomes part of the finished unit. This is the most common method used, especially for high speed and many-units-per-hour production-line conditions.
The casting method is the same idea but instead of a case that stays apart of the unit a mold is use and removed after the potting compound has hardened. Sometimes this is also referred to as encapsulation. This is used when a molded unit is desired.
The encapsulation method is also referred to when a device is dipped into a resin system and a thick coating completely surrounds the unit. Sometimes this is just called a "dip coating". Molded device is a normally described as an encapsulated unit.
The major purpose of potting or encapsulation of a unit is for environmental production.. This could be for moisture protection, protection from chemical attack or sealing off areas. Also potting is used to provide heat dissipation, flame retardance and cushioning from shock. Normally, potting is not the first choice because it is viewed as a messy process that has many problems associated with it. Much of this perception is due to lack of knowledge of what is available, poor design of the unit poor selection of the potting compound and poor design of the manufacturing process. Below is a stepwise planning process that will help you understand what is needed for a successful design.
The design of the part includes three steps. The first step in your planning is the design of the unit or part to be potted or encapsulated. The second step is to make the device friendly to potting. The third is to determine an estimated cost. In this design phase, you design the unit so that it can be manufactured quickly, minimum bubbles entrapped, easy to pour and with a minimum of rejected parts.
The first decision is whether to pot or encapsulate the part. The most popular method of protection is potting a unit in a shell or case. The unit is placed in the potting shell, and the resin system poured around it. When it hardens the shell becomes part of the finished unit. The shell material is a consideration because you are balancing of the ability to bond to it vs. the cost of the shell. Encapsulation is another method that is the use of a mold and the unit is placed in the mold and the resin system pour in it. When hardened, the mold is stripped off and the resin system becomes the outside of the part.
Design the part to be friendly to potting includes easy air release, little shrinkage, and proper use of plastics. Click here for details of Designing Parts for Potting or Encapsulating.
The cost requirements are also outlined in this phase by the estimated amount of the potting material used in the part, cost of the shell or molds.
The conditions that the part will experience in use are defined so the potting compound can be selected. These include; high and low operating temperature; other environmental conditions such as humidity, chemicals and pressure or vacuum; physical requirements such as compressive strength (for bolt holes) and other requirements such as thermal conductivity, flame retardant, Underwriters Laboratories requirements and the color of the hardened potting compound.
Manufacturing of the unit is a large part of the plan since the process is a major decision factor in the resin system selection. Will the process be a hand mix and pour operation or is a meter-mix-dispense (MMD) machine is used. The number of units to be made per hour and when do they need to be hard enough to handle.
Using the above requirements the potting compound is selected. This sometimes requires a balancing of the requirements since all of them cannot be met. If this is the case, the requirements can be reviewed and ranked from the most important to the least important requirements. Some times it helps to select the potting compound and the manufacturing process together in order to give greater flexibility in the choice of the potting compound with the most effective manufacturing process. As an example, the time for the resin system to gel (so the unit can be handled) vs. the time that the units need to be moved or packaged. Can this be done at room temperature or is heat required? See Hardening Process Also in the selection of the potting compound, consideration should be given to the meter-mix-dispense equipment if one is to be used in the manufacturing process. Abrasive potting compounds, large mix ratios (10 to 1 by volume) and high viscosity resin with very low viscosity hardener are not friendly to the MMD equipment. They will work, but high maintenance and down time may occur. See Phase 6 below.
After potting or encapsulating the device, testing will determine if the potting compound selection is acceptable. Temperature cycling testing is the most telling test. In order to make sure the potting compound is fully cured, heating the unit to 150°F (66°C) for 2 hours after an overnight cure at room temperature (if the potting compound is a heat cure system then double the cure time at the recommended temperature). The recommended thermal cycle would be from the lowest temperature to the highest temperature the device will see in use. Run the cycle 20 times with a one-hour dwell time at the lowest and highest temperatures. The faster the rate of change from the high to the low and back, the more severe the test. We suggest that at least 50 units are tested in the temperature cycle to determine a failure rate.
After the temperature cycle, test the unit for its intended use to determine if its performance is affected. If no effect, then the potting compound is suitable. If the potting compound cracks or if the unit does not work a different curing cycle may help or a new potting compound is required. Click for Solutions for Potting Problems.
The equipment is determined (See MMD questions) and bids received. Since lead times are long - 8 to 10 weeks or longer - it is a good idea to do this early in the planning. The potting shells or molds for encapsulation are specified and ordered. The device components are ordered as well.
The last step is the plant trial on production equipment. Fine-tuning of the MMD equipment sometimes takes considerable time on startup. It is recommended that the MMD equipment supplier be given the potting compound before shipment of the machine to your plant so they can work out any problems before the plant trial and that you can observe the operation of the machine before it is shipped to you. It is recommended that both the suppliers of the potting compound and the MMD equipment be present during the trial so any problems can be resolved and problem-solving can be done if the resin system does not harden or perform as previously tested.
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Last Modified: July 20, 2012