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Sealing delicate electronic parts

27 June 2011

What’s the best way of sealing vulnerable electronic parts? Jim Marshall argues that low-pressure moulding is the best solution

Electrical and electronic components are all around us. Whether it is desk or laptop computers, memory sticks, music players or remote controls for the television, you can’t go far without coming across some type of electronic wizardry. But these components are vulnerable to dust, water and other contaminant ingress, not to mention vibration or even physical abuse.

This is where sealing – usually by means of encapsulation - comes into its own. Basically, there are two methods of achieving encapsulation: two component casting/potting using resins and silicones, or low pressure (typically 4-12bar) moulding using hotmelt adhesives. How the two processes differ can be illustrated using the following case study relating to the encapsulation of a sensor.

Previously, the casting process involved eight time-consuming steps: producing a mould case; palletising; inserting the electronics; pre-heating the assembly; encapsulating (a process requiring 16g of resin); setting or vacuuming; curing and, finally, testing. The production time involved with this method was between 2 and 24 hours per unit, not only causing a considerable delay at times of high production throughput, but also demanding space to store the units during curing. In some instances, a heat cure was used to hasten production, but any advantage was offset by higher energy costs. More worrying for this particular manufacturer was the fact that water tests revealed the ingress of moisture, which rendered the product useless. Clearly, another method of encapsulation had to be found.

By switching to a low-pressure moulding technique several benefits were immediately achieved. For a start, just three production steps are now required, namely inserting the electronics into the enclosure, over-moulding and testing. Production time is reduced to just one minute per unit, with no storage space required for a curing cycle. And while the cost of the hotmelt is higher than the resin previously used, just four grams is required, resulting in an actual overall cost saving of 50 per cent. Moreover, as the production cycle is quicker, there are also substantial savings in energy costs.

All that is good news, of course, but there was one other advantage. Testing indicated that the improved encapsulation method also produced a far more water-tight assembly.
The technique has also proved itself in another application involving the corner reinforcement on battery separators. In the past, the contact electronics were first potted with PVC, before being joined to the battery pack.; then a tape was wound around the finished construction. Now, the battery pack and its electronics are simply potted using a hotmelt adhesive - in one single operation.

This low pressure technology can be used to encapsulate smart cards to prevent tampering and damage during everyday use. The process can also be employed to over-mould PCBs that carry surface mounted components. Trials have revealed that material injected at 210°C typically will cool down to less than 150°C by the time it reaches the board, thus avoiding damage.

Low pressure techniques can be readily used for other applications such as moulding grommets in place with precision, replacing previously time consuming manual process. The low pressure alternative also provides a better looking finish, while reducing strain on connecting cables.

Using hotmelts
Most of the moulds used in the low-pressure system are made of aluminium, so there’s the initial cost saving. The combination of a non-abrasive resin and aluminium also means that less force is required when de-moulding the components. Where long runs are involved, steel moulds are recommended, but even then, the inserts coming into contact with the hotmelt can still be made of aluminium.

Once the moulds are in place, the adhesive is introduced into the system. These adhesives are processed at temperatures of 180°C to 240°C, with a temperature application range of -40°C to +140°C. Adhesion to PA, PBT, PVC and similar polar adhesives is very good. The products are colour coded for ready identification, and selecting the right adhesive is important in order to achieve the best results – once briefed on the application, a good hotmelt supplier will be able help you with the selection.

Although mould cycle times are dependent on overall component size and material thickness, typically these range from 10 to 50 seconds. Many mould set machines include permanent water connections on the side of the equipment to allow chilled liquid through the platens.
Ultimately, ‘low pressure’ means that fragile and/or delicate components can be encapsulated without fear of damage and with greater reliability when it comes to excluding damaging substances. Improved productivity can be achieved through faster cycle times, and the single-component material means the process is clean and easy.

Jim Marshall is with Henkel’s Macromelt product division

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