Keep in mind that a UV dielectric is a flat printed film, and the UV energy hits it directly on the surface and only travels a short distance through the printed layer. An encapsulant is placed on top of a component and runs down the side of the component before flattening out onto the substrate. When undercured, you will find that the thin layer on top of the component and the material laying flat on the substrate is cured, but the material on the sides of the component are not. This is because the material on the sides of the component does not get a direct hit from the UV radiation on its surface because it is turned 90 degrees to the substrate. This can be fixed by slowing the belt to allow for more energy for UV encapsulation.
The short answer is no. There are switches that have been in the field for more than a decade with no incidence of failure because of uncured component encapsulant. The purpose of an encapsulant is to mechanically secure the LED to the substrate so that the conductive epoxy joint has less chance of being stressed and broken, and this mechanical anchoring is accomplished by the encapsulant that surrounds the LED.
Some manufacturers will put a center dot of a clear adhesive in the center of an LED to anchor the LED more securely to the substrate, and then when the encapsulant is placed over the component it cannot flow underneath the component.
It is possible to make a UV or thermoset component that is rubbery. We have a rubber- like UV encapsulant that is used for edge sealing large LCD displays on membrane switches. However, these materials are not suitable for use as a component encapsulant. When an encapsulant is used to secure a small component such as an LED onto a membrane switch substrate, the encapsulant is added to provide relief from mechanical shock when the circuit is bent or exposed to temperature cycles that cause the materials of the circuit to expand and shrink at different rates.
If a rubber-like encapsulant were used and the circuit were flexed or bent near the component, the encapsulant would tend to move along with the bend and transfer the mechanical stress directly to the conductive epoxy adhesive that provides contact between the component anode or cathode, and the silver ink traces on the circuit. This can cause cracks or outright failures of the silver epoxy joint.
On the flip side, if the encapsulant is too rigid and the circuit is flexed or bent, then the entire encapsulant dot will break off easily and cleanly, taking the component with it.
For this reason, encapsulants that are used on flexible substrates, such as polyester film, need to be somewhere between a rigid, glassy and a rubbery flexible consistency. A good general rule of thumb (no pun intended) is that if you can see a slight indentation in the encapsulant after pressing your thumbnail into it, then it is the right consistency for use as a component attachment encapsulant for a membrane switch.
This is by far the biggest source of failure on membrane switches. The most frustrating thing about these “latent” failures is that it is virtually impossible to determine what actually caused the failure.
We have seen instances where we have investigated a single LED failure on a finished switch with a customer, and we could not cause failures on any of the other LEDs by mechanically stressing them. In one notable instance, we took the finished switch, stripped the graphic layer off carefully, and then pulled the circuit over the sharp corner of a conference room table and could not get the working LEDs next to the failed one to stop working. Even bending the circuit to about a ¾” radius in the area of the other LEDs did not produce a failure.
Nicomatic, LP (Warminster, PA) reports that probably 90% of LED failures on completed circuits are due to incorrect placement of LED components.
In the case of a latent failure, the effect of a poor surface mount joint or incorrect component placement is that the LED may work perfectly well during final testing. When it is shipped out to the end customer, the joint is stressed, causing a microfracture in the conductive adhesive or at one of the interfaces with the component or substrate, and the LED stops working. This stress can be caused by thermal expansion during shipping or by normal flexing during handling and assembly of the membrane switch into the final component.
The causes of this type of failure are numerous, but can be summed up as:
1) Improperly mixed conductive epoxy adhesive
2) Too little conductive epoxy adhesive used
3) Epoxy adhesive not cured completely
4) Component not placed correctly (offset from conductive adhesive and ink trace, not applied with enough pressure to seat it into adhesive, or canted to one side so that one end seats well, but the other end barely makes contact with the conductive adhesive)
Of these four, the last two are the largest contributors to LED latent failures. If the epoxy adhesive joint is not cured completely, it will not have optimized mechanical strength. If the joint is stressed, a small crack can appear that will push the silver particles apart far enough so that a high resistance junction is created.
If the component is not placed correctly, mechanical stress from handling the circuit can cause a small fracture between the component and the epoxy.
In either case, the LED will show as a failure, but in most cases you can apply slight pressure to the component and it will start working. However, after a while, the small crack will slightly open once more and the LED will stop working.
The best way to minimize the chance for a latent failure to occur is to consider the following recommendations. When surface mounting components on membrane switches:
1) Use a two part adhesive to ensure that the adhesive will continue to cure completely at room temperature after the heat curing cycle.
2) Do not use epoxy adhesives that have high levels of solvent content.
3) Use a center dot of non-conductive adhesive along with the silver conductive adhesive to mount the component for added mechanical strength.
4) Use a UV encapsulant to secure the component to the substrate.
5) Be sure to cure the conductive adhesive completely using the supplier’s recommended guidelines.
6) Follow recommended guidelines for component placement to ensure that optimal contact is made between the component, the conductive adhesive and the substrate and ink trace.
