Silicone Keypad Design Guide

Shrink Rate (Explanation for Tolerance)

There are many factors that contribute to silicone having a varying shrink rate from 3% to 5%. These include silicone durometer, raw mixture, part design, environmental conditions, molding process variance, position of part in mold, and post curing. Abatek uses 20+ years of manufacturing experience to control these variables so that keypad production is consistent.

Because of the variable shrink rate, Abatek is limited to how tightly part dimensions can be held. This also limits how tight assembly stack-up tolerances can be kept, however there are many tips & tricks on this page that explain good assembly design practices.

ISO-3302-1 Class M2 Tolerances

The tolerances used by Abatek for compression molded silicone have been established by an ISO standard. Abatek uses Class M2 tolerances, the tightest available for rubber keypad parts.

Class M1 tolerances are used specifically for o-rings or simple rubber shapes.

Difference Between Fixed and Closing Dimensions

The ISO-3302-1 tolerance has two types of dimensions, fixed and closing. The difference between the two is dependant on if one half or both mold halves are used to create the dimensions.
Closing Dimensions: any dimension whose feature is created by only one side (top or bottom) of the mold.
Fixed Dimensions: any dimension whose features are created from both sides of the mold. T

Openings and Screw Bosses

In most cases, the silicone keypad is sandwiched between the PCB and the front housing. To position the keypad relative to both the PCB and the housing, screw bosses can be used.

Advantage The keypad can be assembled together with the PCB. The pressure applied with the screws can help seal the keypad against the housing.

Disadvantages The keypad must have openings which are more expensive. Also the keypad is not secured to the PCB first so all components must be aligned together.

Spigots

A better solution is to first position the keypad relative to the PCB with spigots, then assemble the PCB+keypad to the housing. Several spigot designs are available. Abatek recommends using push through, non-undercut spigot.

Pull Through Spigots These types of spigots are long and have a thin end. Only the end is initially pushed through the PCB. Then, the operator grabs the end from the underside of the PCB and pulls the rest of the spigot through. One disadvantage is that the ends are prone to tearing if too much pull force is used.

Push Through Spigots: These are shorter and have an opening on the top of the keypad so that a tool can be used to push the spigot through the PCB. The tool helps to easily guide and push the spigot through without damage. When the tool is removed, the hole acts like an undercut feature - further retaining the keypad.

Spigots with Undercuts: Pull through spigots (and even push through) can be made with undercuts, which help retain the keypad. Undercuts are difficult to tool and can lead to higher scrap and keypad cost.

Undercut Wrap

If the assembly has a very thin profile not allowing spigots, an undercut wrap can be used to secure the keypad to the PCB. This method will also help protect the PCB.

Undercuts are difficult to tool and production parts are more likely to tear during removal from mold. This leads to higher tooling pricing, higher scrap and more susceptibility to quality issues.

If undercuts wraps are used, they should not be made into the corners. Abatek will review each undercut wrap design, taking into account material hardness, shape, keypad base thickness and other factors before confirming feasibility.

Making Large Keypads Smaller

The tolerances (per ISO-3302-1) for compression molded silicone becomes wider as the dimension becomes large. With very large keypads, some dimensions will have shrink variance and wider tolerances - which coud cause fitment issues.

For that reason, it is recommended to not design very large keypads and either use separate keypads for high volume applications, or use an expansion joint, which will decrease the tolerance range.

Housing Design

For top layer keypads where the silicone keys are through the housing, it is important to allow:

1) sufficient space between housing and key to prevent constraint (0.2 - 0.4mm)
2) guiding of the key in a vertical motion (extend housing down without touching web)
3) design that prevents the keys from getting caught under the housing or from sticking

See also: Engineer Newsletter Aug 2010 - preventing key sticking

Designing Plastic Caps

Plastic caps must also be designed to prevent constraint of the keypad. If the key-cap is fixed to the keypad, both the keypad and the plastic cap may be forced out of position causing poor actuation feel or even web failure.

By using a loose key-cap design, the silicone keypad can self adjust for small positional misalignment and provide great tactile feel. The lips on the plastic cap prevent it from falling out of the housing and the keypad can provide a slight pre-load to prevent rattling.

Sealing with Silicone

The properties of silicone makes it a good choice to use for sealing. Most applications can benefit from using the silicone keypad to seal with the housing and protect the PCB from the environment. There are two main types of seal designs:

Plastic Lip Seal: Here a plastic lip from the housing comes down on the silicone and seals with the flat keypad base. A silicone perimeter rib is often used to help position and contain the plastic lip.

The sealing lip should deflect the silicone by 0.2mm or 10% of the total keybase thickness (whichever is greater).

Rubber Lip Seal: In this design, the rubber acts like the sealing lip and deflects against a flat portion of the plastic housing. The plastic should deflect (push down) on the silicone lip by 0.2mm

Note: all sealing lips should have adequate pressure from the PCB to provide a good seal. As with all custom applications, testing should be performed to assure the sealing performance of the design.