TECHNICAL GUIDE | ADHESIVE SELECTION
Dome Array Adhesives: PSA, Heat-Bond and Liquid Adhesive Compared for Reliable Keypad Assembly
The adhesive is the least visible and most failure-prone part of a dome array. This guide compares PSA, heat-bond and liquid options across hold, rework, temperature and cost.
Ask ten engineers what holds a dome array to the switch layer, and most will answer "the adhesive" without thinking further. That is usually where the trouble starts. Adhesive failure in dome arrays rarely shows up as a dramatic delamination; it shows up as a dome that shifts 0.15mm over temperature cycles, an intermittent contact after six months, or bleed that creeps onto the contact ring and floats the legs. This guide lays out the three adhesive families Shanyo uses in production - pressure-sensitive adhesive (PSA), heat-bond film, and liquid adhesive - and when each one is the right call.
1. The Job the Adhesive Does
In a dome array, the adhesive performs four jobs at once:
- Holds each dome in exact position relative to its contact pair.
- Seals the dome edges against dust and moisture ingress.
- Provides the spacer function that lets the dome flex without mechanical interference.
- Survives the assembly process: reflow, handling, and the rest of the product life.
Because one material must satisfy all four, adhesive selection cannot be a default. The choice interacts with the carrier (PET or PI), the dome plating, the assembly temperature profile, and the environmental rating of the finished device.
2. Pressure-Sensitive Adhesive (PSA)
PSA is the workhorse of dome arrays. It is a pre-coated adhesive layer on the carrier, applied under pressure at room temperature with no cure step.
- Hold strength: good for standard consumer and industrial applications. Peel strength depends on the carrier and the surface energy of the substrate.
- Temperature: acrylic PSA systems typically hold to 120-150C continuous; silicone PSA extends to higher temperatures but costs more. Above that, adhesive softening becomes a real risk.
- Rework: a genuine advantage - PSA arrays can usually be peeled and repositioned in assembly, which protects yield during prototyping and low-volume builds.
- Cost: the lowest material cost of the three options, and the fastest to laminate.
PSA is the right default for most consumer, industrial and medical panel programs where the assembly does not pass reflow and temperatures stay inside the PSA window.
3. Heat-Bond Adhesive
Heat-bond film activates at temperature - typically 120-180C - and flows into the substrate surface before curing. It creates a stronger, more permanent bond than PSA.
- Hold strength: significantly higher than PSA after cure, with better resistance to creep under sustained load.
- Temperature: survives reflow better than standard PSA; the bond strength holds through thermal cycling that would soften an acrylic PSA.
- Rework: essentially none. Once bonded, the array is permanent; removal damages the carrier. Rework cost must be planned.
- Process: requires a lamination press or heated nip with controlled temperature and pressure, plus the cure time in the line.
Heat-bond is specified where permanence and temperature resistance matter more than rework flexibility: automotive interiors, devices that see engine-compartment temperatures, and assemblies that pass reflow.
4. Liquid Adhesive
Liquid adhesives - dispensed or screen-printed - bond the array during assembly rather than coming pre-coated. They are the least common in dome arrays and the most application-specific.
- Hold strength: can be the highest of the three, especially with epoxy or acrylic structural adhesives.
- Sealing: liquid adhesive can fill gaps and create a more complete environmental seal than a die-cut PSA ring, which helps in sealed outdoor or washdown assemblies.
- Process risk: dispensing must be controlled tightly. Excess adhesive can wick under the dome and change the actuation force; insufficient adhesive leaves edge lift risk.
- Cost: the highest in process cost because dispensing adds a step and requires cure scheduling.
Liquid adhesive earns its place in two situations: assemblies needing a complete environmental seal, and substrates where a pre-coated adhesive will not wet properly (some textured plastics and low-energy surfaces).
Adhesive choice determines dome position stability, sealing and rework behavior.
5. A Comparison Table for Specification
| Property | PSA | Heat-Bond | Liquid |
|---|---|---|---|
| Bond strength | Good | High after cure | Highest option |
| Temperature resistance | To ~150C | Reflow-capable | Depends on chemistry |
| Rework | Yes | No | No |
| Environmental seal | Per die-cut ring | Good | Best |
| Process cost | Lowest | Moderate | Highest |
| Typical use | Consumer, medical panels | Automotive, reflow | Sealed outdoor, low-energy surfaces |
6. Adhesive Bleed: The Failure That Is Easy to Miss
Adhesive bleed is the quiet killer in dome arrays. When the PSA or heat-bond film creeps beyond the die-cut edge - during lamination, reflow, or storage - it can reach the contact ring area. The result is a raised dome edge, an increased actuation force, or a contact that no longer closes reliably.
Bleed control starts with the die cut: the adhesive ring must be dimensioned with a bleed allowance so that even the worst-case creep stays clear of the contacts. It continues with the lamination process: temperature, pressure and dwell time must be matched to the adhesive so the bond forms without squeezing the adhesive out laterally.
Shanyo verifies bleed on new tooling by cross-sectioning sample arrays after lamination and again after thermal cycling. The measurement is part of the DFM package for every array program.
7. Matching Adhesive to the Assembly Process
The assembly process decides the adhesive family more than any other factor:
- Hand or low-temperature assembly, prototyping: PSA, for rework.
- Reflow soldering of the switch layer: heat-bond or high-temperature PSA; verify peak temperature against the adhesive datasheet.
- Washdown or outdoor sealed assemblies: liquid adhesive or heat-bond with a sealed die-cut perimeter.
- Very thin compact devices (<0.35mm total): PSA on ultra-thin carriers, because the added process steps of heat-bond or liquid are harder to control at that thickness.
8. Getting a Recommendation
Send Shanyo the assembly temperature profile, the substrate material, the environmental target (IP rating, washdown, automotive), and the required rework policy. The engineering team will specify the adhesive system and document the rationale in the DFM. Start through the contact page. For the dome side of the design, the selection guide covers force and life trade-offs.
About Shanyo: Qingdao Shanyuan Precision Electronic Technology Co., Ltd. designs and manufactures dome arrays, metal domes, FPC keypads and stamped components. Visit www.yo-shan.com.
