Executive answer
Three thickness questions, three different records
- Write separate records for no-liner material caliper, the lined processing stack and the finished local contact gap. They answer different questions.
- Use a named material example to understand the fields, not to select a universal thickness or turn a typical value into an acceptance tolerance.
- Approve a controlled layer section, removal map and production-intent sample evidence before treating two spacer proposals as equivalent.
Current guidance context
Why a thickness-only RFQ is incomplete
A request for 'a 6-mil spacer' can mean a carrier film, a double-coated tape excluding liners, a lined converted part or a finished contact separation. Those are not interchangeable dimensional records. Before comparing quotes, ask what the number describes, where it is measured and which temporary layers are still present. This small clarification prevents an apparently matching material description from silently changing the layer stack the OEM is trying to approve.
For the printed-contact construction discussed here, Epec describes two opposing circuit films separated by an adhesive/polyester layer with an opening at each switch. The moving circuit closes through that opening. This establishes the anatomy, not a recommended dimension. The guide below turns that distinction into an OEM worksheet; it does not prescribe one spacer thickness for every key or substitute for mounted electrical and mechanical validation.
Read the material fields before copying a number
Two spacer products can use different carriers, adhesive builds or constructions. Do not add a carrier thickness to a total that already includes it, or treat an adhesive-only description as a complete spacer laminate. When two proposals give the same headline thickness, compare the constituent layers and the exact definition of the total before marking them equivalent.
Transcribe the source fields into the material record: SKU, regional document revision, face adhesive, carrier, back adhesive, total tape, liner type, units and inclusion convention. Keep the original rounding. A rounded metric value and its mil equivalent do not create a bilateral tolerance. If procurement needs an incoming-stock limit, ask the responsible supplier for the applicable controlled specification and measurement conditions rather than inventing a range around the published typical value.
Use the exact product and document, not only the adhesive family name. In 3M's February 2026, English-US 7956WDL data sheet, the typical construction is 0.05 mm faceside adhesive, 0.05 mm PET carrier and 0.05 mm backside adhesive: 0.15 mm total tape. The liner is a separate field. The sheet expressly says that these physical data are representative or typical and should not be used for specification purposes.
Map the liner state through each assembly stage
Draw two face labels and name the liner protecting each adhesive surface. Record its state at incoming inspection, die cutting, transfer to the first circuit, bonding to the second circuit and equipment installation, as applicable. Distinguish a liner retained to support a converted part during handling from a permanent layer in the finished design. The converter should confirm the removal order and orientation for the actual SKU; a photograph of printed liner branding cannot provide that information.
At every measurement, write the stage next to the result. A converted sheet with one liner present is not the same specimen as a bonded switch with that liner removed. If a liner, protective film or handling tab is deliberately retained at a later stage, identify its location and purpose rather than subtracting it by assumption. Confirm that each intended adhesive interface is actually exposed before it is bonded.
3M's 7961MP product page lists 11 mil tape thickness without liners and two 4.2-mil liners. For an illustrative handling calculation only, 11 + 4.2 + 4.2 = 19.4 mil when both named liners are included. That arithmetic is not a measured shipping caliper, a tolerance or a finished contact gap. It simply demonstrates why a lined sample and a no-liner material field must not be compared as though they describe the same stack.
Define the finished contact-gap datum on a section
On the project section, mark the intact spacer land, the cut aperture, both circuit films and the conductive surfaces that are intended to meet. The material's total caliper describes a tape layer; a local contact gap describes separation between two conductive surfaces inside an opening. Those are different datums. The actual relationship depends on the approved stack, including contact deposits, local film shape and any additional adhesive or support layers. This distinction is geometric editorial analysis, not a measurement reported by 3M or HYR.
Define the gap in the unpressed state and say whether the switch is laminated, installed on its backing or fully mounted in the equipment. Identify the key family, section location, supporting fixture and any load introduced by the measurement. A micrometer reading across an intact spacer land may be useful for one dimensional question; it does not directly observe the empty aperture or the opposing contact surfaces. A section or another agreed method needs its own datum and specimen description.
Keep key travel and response as separate requirements. A rest-state gap alone does not describe the movement of the overlay, circuit or dome, or the electrical closure behavior under an actuator. Do not select a universal thinner-is-better or thicker-is-better rule from the two named materials. Review the actual construction with the converter, then verify the mechanical and electrical criteria on the specified mounted sample.
Agree on evidence for each dimensional record
Use the worksheet below to keep three records separate: incoming material, the converted processing stack and the local gap in the finished assembly. For each record, identify the drawing or specification owner, the specimen stage, units, measurement locations, method and disposition rule. A supplier data sheet can establish how its published material fields are named; it cannot close an OEM's unknown assembly tolerance or sample acceptance decision.
Ask the converter to return an annotated layer/removal map with its proposal. Ask the equipment team to return the mounting and backing state that affects the approved specimen. If a dimensional method is destructive, agree on the sacrificial sample and what preparation might change; if it is indirect, identify what it infers rather than claiming direct observation. The responsible teams choose tolerances, conditioning and sampling. This article supplies none of those project values.
Hold approval when a proposal gives only 'spacer thickness' without its layer definition, when the sample includes an undocumented liner, or when a gap result lacks its location and assembled state. Resolve the missing definition before comparing prices or authorizing substitution. Material-caliper acceptance, local-gap acceptance and key-response acceptance can reference the same controlled build, but they should remain distinct lines in the inspection plan.
Challenge five thickness traps before approval
These are hypothetical drawing and inspection scenarios, not HYR defects, customer cases or factory test results. Use them to expose missing definitions before the first-article decision.
- The buyer specifies carrier thickness while the quotation describes total double-coated tape. Reconcile the adhesive and carrier fields in a named material record before calling the dimensions equal.
- Incoming inspection measures a lined converted part against a no-liner data-sheet number. Identify every retained liner and the inspection stage, then use the agreed specification for that specimen.
- The team writes the nominal tape caliper as the contact gap without locating the conductive faces. Add the contact aperture and actual surface datums to the section; request finished-sample evidence.
- An inspector squeezes a flexible aperture while trying to measure a rest-state gap. Define the support, contact load and method so the result refers to the required unpressed state rather than an uncontrolled deformation.
- A replacement stock keeps the same stated total but changes the carrier, adhesive build or liner sequence. Review the changed layers and assembly steps, and decide which dimensional and functional checks must be repeated.
Send a controlled spacer RFQ, not a catalog value
Send the proposed material record, controlled circuit and spacer artwork, aperture geometry, layer section, removal map and equipment mounting section together. Mark whether the question concerns raw-stock caliper, a converted-part measurement or a finished contact gap. Ask for a proposal that names the material, maps the temporary layers and flags any dimension that cannot yet be confirmed. An open item with an owner is more useful than a copied catalog value presented as a pass.
Before approval, agree on the evidence bundle: current material documentation, approved stack and aperture revision, liner-removal instructions, representative measurements in the named stages and a production-intent mounted sample with separate electrical and key-response results. Tie each record to the same build revision. Require an impact review when material construction, print buildup, aperture geometry or lamination and mounting conditions change; do not assume one original sample covers every later substitution.
For an HYR inquiry, use the secure RFQ to share the engineering drawing and unresolved thickness questions. Request human project review of the proposed construction and acceptance plan. Neither this worksheet nor the parent-company assembly photograph promises an HYR material choice, verified gap, tolerance, key life, price, delivery date or completed equipment approval.