How to specify an MCH heater

Start with the four numbers you cannot skip

A complete specification starts with four inputs, and most RFQs are missing two or three of them. Supply voltage and its tolerance band come first, including the battery cut-off voltage if the product is cell-powered. Second is the target power or the heat-up time, and — critically — which of the two is the hard constraint. Third is the working temperature together with the maximum allowable surface temperature, since these set the material and the protection scheme. Fourth is the available space: length × width × thickness, the mounting method and where the leads exit. With those four, a circuit design, drawing and price can be produced without a second round of questions. If only part of the list is known, the remaining items can be proposed numerically using R = V² / P and the published voltage and resistance windows rather than guessed.

• 1 · Supply voltage and tolerance band, including battery cut-off
• 2 · Target power or heat-up time, and which one is the hard constraint
• 3 · Working temperature and maximum allowable surface temperature
• 4 · Available space L × W × T, mounting method and lead exit

Electrical definition: voltage, power and cold resistance

With the rail fixed, the circuit follows from R = V² / P. The site window covers 3.7–220 V and 5–300 W (custom parts up to 3,000 W), with cold resistance specified between 0.3 and 1,500 Ω at 25 °C, ±10%. That tolerance is not cosmetic: at a fixed voltage a ±10% voltage band changes power by roughly ±20%, and a resistance 10% low is a part 10% high in power. Always state the nominal voltage together with its band and the control method — PID, on/off or thermostat — because each treats the tolerance differently. The cold resistance is the one electrical parameter that can be verified at incoming inspection without a hot stage, so it is also the practical proxy for rated power. Use four-wire (Kelvin) measurement at 25 ± 1 °C after the part has stabilised; two-wire clips add 0.1–0.3 Ω, which is a large error on a low-voltage part.

• Use R = V² / P at the nominal voltage; state the voltage tolerance band explicitly
• Published window: 3.7–220 V, 5–300 W (custom to 3,000 W)
• Cold resistance 0.3–1,500 Ω at 25 °C, ±10%; a ±10% voltage band moves power ~±20%
• Verify cold R with four-wire Kelvin measurement at 25 ± 1 °C

Thermal definition: temperature, area and watt density

Temperature alone does not define the heater; the heated area does. Define the working temperature and the maximum surface temperature, then compute watt density as rated power divided by the printed resistor footprint — not the outline. A 40 × 20 mm chip at 24 V / 60 W with the resistor covering 32 × 16 mm has 5.12 cm² of heated area and 11.7 W/cm²; at 100 W the same outline gives 19.5 W/cm². Treat 45 W/cm² as the ceiling. The ceramic itself is rarely the limit: with q″ = k · ΔT / t, k = 20 W/m·K for 96% alumina and a fired thickness of 0.635 mm, each kelvin across the plate carries about 3.15 W/cm², so 45 W/cm² needs only about 14 K of gradient. State which area the watt-density number refers to on the drawing.

• Give working temperature and maximum allowable surface temperature
• Compute watt density on the resistor footprint, not the outer outline
• 45 W/cm² is the ceiling; q″ = k · ΔT / t gives ~3.15 W/cm² per K for 0.635 mm
• Mark the resistor footprint on the drawing and state which area the number covers

Mechanical definition: outline, border and tolerance

The mechanical sheet should carry more than an outline. State length × width × thickness within the published envelope (up to 100 × 100 mm, 0.7–2.0 mm thick), plus mounting and lead exit. Mark the unheated margin: the distance from the resistor edge to the plate edge plus the termination area can be 25–35% of the outline on a small chip, so an outline-based watt-density calculation flatters the design by exactly that much. Allow dimensional tolerance for the ceramic body and the lead exit rather than treating it as an afterthought on a part that must fit a machined pocket. Where an internal corner is unavoidable, a 0.3–0.5 mm fillet is a cheap way to remove a stress concentrator. Optional holes, slots, chamfers and designated heating zones belong on the same drawing.

• Outline up to 100 × 100 mm, thickness 0.7–2.0 mm; include mounting and lead exit
• Mark the unheated margin — 25–35% of the outline on small chips
• Allow tolerance for the ceramic body and lead exit; add 0.3–0.5 mm fillets at internal corners
• Specify holes, slots, chamfers and designated heating zones on the drawing

The thermal interface is a specification

Watt density is meaningless without the thermal path. State what the heater touches, the flatness of both surfaces, the clamping method and the interface compound, because the contact to the load usually dominates the total thermal resistance. Industry-practice starting points are 3–8 W/cm² for free convection in still air, 15–30 W/cm² clamped to a metal block with a compliant interface, 30–50 W/cm² with flowing liquid on the cold face, and 45–60 W/cm² for intermittent duty below 30% — these are starting points, not guarantees. The decisive number is the one your own thermal model produces for the mounting. At commissioning, put one thermocouple on the plate and one on the load; if the plate sits 60 K above the load at steady state, fix the interface before asking for a lower watt density. The heater is doing its job; the mounting is not.

• Define the contact surface, flatness, clamping method and interface compound
• Starting ranges: still air 3–8, clamped metal 15–30, flowing liquid 30–50, intermittent 45–60 W/cm²
• Contact resistance usually dominates; improve the interface before lowering power
• Verify with a thermocouple on the plate and one on the load at rated voltage

Leads, terminations and protection

Terminations are part of the specification, not a purchasing detail. State the nickel lead diameter (Ø0.25 / 0.3 / 0.5 mm), length, colour and whether the end is a terminal, connector, FPC or solder tab; for battery parts a 0.55 mm minimum thickness may apply. Because MCH does not self-limit, list the protection explicitly: thermal fuse, thermostat / TCO, NTC or thermocouple feedback, and any IP rating for the assembled module. Dielectric should be confirmed at system level rather than only at component level — the published figure is 4,500 V / 1 s with leakage below 0.5 mA and insulation resistance above 100 MΩ. Also size the fuse, relay and traces for the cold-start current: printed tungsten paste shows roughly 2–3× hot-to-cold resistance at 600 °C, so a 230 V / 100 W part with 529 Ω hot and 0.43 A steady can draw about 1.3 A on the first cycle.

• Nickel leads Ø0.25 / 0.3 / 0.5 mm; length, colour, terminal, connector or FPC
• Protection: thermal fuse, thermostat / TCO, NTC / thermocouple, IP rating
• Dielectric 4,500 V / 1 s, leakage < 0.5 mA, insulation > 100 MΩ — confirm at system level
• Size the fuse for cold-start inrush; printed paste is ~2–3× hot/cold at 600 °C

Compliance and documentation

Close the specification with compliance and the documents the release will need. State RoHS / REACH and lead-free requirements, the required material declaration, and which test reports must accompany the part. For automotive or medical programmes, name the structured documentation expected (for example a PPAP-style package) at the RFQ stage so it can be written into the project plan, rather than requested after tooling is released. Ask for the part-specific datasheet — 2D drawing, cold resistance and tolerance, R–T curve, rated power at your voltage, dielectric and insulation test values — and for STEP / DXF once the design is released. Third-party testing can be commissioned for specific market requirements. Lot traceability and batch test records should be specified if the assembly is delivered as a single part number.

• RoHS / REACH, lead-free declaration and required test reports
• Name PPAP-style documentation at RFQ stage for automotive / medical programmes
• Require the part-specific datasheet; STEP / DXF after design release
• Specify lot traceability, batch test records and third-party testing if needed

Common mistakes

Three omissions cause most of the rework. Specifying power without defining the heat sink or thermal path makes watt density meaningless. Treating a nominal voltage as if it had no tolerance band ignores that a ±10% rail changes power by roughly ±20%, and that the battery cut-off voltage is the worst case on a cell-powered product. Treating dimensional tolerance as an afterthought on a part that has to drop into a machined pocket leads to a heater that fits the drawing but not the assembly. A fourth, subtler error is calculating watt density on the outline rather than the resistor footprint, which hides a 25–35% margin on a small chip. Confirm the dielectric requirement at system level, not just at component level, and re-check the whole sheet against R = V² / P before release.

• Power density is meaningless without the defined thermal path
• A ±10% voltage band changes power ~±20%; include battery cut-off
• Allow tolerance for the ceramic body and lead exit on machined-pocket fits
• Use the resistor footprint for watt density; confirm dielectric at system level

What we send back

A complete specification earns a complete reply. The deliverable is a part-specific datasheet: a 2D drawing, cold resistance with its tolerance, the resistance–temperature curve, rated power at your voltage, dielectric and insulation test values, a material declaration, and a recommended control / protection scheme. Where the heater is supplied as an assembly, the same datasheet covers the housing, thermal interface, lead set, connector and protection parts as one part number with 100% end-of-line testing. If the checklist is only partly filled, the missing items are proposed rather than guessed, using the published voltage, resistance, dimension and watt-density windows. STEP / DXF can be supplied once the design is released, and a pilot lot can be built before production tooling is committed.

• Part-specific datasheet: 2D drawing, cold R + tolerance, R–T curve, rated power
• Dielectric / insulation values, material declaration, control / protection scheme
• Assembly-level option: housing, leads, protection as one part number, 100% EOL test
• STEP / DXF on release; pilot lot before production tooling

Specifications

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