Key takeaways
- A heat sink RFQ needs the thermal duty, not just a shape and a material.
- The interface, airflow and mounting data decide the fin geometry as much as the heat load does.
- The process route should be open at RFQ stage so the supplier can quote extrusion, casting or bonding on merit.
- Send volume, tooling ownership, finish and inspection requirements together, or the quotes will not be comparable.
Start with the thermal brief
A heat sink is not a bracket with fins. It is a thermal component, and its price depends on how much metal it needs, how the heat spreads and how the part is made. An RFQ that sends only a STEP file and an alloy will get a quote for a shape, not for a thermal duty.
The brief should state the heat load in watts, the ambient air temperature, the available airflow and the allowable component temperature. Add the duty cycle if the load is not continuous. These inputs let the supplier check whether the proposed geometry can meet the target or whether the design needs more surface area, a thicker base or a different route.
If the thermal performance is not yet fixed, say what is known and what is open. A supplier can then quote a range and flag the variables that will move the price.
The thermal inputs to send
Give the heat load and the allowable temperature rise. For illustration, a 50 W load with a 25 C rise above ambient is a different problem from a 50 W load with a 10 C rise. The first may suit a modest extrusion; the second may need a larger base, a different fin density or forced air.
State the airflow: natural convection, fan assisted, ducted or immersed in a cabinet. Give the approach velocity if a fan is used. A heat sink selected in still air can behave very differently in a closed enclosure where the air recirculates and the ambient rises.
Add the interface material, the mounting pressure and the flatness requirement. A thermal pad, a paste and a phase change material each need a different surface finish and a different contact pressure. If the interface is not defined, the thermal resistance cannot be calculated with confidence.
Ambient is not always room temperature
Inside an enclosure, the air around the heat sink may be 15 to 20 C above the room. State the local ambient, not the laboratory condition.
Geometry and interface data
The drawing should show the base footprint, the fin height, the fin thickness, the fin pitch and the base thickness. It should also show the mounting holes, the keep-out zones and the direction of airflow relative to the fins. A fin pattern that is efficient in one direction can be poor in another.
The interface face needs its own callouts: flatness, surface roughness and the position of the heat source footprint. A thin base spreads heat sideways, but only if the source is close enough and the base is thick enough. Mark the source outline on the drawing so the supplier can check the spread resistance.
If the heat sink is also a structural part or an electrical enclosure, say so. That changes the wall thickness, the alloy and the inspection. A thermal part that carries a connector or a PCB often needs machining after casting or extrusion.
Keep the process route open
There are several ways to make a finned aluminium part. An extrusion gives a constant cross-section and low tooling cost for a simple profile. A die casting gives a three-dimensional shape with bosses and mounting features in one part. A bonded or skived heat sink gives a high fin density where an extrusion cannot reach it. A machined part suits a small quantity or a tight interface.
If the RFQ names one route before the supplier has seen the volume and the geometry, the quote may be expensive for no reason. Describe the function and the constraints, then ask the supplier to propose a route with the tooling and the lead time.
For a casting route, include the draft, the wall thickness and the machining allowance. For an extrusion, include the profile envelope and the cut length. For a bonded assembly, include the joint method and the flatness after bonding. The route changes the data the supplier needs.
Material, finish and insulation
Aluminium 6063 is common for extrusions and 6061 for machined parts. Die castings use alloys such as ADC12 or A380, which have different thermal conductivity and machinability. If the alloy is fixed, state it. If it is open, state the thermal and mechanical requirement and let the supplier propose.
Finish matters for corrosion, appearance and electrical insulation. Anodising adds a thin oxide layer and can be decorative or functional. Powder coating adds thickness and thermal resistance, so it should not cover the interface face unless the design allows for it. Conversion coating is a thinner option where conductivity is not required.
If the part must be electrically isolated, give the dielectric requirement and the creepage distance. If it must be conductive, say where the contact surfaces are and which finish is acceptable. A general note to anodise the whole part can accidentally insulate a face that needs to conduct.
Mechanical and environmental data
Give the mounting method, the torque, the fastener size and the load the part must carry. A heat sink clamped to a PCB with two screws sees a different stress from one that supports a transformer. Vibration, shock and thermal cycling should be stated if they apply.
State the operating environment: indoor, outdoor, humid, salt laden, industrial or washdown. That drives the alloy and the finish. A part that looks fine in a dry lab can corrode at a seaside site or in a washdown cabinet.
Add the IP requirement if the heat sink is part of an enclosure, and say which surfaces are gasketed. A casting with a sealing groove needs a different level of flatness and porosity control from a plain finned block.
Volume, tooling and inspection
Send the annual volume, the release size and the expected programme life. Tooling cost is divided over the volume, and a die casting tool that is sensible at 50,000 parts a year may be wrong at 2,000. State whether the tooling is to be dedicated, shared or owned by the buyer.
Say what inspection you need. A dimensional report on the first article is normal. A thermal test may be needed on a sample, but agree the method and the fixture. If the part is a pressure boundary or a sealing surface, add the leak test and the porosity acceptance.
Include the delivery destination, the Incoterm and the packaging requirement. A heat sink with thin fins is easily damaged in transit, so the packing method is part of the cost. Ask the supplier to quote the packaging explicitly rather than assuming a standard carton.
| Item | What to send | Why it changes the quote |
|---|---|---|
| Thermal duty | Heat load, ambient, airflow, allowable rise | Sets the metal and surface area |
| Interface | Material, pressure, flatness, roughness | Sets the base and the finish |
| Geometry | Base, fins, mounting, keep-outs, airflow direction | Sets the process route |
| Process | Open route or a named route with constraints | Sets tooling and lead time |
| Material | Alloy and temper, or the thermal requirement | Sets conductivity and cost |
| Finish | Anodise, coating, masking, insulation | Sets cost and thermal resistance |
| Volume | Annual demand, release size, life | Sets tooling amortisation |
| Inspection | FAI, thermal test, leak test, porosity zones | Sets the quality plan |
A quote is only comparable when every supplier prices the same scope. Send the same pack to each one.
Frequently asked questions
What is the most important input in a heat sink RFQ?
The thermal duty: heat load, local ambient, airflow and allowable temperature rise. Without those, the supplier can only quote a shape, not a heat sink that meets a target.
Should I specify extrusion or die casting in the RFQ?
Only if the route is fixed by the design. Otherwise describe the function, volume and constraints and ask the supplier to propose a route with tooling and lead time. That keeps the quotes comparable on scope.
Does anodising affect thermal performance?
Yes, a coating adds a thin thermal resistance at the interface and on the fins. It is usually small, but it matters on a tight design. Mask the interface face if the design requires direct metal contact.
How do I compare heat sink quotes from different processes?
Normalise the scope: same thermal duty, same interface, same finish, same volume, same inspection and same Incoterm. Then compare the total cost, including tooling, packaging and the thermal test.
Sources and further reading
- JEDEC JESD51, Thermal measurement of semiconductor packages
- ISO 2768-1:1989, General tolerances, linear and angular dimensions
- ISO 8062-3, Geometrical product specifications for moulded parts, casting tolerances
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Have a heat sink drawing ready to quote?
Send the thermal brief, the drawing and the volume. The engineering desk will reply with the missing inputs and a route recommendation before the RFQ goes out.
Send a drawing