A through-hole power device can be constrained by both the PCB and a heat sink. The order in which those constraints are established matters. Define a mounting sequence that follows the package manufacturer’s instructions and does not rely on soldered leads to pull a misaligned package into position.
Draw the two mechanical interfaces
Show the board holes, package leads, heat-sink mounting surface and fastener location. Include the assembled tolerances that determine whether the part reaches both interfaces naturally. A nominally aligned CAD model does not establish how actual parts fit at tolerance limits.
Infineon’s AN-1012 gives a fastening-before-soldering instruction for its TO-220 example and discusses supported lead forming. Treat this as package-specific guidance: confirm the current instructions for the selected device and mounting construction before defining the production sequence.
Identify when each interface becomes fixed
| Operation | Question |
|---|---|
| Lead preparation | Is the supplied or approved formed geometry correct? |
| Mechanical placement | Can the package and board align without forced bending? |
| Heat-sink attachment | Is the final device position established as required? |
| Soldering | Are the leads joined in the approved mechanical state? |
| Later assembly | Does another operation move either fixed interface? |
Observe an actual assembly with the production tools. Small manual adjustments that never appear in a work instruction can reveal where the nominal sequence depends on operator force.
Investigate an alignment mismatch directly
Measure the relevant parts and compare them with the released tolerances. Preserve examples that do not fit naturally. Do not enlarge a hole, rebend a lead or loosen a heat sink informally to make a single unit pass.
For example, if the fastener can only be inserted after pushing the package sideways, determine which interface is displaced. Soldering first may hold the part in place temporarily while concealing the same dimensional conflict.
Where lead compliance is part of the approved construction, define the formed geometry and tooling. An unspecified bend is not a substitute for a reviewed tolerance and loading arrangement.
Verify the completed sequence
Inspect lead exits, package position, solder joints and the heat-sink interface after the full route. Include the relevant thermal and electrical checks in qualification. A good solder fillet alone cannot establish that the package experienced acceptable mechanical loading.
Retain the sequence when changing board thickness, heat-sink supplier or mounting hardware. These changes can alter alignment even if the schematic and semiconductor remain unchanged.
The insulating-hardware guide covers the mounting stack, while lead-forming review illustrates why preparation and soldered-state adjustment are different operations.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist