How do I make sure the MOSFET is aligned in the z-axis to properly mount with the heatsink in JLCPCB assembly?
These devices would be fitted by hand so the question probably does not arise.
See (2), (3) and (4) in:
[https://support.jlcpcb.com/article/83-smt-assembly-faqs](https://support.jlcpcb.com/article/83-smt-assembly-faqs)
However, have you checked that JLCPCB can supply the heatsink **and** the transistor?
If they cannot supply the heatsink then it is probably safest to not have the transistor fitted by JLCPCB.
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