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
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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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
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.