Guides / Quality
How to Specify Surface Finish on Semiconductor Parts
September 21, 2026
Surface finish is the cheapest thing to write on a drawing and one of the most expensive to get wrong. Ra is an average of surface height deviation over a sampling length. It is not a measure of cleanliness, it is not a measure of flatness and it does not describe the direction of the machining marks.
Which surfaces need which number
These are the ranges that come up on semiconductor hardware. Treat them as orientation, not as a specification to copy.
| Surface | Typical range | Why |
|---|---|---|
| Gas wetted internal passages | around 8 Ra microinch, electropolished | Less moisture and particle retention, faster purge |
| Vacuum facing surfaces | around 16 Ra microinch | Lower outgassing load |
| O-ring grooves | around 16 to 32 Ra microinch | Seal integrity without tearing the elastomer |
| Mounting and structural faces | as machined, 63 to 125 Ra microinch | Function does not need better |
| Cosmetic outer surfaces | bead blast or anodize finish | Appearance and handling |
The pattern is simple: only surfaces that touch the process need process numbers.
Why a blanket note is expensive
A note that says 16 Ra all over does three things. It prices the part at the tightest number on any surface. It forces extra operations on faces that never needed them. And it gives the inspector a reason to reject a part that works. On a plate with twenty surfaces, that one note can add more cost than the material.
Name the surfaces. A finish callout on the face, or a table on the drawing with the surface reference and the number, removes the whole argument.
Direction matters more than the number sometimes
On a sealing face, a scratch that runs across the seal line leaks even when the Ra passes. On a sliding surface, lay direction changes wear. Ra is blind to both, so when direction matters, say it: lay perpendicular to the seal line, or no radial tool marks on the sealing face.
How it should be measured and reported
Require these on the report:
- Instrument type, contact or optical
- Cutoff length used
- Number of readings per surface and where they were taken
- The surface reference from the drawing, so the number can be matched to a face
A shop that can produce that report has a roughness tester and a method. A shop that returns a single number for the whole part is guessing.
Finish interacts with everything else
A surface specified after anodize is not the same as one specified before it, because the coating changes the texture. Electropolish removes material and softens edges. Both effects are covered in hard anodize or electropolish, and the flatness side of the same argument is in flatness, parallelism and GD and T on plates.
If you want quotes where the finish callout is actually priced rather than assumed, send the RFQ with the surfaces named and the numbers attached to them.
Frequently asked questions
Is a smoother surface always better?
No. Smoother costs money and time, and past a point it buys nothing. A sealing face, a gas wetted passage and a mounting bracket have three different requirements. Specify each surface for what it does.
What is the difference between Ra in microinch and micrometre?
They measure the same thing in different units. 8 Ra microinch is about 0.2 micrometre. Mixing units on one drawing is a common and expensive mistake, so state the unit next to every number.
How do I know the supplier measured it correctly?
Require the instrument type, the cutoff length and the number of readings per surface on the report. A roughness number without a cutoff is not a measurement, it is an opinion.
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