Guides / Quality

Flatness, Parallelism and Datums on Semiconductor Plates

September 21, 2026

A plate arrives. Every hole is in position, every thickness is in tolerance and the plate rocks when you set it down. Nothing on the drawing was violated, because the drawing never said the thing that mattered.

Size tolerance and form tolerance are different questions

A thickness tolerance of ±0.05 mm says each measured point falls in a band. It says nothing about whether the surface is flat, whether the two faces are parallel, or whether the part is twisted. Those are form controls and they have to be called out separately: flatness on the face, parallelism between faces and where it matters, a profile control.

Datums are the whole argument

A datum scheme tells the shop how to hold the part, how to measure it and how it will be used. Without one, the shop picks its own, which is rarely the one your assembly uses.

A workable scheme for a jig or mounting plate:

Primary datum: the mounting face, the one that touches something in use
Secondary: a locating edge or a dowel hole pair
Tertiary: a second dowel or edge that sets rotation

Then tie hole positions to that scheme. If the assembly locates on two dowels, position the dowels tightly and let the clearance holes stay loose. That single decision usually cuts price more than any material change.

Free state, constrained and the measurement method

A thin plate behaves differently under its own weight than it does bolted down. Say which condition the number applies to, and if it is constrained, give the bolt pattern and the torque. Also say how it is measured, because a CMM touching nine points and a surface plate with an indicator on a grid are not the same test.

Require these lines on the report: method, number of points, the datum scheme used and whether the part was free or constrained.

What it costs to tighten

Requirement	Typical process	Relative effort
0.1 mm flat	Normal milling	Baseline
0.05 mm flat	Stress relieved plate, rough, rest, finish	Moderate increase
0.02 mm flat	As above plus careful fixturing and light finishing cuts	Noticeable increase
Under 0.01 mm flat	Grinding or lapping, often another supplier	Different price class and schedule

The material side of this is in choosing an aluminum alloy and the cost side in what drives the cost of a semiconductor test jig.

The cheapest fix is usually the drawing

Tolerance the face that touches something. Leave the rest. Use one datum scheme across a family of plates so the shop can fixture them the same way. Say free state or constrained. Ask for the measurement method on the report. None of that costs anything at the drawing stage, and all of it removes a rejection argument later.

If you want plates quoted by shops that will state their measurement method up front, send the RFQ with the datum scheme and the form controls on the drawing.

Frequently asked questions

Why does my plate measure flat at the supplier and bow here?

Either it was measured constrained and is being used free, or the material was not stress relieved and moved after unclamping, or it is being bolted to a surface that is not flat. All three are common and all three are avoidable.

What flatness can a shop actually hold on a 400 mm plate?

With stress relieved material and a rough, rest, finish sequence, numbers in the 0.02 to 0.05 mm range are routine. Tighter is possible with grinding or lapping, which is a different process, a different supplier and a different price.

Should flatness be free state or constrained?

State which one you mean. If the plate is bolted down in use, a constrained measurement with the bolt pattern and torque stated is more honest. If it sits on three pads, free state is the right call.

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