Zoom Lens for FPC Inspection: When Continuous Magnification Beats a Fixed Telecentric

FPC (flexible printed circuit) inspection usually starts as a telecentric problem: measure a trace width, check a pad, look at a 10 µm defect. A telecentric lens is still the right answer when the part is flat, the magnification is fixed and the measurement must be dimensionally honest. A zoom lens becomes the right answer when the same station has to look at a 40 mm connector area and a 0.5 mm bond finger in the same cycle.

This page is about the second case, and about the numbers that decide it.

What a zoom lens changes on an FPC line

One station, several fields of view. Instead of a fixed FOV per lens, the magnification can be changed between recipes. A 6.5X or 12.5X body covers a wide search field at 0.58X and a bond-finger detail at 7.5X.

Resolution becomes magnification-dependent. At low magnification the optical resolution is coarser than at high magnification. This is the trade-off: the same lens is fast for finding a defect and adequate for confirming it, but the resolution at the low end does not improve by zooming digitally.

WD stays fixed. Unlike a zoom microscope, the zoom lens keeps a fixed working distance across the range. On an FPC fixture this is what makes the mechanics simple.

Models that cover the FPC range

All values below are transcribed from the published product pages. Blank cells mean the value is not published for that model — treat as 待确认, not as zero.

Optical data at each magnification

Read the table by column, not by row: the resolution at 1X is what you get for a 20 µm trace; the resolution at 5X is what you get for a 5 µm feature.

PMS-LZH-14105K

PMS-LZL-12100

Where the zoom route stops working

• Dimensional measurement of trace width. A zoom lens is not telecentric; if the trace height varies, the apparent width changes with focus. Use telecentric for that step.
• Sub-5 µm defects. Check the published resolution at the magnification you plan to use, not the maximum.
• Large flat panels. Beyond the image circle, vignetting and distortion rise; a line-scan lens is the better platform.

FAQ

Can a zoom lens replace a telecentric lens for FPC inspection?

Only for the detection steps. For dimensional measurement of trace width or pad position, keep the telecentric lens — a zoom lens is not object-space telecentric.

What magnification do I need for a 10 µm defect?

The rule is that the feature should span at least 2–3 pixels. With a 3.45 µm pixel camera, 10 µm spans about 3 pixels at 1X — marginal; the published optical resolution at that magnification decides whether it is detectable.

Does changing magnification move the focus?

On a parfocal zoom lens the focus should hold across the range. If it does not, the lens needs parfocality adjustment before it goes on the line.

Is motorized zoom worth it on an FPC line?

When recipes change frequently, yes: the magnification becomes a recipe parameter instead of a mechanical setup step.

Next step

If the requirement is detection first and measurement second, the zoom route is cheaper. If the trace width must be certified, keep both.

• Motorized zoom lens complete guide
• FPC 10 µm defect inspection (telecentric route)
• Magnification and FOV calculator (2X–50X)