En impedanskupon til flex PCB er et testspor på samme produktionspanel som det rigtige fleksible kredsløb. Den repræsenterer et højhastighedsnet og måles efter fremstilling med TDR. Se time-domain reflectometry; IPC giver kontekst, men tegningen skal definere accept.
Kort fortalt
- The coupon must stay on the same production panel as the real FPC and represent the same manufacturing conditions.
- The target is usually 50 Ω, 90 Ω, or 100 Ω, and ±10% is a common production tolerance.
- The TDR report should include lot number, measured value, calibration date, and stackup revision.
- IPC-6013 and IPC-2223 provide standards context, but they do not replace project drawing notes.
- Reviewing the coupon before mass production is cheaper than explaining deviations after production.
Hvad er en impedanskupon
En impedanskupon til flex PCB er et testspor på samme produktionspanel som det rigtige fleksible kredsløb. Den repræsenterer et højhastighedsnet og måles efter fremstilling med TDR. Se time-domain reflectometry; IPC giver kontekst, men tegningen skal definere accept.
Hvorfor FPC er mere følsomt
I FPC er polyimid, lim, coverlay og kobber meget tynde. 12.5 µm lim eller 10 µm sporbredde kan flytte flere ohm. Derfor er CAD-værdien 50 Ω eller 90 Ω ikke nok uden kupon fra det rigtige panel.
Related reading: flex PCB impedance control guide, flex PCB materials guide, IPC-6013 RFQ checklist。
"For flex, I need the coupon from the actual panel. A 18 µm copper layer, 25 µm adhesive layer or shielding film can move the measured impedance before assembly."
— Hommer Zhao, Engineering Director at FlexiPCB
Hvornår kræves kupon
Kræv kupon for USB, MIPI, LVDS, PCIe, RF, differentialpar eller stigetid under 1 ns. En RFQ på 600.000 stk/år stoppede på manglende tekniske data; FPC får samme risiko uden stackup og kupon.
Kravtabel
| Item | Requirement | Why |
|---|---|---|
| Geometry | same width and spacing | 10 µm can change impedance |
| Reference | same plane or shield | return path controls field |
| Dielectric | same PI, adhesive, coverlay | thin FPC dielectric dominates |
| Copper | same type and final thickness | etch and plating affect width |
| Length | 75-150 mm if possible | stable TDR region |
| Tolerance | usually ±10% | ±5% needs early review |
TDR evidence
The report should show target, tolerance, average value, range, equipment, calibration date, panel reference and lot number. Some buyers connect this to ISO 9000 traceability, but the purchase order still needs the exact acceptance rule.
"Sending stackup, coupon target and TDR requirement with the first RFQ can save three calendar days because the supplier does not have to quote from assumptions."
— Hommer Zhao, Engineering Director at FlexiPCB
Drawing note
Use a specific note: provide a panel coupon for 90 Ω ±10% differential pair on L1 referenced to L2 ground, with the same width, spacing, copper and coverlay as production. Measure by TDR and include lot-level report.
"A useful impedance report is simple: target, tolerance, measured value, lot number and no unexplained deviation."
— Hommer Zhao, Engineering Director at FlexiPCB
Ofte stillede spørgsmål
Do all flex PCBs need coupons?
No. Low-speed FPCs can use continuity and dimensional inspection. Add coupons for 50 Ω, 90 Ω, 100 Ω, ±10% or high-speed interfaces.
What tolerance is realistic?
±10% is common for production FPC. ±5% may be possible with tighter material control and supplier review before layout release.
How long should the coupon be?
Use 75-150 mm when the panel allows it. Shorter coupons can work, but launch effects become more visible in TDR.
Can one coupon cover multiple targets?
Only when geometry and reference conditions are identical. A 90 Ω pair and a 100 Ω pair usually need separate structures.
Which standards should be cited?
Use IPC-6013 for flexible board qualification and IPC-2223 for design context, then add the project target, tolerance and report rule.
What belongs in the RFQ?
Send Gerbers or ODB++, stackup, controlled net list, target, tolerance, coupon requirement, annual volume and requested inspection evidence.
Final Recommendation
Treat the coupon as part of the product. For USB, MIPI, LVDS or RF flex circuits, contact FlexiPCB or request a quote before tooling.
