PTS Coupons for TH Process Control

The parametric test system (PTS) was created by Hewlett-Packard’s Printed Circuit Division in 1987 based on early H-P coupons used in production since 1972. Those early coupons focused on inner-layer shifting, by using the copper on I/Ls shorting to a PTH, moiré patterns, and hole quality cross-sections. Additional influence came from a parametric printed circuit board used as a training and process vehicle for the first NanYa PCB Facility in Taiwan (circa 1983). This PCB had various design-rule technologies on it and provided feedback on how the process was improving.

The H-P PTS was a group of seven coupons that could be placed on production panels or used on parametric panels to provide a snapshot of the capability of the processes. The initial seven coupons (Figure 1) were designed to test:

• (a) Outer-layer registration

• (b) Inner-layer registration and shifting

• (c) Conductors/pads open and shorts

• (d) Plated through-hole and I/L conductors continuity

• (e) Artwork defects

• (f) Soldermask registration

• (g) Etch factors

The coupons were all designed to be tested by facility continuity testers using a bed-of-nails open-short testing machine. In H-P’s case, the tester was an ATG2000 grid tester. The testers fault-file was captured by an H-P workstation and stored. Each coupon had a stored perfect response or netlist that was compared to the fault file and the opens and shorts were translated to dimensional shifts or other parametric data. The RS/1 statistics program was used to produce control charts and statistical reports, as well as historic data. To provide for process control, the coupons were reduced in size and small stand-alone coupon testers were built to allow operators to check the process immediately as a confidence indicator. These home-built milliohm meters worked with a simple 1-amphere power brick, a 4-digit digital panel meter and a machined-Plexiglas coupon holder with 8-spring-loaded gold pins wired to a 4-position rotary switch in a 4-wire Kelvin measurement scheme.

The Gerber artwork for these 7 PTS coupons are in the ZIP file at the end of this BLOG.

Tags: conductors, coupons, etch factors, gerber, PADS, Parametric test system, PCB, plate through hole, PTS, soldermask