HKPCA Journal No. 10
1/5
Controlling Multilayer Registration
Jim Dermody
Operations Technology, Inc.
T H E P R 0 B L E M
How does one optimize the multilayer fabrication process for best registration of layers and drill
patterns? As advances in all aspects of circuit and component technology increase the density of
interconnections, more accurate means of layer registration are required. At these densities, the
accumulated errors - from plotter calibration and film stability - from artwork and post etch tooling -
especially from lamination - and also from drilling - require careful attention to maintain yields at a
profitable level.
When assessing the quality of panels in trial runs, the timeliness and accuracy of corrective
measures are essential. Leading PCB manufacturers are now turning to automatic measuring
systems employing X-ray imaging to quickly determine the corrections needed. Once those
corrections are applied, verification is required
to maintain quality and catch process drift
before it impacts yields.
Once the panel is laminated, the relative
movements between innerlayers are fixed.
Following that step in the process, the sooner
the amount of misregistration is determined,
the sooner corrective action can be taken.
Historically, multilayer PCB fabricators have
attacked the registration problem from several
directions and achieved success in varying
degrees. Tooling can only maintain registration
to a certain precision. During the lamination
press cycle, the layers shift and stretch –
often dramatically. Uneven temperature and the dynamics involved when the pressure phase is
initiated can cause problematic distortion of the layers.
A range of techniques has been devised to quantify the amount of misregistration present in a
laminated panel:
Optical and laser-based probe techniques are of no use, because they cannot penetrate the copper
bottom and top layers.
The time required to prepare and measure cross-sectioned samples can be daunting. At the least,
the sampled sections must be prepared and measured for both X and Y axes. Proper preparation
of cross-sections is something of an art. When the best skills are not applied, the results can be
misleading. When using measuring microscopes to determine the amount of mis-registration,
human error further reduces measurement accuracy. Operator guided measurements always incur
some degree of subjective judgment. Short-cut methods that skip the potting and polishing very
often give deceptive results. Even those methods that involve angled cutting through coupons to
reveal the layers have their problems. The inherently “noisy” imaging characteristics of the cut
material make measurements uncertain. The copper is often smeared while being cut, further
reducing the accuracy of the measurement.
