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DRILLINGRegistration Techniques for ADVANCED TECHNOLOGYPCBs
Or, so you think you can really do two-track routing? byMICHAEL BARBETTA andJOE DICKSON
This article will review all of the essen-tial steps involved in the proper regis-tration of today’s advanced technologyprinted circuits. We’ve included a regis-tration template that will help PCBmanufacturers grade the capabilitieswithin their own factory, and we’ll dis-cuss steps needed to improve the apti-tude of the systems and processes.The correct registration and align-ment of through-holes, blind andburied vias, and soldermask to theinnerlayer and outerlayer pads isbecoming increasingly difficult withtoday’s tight pitch product, especiallyon high layer-count boards and/orlarge form-factor PCs. Include HDI,sequential lamination, via-in-pad andother complex design requirementsand you assure a challenge to a facili-ty’s registration process.Many producers of PCBs do notfully realize all of the processes thatcontribute to registering a board fordense-featured ASICs and tight tracespacing. Telecommunication products– with their high layers and large panelsizes – are adding 0.8 mm and 0.5 mmpitch devices into their designs. Toimprove signal speed and keep the sizeof the cabinets within spec, they arelooking to introduce two-track routingbetween the BGA/CCGA pads.The latest movements into denserouting techniques have moved thepotential drill-to-metal CAF concerns
into the forefront, ahead of drill-to-drillCAF potential. Low CAF and non-CAFmaterials may help with this concern,but these will also increase materialcosts. In the past, breakout and annularring requirements were for holewallreliability concerns. Now the annularring requirement is an inviolable refer-ence for potential CAF concerns.Before getting into specific areas ofinfluence, it is important to understandwhat is involved in registration, specif-ically innerlayer registration to the pri-mary drill process. The best way tostart on a plan of continuous improve-ment is by bucketing each area, thenmeasuring and calculating the influ-ence. Many PCB manufacturers simplyadd the tolerances of the equipmentpurchased to complete the registrationprocess and call the rest of the variance“material movement.”Some key areas that can influenceregistration but are frequently ignoredare:The rate of rise monitoring ofthe PCBs in lamination (not platentemperature).Trying to adjust forscaling influences without monitoringthis process creates a constant move-ment of the material influence target.Drill depth into backup (smallholes).The registration influence can’tbe measured by a manual caliper (visu-al depth scope), but the drill wanderinfluence can be measured.
Incorrect scaling influences dueto scripting errors by CAM opera-tors. This is a frequent issue if theentire scaling scripts are not complete-ly automated. A small typo can changethe scaling script to zero, but still showscale values on the film. Two-camera PEP alignmentwithout slope compensation. Thesuppliers of this technology have beentrying for years to get PCB manufac-turers to upgrade, but the old equip-ment in many shops was purchasedused, without much real understandingof the process and limitations. If thetargets are offset in one axis (as is thecase in the most popular models), thena scaling influence reset is required or ashift will occur. The PEP will not cap-ture this shift.Hard and soft tooling influencesin drilling. While hard tooling has theadvantage of being automated, with-out vision alignment to the drill head,it does add additional variation.Use of post-lamination x-raysystems incorrectly or not under-standing resolution vs. accuracy.Trying to register multiple depths ofinnerlayers at once without under-standing what resolution error isinvolved is a common pitfall.Lead time after scaling.Thismust be minimized and consistentlycontrolled, especially in high-heat,high-humidity environments. Fre-
PRINTED CIRCUIT DESIGN & MANUFACTUREDECEMBER 200438
