Post Lamination X-Ray of laminated pressed panels not only offers the opportunity to save a misregistered panel during drilling but also provides a method of predicting the registration possibilities before the panel is out of CAD.

The need to know what is happening before, during and after lamination has always been important to multilayer manufacturers so that they can adjust their process to attain the best possible registration. Cross sectioning of the panel could provide some answers however sections only show relationships in one axis and sectioning is a slow tedious process tool. Typical optical measurement systems show deviations from nominal, but only for inner layers. The best method of analyzing registration after lamination is to x-ray the panel.

X-ray has been used in analyzing multilayer printed circuit boards for many years. The old method of using x-ray involved a small sampling of laminated panels that were drilled and then brought to a manual x-ray unit to check how good the drill alignment was to the inner layers. No real measuring technique existed so a person, hopefully with a good eye, was designated to guesstimate what offset to put in the drill program. This confusing operator dependent method often led to more problems than it cured. Although x-ray methods have improved, any method that involves making shifts based on a small sampling is making an assumption that all the laminated panels are exactly the same. The best method of X-Ray is to optimize every multilayer and drill in new tooling holes for the drilling machine. This method of “custom drilling” each board means that every multilayer is optimized and compensated for stretch, shrink, skew or any shift which would affect drill registration to all the layers. New tooling holes are drilled in the optimum drill position for each individual panel achieving a “best fit” without modifying the drill program.

After new tooling holes are drilled into the panel in the optimum position, these panels can be stacked and drilled without the need for additional drill offsets. Since each panel has its own tooling corrections in relation to its internal registration, even two panels which differ in internal registration can be stacked and drilled.

Even though the panels may be offset to each other a small amount (according to their own registration characteristics) when pinned on the new optimized tooling, each panel is in effect “custom drilled”.

Another advantage of x-ray drilling panels before final drill is a major improvement in drill machine utilization. Drill set up time is reduced and the time spent waiting for information from the manual x-ray is virtually eliminated. The down time from drilling a first article then analyzing and estimating the shift with x-ray and putting off set corrections in the drill is totally eliminated and the actual drill utilization increased by that amount.

One effective method used for determining inner layer registration utilizing a Post Lamination X-Ray Drill is to place special targets on every layer in the exact same location layer to layer. Two targets on centerline or four targets near the corners can be used. These common targets form a pad stack which in a perfect world would be the same diameter as the individual targets. When viewing the laminated panel, of course, this is not the case. Instead of perfectly round targets, a blob or out of round stack of targets appears.

The algorithm used to view the pad stack looks at the outline of the target stack and determines a new positional location of the target mass. This is done with either two targets near centerline or four targets near the corners.

Another advantage of x-ray drilling panels before final drill is a major improvement in drill machine utilization. Drill set up time is reduced and the time spent waiting for information from the manual x-ray is virtually eliminated. The down time from drilling a first article then analyzing and estimating the shift with x-ray and putting off set corrections in the drill is totally eliminated and the actual drill utilization increased by that amount.

The machine is actually measuring the target stack and therefore if the diameter of the stack exceeds the set allowable tolerance it will be rejected and not processed. This inspection mode allows the machine to be used as a very fast 100% inspection machine. If any panels exceed the limits set, the operator can go through layer analysis to find out the layer or layers that shifted and by how much. If the target stacks can be optimized so that all the targets are within the pre determined tolerance limit then the panel is drilled with holes for pinning the panel to the drill.

Layer analysis is an important added advantage of the Post Lamination X-Ray Drill since it gives the PCB manufacturer a last chance look at the registration before drilling and final processing the board. The layer analysis coupon is made up of a single (larger) target that is common to every layer. When the package is laminated it is these stacked up targets that are used for optimization and drilling of the tooling holes. Layer analysis requires targets in addition to the common targets. These LAP (layer analysis package) targets are specific to every inner layer and when laminated comprise a coupon which represents every layer. A measurement can be taken from each position in the coupon and the registration of all the layers to each other can be displayed.

This type of analysis offers a level of process control for the fabricator. Layer shifts which can be attributed to material growth or shrinkage, registration tooling issues, lay up problems or shifts during pressing can be identified and addressed. Trend analysis of each layer in the panel and how it relates to the same layer in all the other panels can be graphically displayed allowing changes to be made.

This layer analysis information can be presented in tabular form for each panel or in a few different graphic representations. In any case this information can now be used to determine out of spec cores, to do artwork scaling or drill scale factors before the panel is even put on the drilling machine. It is important to note that the collection of data from the X-ray machine is real time and immediate. Other post lamination registration checks rely on drilled and etched holes to measure the layer shift and ultimate registration. To collect relevant data from this type of test requires a large sampling to eliminate the effect of drill error on the results.

Optimization:

The pressed multilayer represents a work in progress. One would like to know where the layers are with respect to each other, but would also like to predict the fit to later processes, especially mechanical drilling, laser drilling (or laser drilling mask exposure) and outer layer exposure.

There is a need to go beyond Layer analysis, which identifies the shifts and changes from layer-to-layer in a package. Just as it is good to know the attributes of each layer with respect to other layers, it would be good to know the attributes of each composite multilayer assembly within a lot with respect to other layers and panels in the same lot (Run Analysis) or in similar lots made at different times, with different environmental factors or different materials (process related effects) (Statistical Process Control on the lamination process).

By Viewing the data in “Run Analysis” one can easily view the relationship of one layer to another over a period of time or a specific lot of panels.

It is also important to know the characteristics of each layer design with respect to time, environment and material differences. (Statistical Process Control on the manufacture of layers)...

A combination of X-Ray Drilling and Inspection makes it possible to do it all:
• Optimize a laminated panel for drilling or exposure
• Analyze layer-to-layer fit of each panel
• Analyze size differences from nominal of each layer
• Analyze panel-to-panel deviations
• Provide SPC data with which to guide decisions affecting process, material and environmental factors of manufacture
• Provide data with which follow-on processes can be optimized to provide best-fit
• Provide SPC data on lots to show effects of press parameters, pre-preg materials and other process related effects on dimensional integrity.

Any method or process of measuring internal layers and layer to layer registration such as continuity testing , counter-sink drilling to expose layers, cross-sectioning or even conventional X-ray inspection is too slow to use as a 100% process control method. Many of these alternatives to a Post Lamination x-ray drill and inspection system are limited in their ability to deliver data and often give information too late in the process to be used as a production tool. The key to successfully integrating x-ray into your process is immediate results (drilling new tooling holes) and timely reporting of data (Layer Analysis, Run Analysis and SPC).

The Past, Present and Future of X-Ray Inspection and Drilling

Early x-ray systems were used only to show a qualitative image of layer-to-layer. Real time systems became popular in the 80s. X-Ray optimization became an interesting technology in the 90s. Today, at the beginning of the 21st Century, X-Ray inspection in conjunction with optimization is the best technology for informed decisions related to process control, environmental control and material selection. Tomorrow, such inspection on all production lots will become a necessary part of making large multi-layers with 30+ layers with annular rings of 25 microns on boards which mix high numbers of mechanically drilled holes with large numbers of HDI holes. Nothing else will do the job because such boards must be made of inner layers that can be made to fit to each other within a total tolerance of 50 microns. Such a task requires a predictive capability few people have today and requires process control which tightly controls environment and process parameters based on measurement of layers and panels and the construction of an information feed-back system. We are at the limit of material capability, but are forced by price to continue to use essentially the same materials and processes of the past to make unbelievably tight tolerance circuits. . X-Ray Inspection-Drilling and SPC tools help the Multilayer producer bridge the technology gap from today to tomorrow.

Changing Data Into Information

Machine data can be used to feed a predictive engine. My taking into account machine measurements, and correlating these measurements to materials, press cycles and distortion related variations one can accurately predict movement. However, scrap review does not increase bottom-line profits. Accurate information must be delivered accurately during pre production engineering phase. Today, it is possible at time of Zo simulation to now review and perform registration simulation. This closed loop technology ensures much reduced registration scrap, improved delivery performance and higher profits.

For more information Visit :

Michael Angelo (President, Multiline Technology Inc)

E-mail: mangelo@multiline.com
Telephone: 631 249-8300

Leigh Eichel (Vice President, Direct Logix Inc.)

E-mail: leigh@directlogix.com
Telephone: 617-212-8691