Data Sheets Are Incomplete Guides
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The document discusses how data sheets provide incomplete information for complex circuit board designs. It recommends consulting a manufacturer to determine the optimal materials and fabrication processes. While multiple materials may have similar specifications on paper, an experienced manufacturer is needed to identify which will actually meet requirements like tight tolerances, controlled impedance, or etching processes. The document emphasizes how data sheets alone cannot account for the intricacies of achieving specifications like IPC Class 3A tolerances, and that only certain materials are suitable when both tight tolerances and etching are required.
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Data Sheets Are Incomplete Guides
- 1. Data Sheets Are Incomplete Guides DESIGN FOR MANUFACTURING The Leader in QuickTurn HDI PCBs www.protoexpress.com
- 2. Data Sheets Are Incomplete Guides •Consult a manufacturer before beginning any complex board design •Consider speaking with your manufacturer for projects involving controlled impedance, unusual operating environments, high voltage, high current, or tolerances tighter than IPC Class 2. •Your manufacturer can help you determine the optimum material set and stackup. © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 2
- 3. Data Sheets Are Incomplete Guides •Information available in data sheets is just a starting point for predicting material behavior during fabrication •Competing materials may have similar characteristics but only an experienced manufacturer can identify the best fit for your project •Consider, for example, polyimides, which serve high-temperature applications and have low loss © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 3
- 4. Data Sheets Are Incomplete Guides •Military and aerospace customers sometimes specify a particular polyimide material, though often they simply call out one of the three IPC slash sheet numbers that pertain to those materials, such as IPC-4101/41. •There are many comparable materials from several suppliers that all conform to that standard © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 4
- 5. Data Sheets Are Incomplete Guides •Mil/aero customers typically require fabrication to satisfy Class 3A requirements, the most stringent IPC tolerances. •It is routine for those customers to also specify positive etchback for through-hole vias © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 5
- 6. Data Sheets Are Incomplete Guides •Etchback results in a three-point connection between a via barrel and inner planes, something like a mortise- and-tenon joint in carpentry. •The etchback processes used to bolster the connections are well established, but somewhat complicated, requiring precise control of various chemistries and exposure times • A small area at the upper and lower surfaces of each inner copper plane is uncovered by using a plasma etching system to remove of up to 3 mils of resin between them © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 6
- 7. Data Sheets Are Incomplete Guides •Following etchback, there are upward of 30 baths and rinses leading to the electroless deposition of copper as a seed to bind copper plating to the holes and to the areas of the inner layers within the bores that were exposed by etching back the dielectric. •How do those minute details about etchback relate to seeking design advice from a board manufacturer at the outset of a project? © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 7
- 8. Data Sheets Are Incomplete Guides •There are many alternative polyimide materials whose published specifications are very similar to one another. •If you were to pick one independently based on those specs, architect a stackup, and devote months to perfecting a design that works fine in simulation, you would likely wind up with an unmanufacturable product, if etchback and Class •3A tolerances are key requirements © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 8
- 9. Data Sheets Are Incomplete Guides •Why? Because IPC Class 3A stipulates that wicking cannot exceed 50 microns (just under 2 mils). •Wicking refers to the migration of copper into the dielectric. •The plasma etchback, alkaline bath, and the chemicals that remove the glass fibers must be very carefully controlled to minimize wicking. © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 9
- 10. Data Sheets Are Incomplete Guides •Compared to the Class 3 limit for wicking, which is very demanding, the Class 3A tolerance is 40 percent tighter. •If you want through-hole etchback and fabrication to meet 3A requirements, only certain of the polyimides that conform to IPC-4101/41 can meet the wicking tolerance. •It takes an experienced manufacturer to know which material is suited for such needs. © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 10
- 11. VISIT SIERRA CIRCUITS CALL SIERRA CIRCUITS TO ARRANGE A TOUR » Amit Bahl, Director of Sales and Marketing Sierra Circuits, Inc. 1108, West Evelyn Avenue Sunnyvale, CA 94086 Phone: 408-735-7137 Fax: 408-735-1408 Toll free: 800-763-7503 Office Hours: 8 a.m. to 5 p.m. Pacific Time, weekdays TO KNOW MORE ABOUT OUR PCB MANUFACTURING & ASSEMBLY CAPABILITIES VISIT www.protoexpress.com © 2012 Sierra Circuits, Inc. All Rights Reserved. | www.protoexpress.com 11