Rethinking Inspection for the UHDI Era
Increasing circuit density is pushing traditional inspection, verification and validation methods to their limits and creating a need for new approaches.
When does technology advance to such a degree that the industry needs to pause and rethink how it produces products and then verifies and validates that those products will reliably perform as intended and marketed?
Over the decades I have spent in the electronics industry, I have marveled again and again at the technological advances – and the pace of those advances – that continually take place in our industry. From single-sided circuit boards in the late 1950s to multilayer boards, then microvias, high-density interconnect (HDI) circuits, and emerging ultra-high-density interconnect (UHDI) and organic substrates, the pace of innovation has seemed breathtaking. And this pace is not limited to circuit boards; memory has transitioned from circuit cards in the 1960s to high-density chips that are now common and continue to advance technologically.
The demand for more capability in smaller packages drives the innovation that enables continually advancing technology. And as technology advances and package sizes shrink, the industry keeps asking for more of both. While design engineers keep advancing designs and material suppliers keep developing newer and more advanced materials to enable those designs, manufacturing processes and verification and validation methods too often remain unchanged and simply get pushed to greater extremes.
One example of product verification and validation can be seen in most quality departments in our industry. On a shelf in the back corner of the room sit optical microscopes that can magnify up to 60x. Next to those may sit one that can magnify up to 200x. However, at workspaces where employees perform inspection measurements, electronic microscopes can provide magnification of 1000x or more. The technology is basically the same, but with the small features of current technologies, the old 60x and 200x microscopes just don’t cut it.
The same can be said for many other longstanding verification and validation methods, such as cross sections. The process of making a cross section is the antithesis of what the electronics industry is about. The process involves taking a circuit board, cutting a piece with a saw, placing it in a mold and pouring resin over it, grinding and polishing it after the resin sets, cleaning what is referred to as a “puck,” and then examining and measuring it. So how well does this inspection technology, or the use of ever-increasing microscope magnification, work to verify and validate processes or products in the world of ultra-high-density circuitry? Most likely, marginally at best.
Very possibly, the level of technology that our industry is producing and further developing has reached a point where, together as an industry, we need to pause and rethink verification, validation and quality inspection: why we do it, how best to do it, and what new tools we should develop to assure consistency.
For such a paradigm shift to take place, the industry must start by reviewing why it does what it does. With advances in technology, what needs verification: reliability, robustness or consistency? Considering a product’s size, thickness and materials, what is its most vulnerable attribute, requiring the greatest focus and effort to verify and validate?
Do different types of technology require radically different test procedures and equipment? The industry has inspected microvia printed circuit boards using basically the same equipment and processes as traditional multilayer circuit boards. Possibly, the too-frequent microvia failures result in part from the processes and equipment used to inspect the circuit.
As more high-density and ultra-high-density technologies become mainstream in electronics, using equipment and inspection processes developed for standard products to verify and validate them may miss critical failures, similar to what has occurred with microvias. Possibly, the entire inspection, verification and validation process needs development not after a product has been designed, but while the product is being developed.
The same can be said about in-process inspection. As circuit density, especially on innerlayers, increasingly tightens, the industry should rethink traditional visual and cross-section analysis. The cutting or grinding tolerances involved in creating a cross-section alone may be greater than the area under inspection. Equally, the time required for traditional in-process inspection delays corrective process changes.
So, how will our industry assure reliability, consistency or robustness as the technologies it produces become ultra- or ultra-ultra-dense? A wide variety of sensors and the ability to more easily create software are now available that simply did not exist a decade or several decades ago when the industry established traditional test, verification and validation protocols. Now would be a good time for the industry, working together, to pause, rethink and create a better inspection paradigm for the increasingly dense and small technologies of today and the future.
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Peter Bigelow has more than 30 years’ experience as a PCB executive, most recently as president of FTG Circuits Haverhill; peterbigelow@msn.com.