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Features Articles

QFNs have advantages over BGAs, but preconditioning specs and test methods are in order.

Why board stretch makes printing tight panels like nailing jelly to a wall.

Recently our company developed some new test printed circuit boards that push the boundaries of today’s technology. These boards – very similar to what one would imagine a smartphone board would look like – are four-up, panelized 250 x 120mm boards packed with several leading-edge components, including metric 0201s and 0.25mm CSPs. As a process engineer who thrives on future technologies, this is really exciting stuff for me.

As our team was developing the print program for these little gems, we weren’t getting anywhere fast in terms of alignment. With features this small, there is absolutely no room for error, and the tolerances for the board and the stencil leave little wiggle room. Add in inevitable board stretch (discussed in a previous column) and, well, it was an exercise much like nailing jelly to a wall: it just wouldn’t stick. One part of the board would be absolutely bulls-eye, but then panel #4 would be out of alignment. When panel #4 was acceptable, then panels #2 and #3 were off. Why? Because there is this blasted thing called Theta!
In my experience, most customers just want to focus on standard x and y alignment. In all fairness, for larger pitches like 0.4mm CSPs, aligning for x and y is usually fine. But, with these highly miniaturized devices, rotation absolutely has to be factored in to accommodate for the ultra-fine pitches and board stretch (and sometimes stencil stretch as well). Using x and y only probably won’t get you there, and that’s certainly what we found with our new test boards. To address the challenge, we used a three fiducial capture software functionality. Using three fiducials, even though it may take a bit more time, ensures true triangulation because three points are now known. There is much better alignment and better rotational alignment as well. The minor time sacrifice will be worth it in the end.

Three fiducials, however, probably won’t be the end of the alignment story with finer-featured devices, and you’ll still be hammering away at that jelly. For this product, all the alignment options in our print platform’s software had to be used. In addition to three fiducial capture, we relied on experience with semiconductor printing and the technique used for wafer bumping. The rule with wafer bumping is to first sort out the rotation and not worry about x and y. If there is any mismatch of the artwork on the board to the stencil, find a happy medium where everything is off by the same amount. Once that’s done and rotation is defined, it’s a simple case of dialing it in to x and y. Too often, process engineers will try to do x and y and then rotation or, worse still, all of it together.

Of course, there may be the case where the board stretch is just too great and simply cannot be properly aligned to the stencil, particularly with highly miniaturized devices. In fact, robust SPC tools can flag boards that are outside of tolerance. The software takes data from the fiducial capture, measures board stretch and can issue an alarm if the tolerance is outside of the specified range. The reality is that board stretch has been accepted because there is some give in the process with larger (relatively speaking) components. But, things are getting really tight – as evidenced by our recent test board experience – and the give is giving way. With the tools we used, good alignment is certainly achievable, and frankly, it’s what will have to be done for sub-0.3mm CSPs and the like. You have to deal with the cards you’re dealt, and right now that means some board stretch. As I noted in a previous column, though, PCB manufacturers will have to solve the board stretch issue as the industry migrates to these smaller devices – and it’s not far off. Otherwise, we’ll be looking at processing singulated PCBs.

As far as alignment goes, the vast majority of manufacturers today are only doing x and y offsets, and for current technology, that should be satisfactory. But I caution anyone reading this column to brush up on their platform’s capabilities and understand how to use rotation and multiple fiducial capture. Without question, 0.25mm CSPs are on their way, and you don’t want to be nailing jelly to a wall.

Clive Ashmore is global applied process engineering manager at DEK International (dek.com); cashmore@dek.com. His column appears bimonthly.

Peter Bigelow

Those that squeeze suppliers to compensate for internal deficiencies are signing their own death certificates.

Mike Buetow

As it turns out, software developers and journalists have a lot in common.

Will future developments in packaging and assembly be joint efforts?

Lessons from the Skunk Works management style still resonate today.

Clarence L. “Kelly” Johnson (1910-1990) was a legendary aircraft designer and aerospace program manager. He and his Lockheed teams, known organizationally far and wide as the Skunk Works, were responsible for creating some of the most celebrated military aircraft in history, including the F-104 Starfighter and the SR-71 Blackbird Mach 3 Reconnaissance Aircraft. Lesser known but still renowned and enduring within the aerospace community are Johnson’s 14 Rules of Management, honed from more than 40 years in the pressure cooker environment of shepherding high-profile, yet top-secret, government contracts from conception to completion.

Johnson’s life and career offer valuable lessons that can be applied to any business, including test engineering.

A good example of this is the use of plainspoken clarity and stick-to-it-iveness as a day-to-day business practice – in written, spoken, and digital words.

Why? Because in our business, what often passes for plain speech and follow-through is more like jargon and lip service. Buzzwords are frequently employed to obscure, confuse and mislead, often in the service of diverting attention, rather than expanding the frontiers of knowledge.

Which is a pity, considering how many of us live and work in the technological center of the English-speaking universe (Silicon Valley). Home of allegedly well-spoken, smart people. In law-abiding, image-obsessed California. Where many of us still harbor barely suppressed yearnings to keep it the acknowledged center of digital supremacy it has always been. Somehow we lost our way, and we need to regain it.

Kelly Johnson’s lifelong motto was “Be quick, be quiet, be on time.” Short. Sweet. Indisputably clear. His 14 Rules of Management amplified that philosophy.

Johnson firmly believed that world-beating results were best achieved by small, highly-talented, firmly-focused, authoritative teams, holding oligarchic responsibility for delivering an extraordinarily complex working product (more often than not a supersonic aircraft) on time, within budget, and to customer (usually the US Air Force) specifications. Those teams managed documentation systems kept deliberately simple and extremely flexible. Procedures were subject to ruthless reinvention as circumstances demanded. Meetings were limited to small gatherings, often no more than a handful of people, to a maximum of 15, thus encouraging open participation and rapid feedback to design changes. Blowhard reports were discouraged; in fact, Johnson hated reports exceeding 15 pages in length. Reporting relationships were short. The customer was kept well – and regularly informed. Costs were carefully monitored. Surprises were minimal. Trust was all: between design chief and team; between prime and subcontractors; between customer and prime.

Johnson’s rules were the result of necessity. Most of the design, technology and materials for revolutionary aircraft like the SR-71 had to be developed from scratch.
The physics behind the specifications demanded it. National security inspired it. The Lockheed team was in uncharted waters, and had to apply its skills to making its own charts. A rigid system of design and contracting rules would never have enabled the crown jewel of American aeronautics, a plane conceived to outrun and outsoar everything shot at it or flown in pursuit after it, to see the light of day. Or night, where the SR-71 often operated. From this fearless willingness to invent anew when confronting technical obstacles, great things were accomplished. Skunk Works designs were significant contributors to the winning of the Cold War.

Back to Earth: What lessons can we test engineers draw from this history?

1. Keep It Simple, Stupid (KISS, another Johnson innovation). Follow a 20th Century variant of Occam’s Razor (i.e., given the choice between a simple path of thinking or action and a more complicated path, choose the former). Example: Avoid being hamstrung by overly-annotated Statements of Work (SOWs) that read like combat after-action reports. I recently received an 18-page SOW from a customer intent on defensively micromanaging every byte, bit and pogo pin of an in-circuit test fixture and program development because of the neglect of a previous supplier. It was a Dutch dyke-plugger’s manifesto in its attempt to catalog every engineering goof of the preceding 10 years. The sins of old are visited on the new.

2. Listen to the Customer. Really. Listen carefully to what the customer wants, however harebrained it may initially sound. Customers have legitimate needs, wants and biases, often borne of bad past experiences. (That’s why they are in your office, right?) Customers also have half-formed (some would say half-baked) ideas, occasionally needing the guiding nudge of hard-bitten expertise to make them real. Either way, respect them. Just take notes and suppress the snarky opinions. There will be ample time to pass judgment later.

3. State what you are prepared to do, then do it, and keep the customer informed while you’re doing it. Avoid the impression your work relies heavily on Black Magic (unless, of course, it does). Too often in the test business, the impression is given, with well-measured condescension, that a genius is at work and is not to be disturbed, by anyone, until the masterpiece is unveiled. And heaven preserve the poor Buyer who simply and reasonably wants a clear response to the question of when their product will be done. The bolder among them might even hazard a query about how it works. Such effrontery is often met today with the email equivalent of a malevolent stare, often in 140 words or fewer. Genius works that way. Rather than promoting hostilities, better to state your intentions in your quote, then back them up in the execution once the order is yours. Back them up again with accurate coverage reports. In all things make clear to the customer exactly what it is they are paying for. Look past the customer’s doltish behavior; they are Our Dolt. Patience, until further notice, is still a virtue. And their checks are still cashable.

Truth in advertising is the exception nowadays. Make it your strength.

4. No less important, declare unequivocally what you will NOT do. Kelly Johnson’s Rule #10 states this definitively: “The specifications applying to the hardware must be agreed to well in advance of contracting. The Skunk Works practice of having a specification section stating clearly which important military specification items will not knowingly be complied with and reasons, therefore, is highly recommended.” Don’t attempt to hide this uncomfortable truth. Better to address and enumerate the technical shortcomings head-on and early, before the recriminations start.

5. Above all else, eschew obfuscation. Say that 10 times fast. Worship clarity in all things, and put theory into practice. (Sounds insultingly basic, but so many of us don’t do it.) This is your lodestar. Everything flows from this. Observe this rule and you will separate yourself from the herd in a business where vagueness is deployed to competitive advantage.

Common sense yet again.

For some, anathema. For others, refreshing. Choose wisely.

Any takers? From somewhere on high, Kelly’s watching.