Not sure how to tackle an alignment issue? These tips might help set you straight.
In today’s slang, a “life hack” is any trick, shortcut, or proven workaround for a given task that increases productivity and efficiency. And, as regular readers will be acutely aware, this column’s focus is all about improving printing productivity and efficiency. So, I thought a series of “screen-printing hacks” might be helpful to engineers, no matter the level of experience. Throughout 2020, this space will periodically delve into various screen-printing hacks; an issue central to a good print outcome will be identified, and I’ll cover some ways to get the process back on track if something goes awry. The first installment of our screen-printing hack series is alignment.
The goal for the printing process is 100% alignment; the solder paste must align with the feature (the pad) on the substrate. When the solder paste inspection (SPI) system indicates this is not the case and offsets (paste not centered on the pad) are present, an alignment issue is most likely the culprit. Where do you start? Here’s a list of the most common causes and potential fixes; i.e., hacks:
Machine calibration. First things first. When was the last time the printer was calibrated? The truth is most operators and line engineers can’t answer this question, which is a bit scary. Nevertheless, when alignment is off, the first thing to do is run SPC for an alignment check on the system. It’s literally a 10-minute exercise that could save countless hours and lost yield. If it doesn’t pass, do a machine calibration – stat! (An annual maintenance program wouldn’t hurt, but I digress.)
Fiducial issues. Potential issues and fixes here are many. In short, make sure fiducials are well-defined, and use as many as possible given the allowable cycle time. To get a read on fiducial readiness, interrogate the fiducial and look at the score (out of 1,000 on our company’s system). A score less than 70% indicates something’s not right, so go down the list.
• Correct size and shape: The machine is looking for something 1.0mm, but if the board fabricator has made it a bit smaller, then the alignment might not be centered. Or, if the fiducial mark isn’t round (most are, although there are the occasional diamonds and squares) and ends up a bit egg-shaped, it can also affect solder paste deposit location. Again, the fiducial score is a great tool for identifying any fiducial shape issues.
• Color: If the fiducial is gold flash, ensure there is a dark background so the camera can see it and accurately find the edge. If the fiducial color is too close to that of the background/substrate color, the camera may struggle to locate it.
• Quantity: A minimum of two fiducials are needed to get triangulation, but some systems accept three or four fiducials. In my book, and especially in the age of high-density designs and miniaturization, the more the better for accurate alignment. The cycle time tradeoff to using more is negligible, so I say go for it, especially with complex boards.
Rotation. So, the machine is in calibration, fiducials check out and offsets are still present. Now what? I suggest rotation. Theta offset is historically an adjustment many operators/technicians tend to shy away from. However, it is often a necessary evil. Dialing in offsets is paramount for good alignment. Estimating x and y offsets is relatively straightforward. Rotation, however, takes a little more reasoning. If you chose to do this manually, my advice is to start by dialing out the theta and then move to the x and y. Full disclosure: I use our company’s automated printing process control technology that, when enabled, makes the rotation adjustments for you. That’s the ultimate hack!
PCB fabrication issues. If, after all these checks, solder paste is still not on the pad, start investigating the PCB fabrication. Our company recently saw an issue with some mobile phone substrates. The solder-mask-defined pads were not, shall we say, very well defined, and the pad openings were not exactly where the Gerber file said they should be. If that’s the case, then you have a good game because the problem could be anywhere. In this situation, I’d use a tool on our machines called “video model.” Instead of relying on fiducial alignment, choose a unique pad defined by the solder mask (i.e., not one in the middle of a CSP!), and use the video model as the alignment point instead of the fiducial. To be fair, this is a heck of a hack and a last resort. But, if the issue is a solder mask definition problem, this is the way to go.
While a whole host of potential mechanical interactions can impact alignment, the above are my top contenders. Next up on the hacking front: bridging.
firstname.lastname@example.org. His column appears bimonthly.is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com);