SCREEN printing
One for All
Precision mass-alignment of singulated substrates.
DISCUSSION AROUND SMALLER devices, complex designs and manufacturing challenges as a result of miniaturization: a never-ending story, isn’t it? Truth is, just when it appears the industry has hit a wall in terms of capability, we find a way forward. Yes, miniaturization is rolling on, and the industry continues to overcome perceived obstacles, this time enabling a higher accuracy approach to mass processing of singulated substrates.

Several years ago, the general thinking was components would keep getting smaller. The prevailing view was that by this time, the metric 03015 and the metric 0201 would be working their way into mainstream production. Although the processes to accommodate these small devices have long since been developed, it will likely be some time before they appear on a majority of BoMs. What is happening, though, is manufacturers are trying to eek out slightly more with standard 01005s by placing them closer together, creating a much narrower gap from the edge of one component to the edge of the next. (See “Screen Printing,” December 2020.) These narrow gap designs – which today see pitches of approximately 100µm with 75µm on the horizon – in combination with the other elements of miniaturization require much tighter alignment tolerances in the stencil printing to ensure solder paste hits the pad target.

view of carrier
Figure 1. Substrates are transported into the printer on a carrier and align with individual towers.
view of carrier with PCBs raised to print height
Figure 2. PCBs are raised to print height using specialized motors and customized towers to enable precision alignment.
Achieving pinpoint printing alignment on miniaturized narrow gap assemblies would be completely doable if the standard protocol were to print one substrate at a time. However, to achieve required volumes and cost-efficiencies, boards are most often processed in a panelized fashion; this is particularly the case with smaller, mobile-device-type PCB assemblies. Because FR-4 substrates can succumb to fabrication-induced board stretch, bow/twist and a host of other issues that impact printing alignment, the printer generally conducts a “best fit” alignment routine, taking the global fiducials and aligning the stencil to the entire panel. If the board has no issues, best fit is a very good fit, and it’s off to the races. Conversely, if there is stretch on the PCB across the circuits, then a “best fit at best” alignment is the only option. In some cases where board stretch is present, alignment can be up to 100µm off the paste to pad across a 120 mm-long board. Twenty years ago, a 100µm deviation was fine, but definitely not in today’s narrow gap, 150µm-wide pad dimensional reality. So, how do we solve the dilemma of multi-substrate processing with individual substrate alignment using a single stencil?

Singulated substrate, mass printing is the clear solution. Printing singulated substrates in a tray format has been employed in semiconductor packaging for some time. Through use of specialized carriers and tooling, individual substrates organized as multiples in a single carrier are transported and aligned for processing with whatever printing media is required: solder paste, die attach paste, flux or thermal interface materials. Several mechanical solutions exist in this area. For consumer electronics, however, constraints of the substrate thicknesses and outline dimensional tolerances are not compatible with traditional mechanical mechanisms.

This reality has led to innovations in mass-printed, singulated substrate processing. Using optical alignment and specialized actuators, a high-speed technique for printing multiple substrates with individual alignment in a panel format is one solution for very high-density PCBs. With this technology, PCBs are placed individually into a multi-substrate carrier plate, and each PCB is married to its own tower, which is powered by an actuator facilitating individual movement in x, y and θ. Fiducial measurements of the stencil are taken to ensure stencil to tower alignment, after which substrate-loaded carriers are transported into the printer, PCB fiducials are identified, and individual towers simultaneously raise and custom fit the PCBs to the stencil (FIGURES 1 and 2). The printing operation takes place, and the substrates are lowered back into the carrier.

There are, as noted, many approaches to singulated substrate processing, and each has benefits for specific applications. For high-volume, high-speed, precision narrow gap printing, however, the latest development offers a viable solution to the board stretch, miniaturization dilemma. Once again, the electronics industry has enabled small-dimension processing to take a big step forward.

Clive Ashmore headshot
CLIVE ASHMORE
is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.