SCREEN printing
A Different Take on Tooling
Reconfigurable with dedicated-like support.
AS BOARD COMPLEXITY has increased with decreasing pitches, thicknesses and component sizes, ensuring support for thin, high-density substrates – essential to cost-effective, pinpoint accuracy stencil printing – continues to pose challenges. Using vacuum to secure miniaturized assemblies is, for the most part, a successful technique but requires the use of dedicated tooling plates, which can be costly. Considering the quantities of dedicated tooling blocks needed in a high-volume manufacturing environment, finding a suitable, lower-cost alternative has been a longstanding ambition. And, while commercialized automatic pin-based tooling systems are a good option for some applications, they are not as effective for high-density, thin boards.

How, then, do we bridge the gap and provide similar quality substrate support without requiring a dedicated tooling plate for each product and each SMT line? One solution lies in a high-flow vacuum system that supports the PCB – no matter how densely populated – through an almost counterintuitive use of airflow, low-pressure vacuum and reconfigurable metal plates. (FIGURE 1). The plates – which are tooling height, approximately 2.0mm thick and constructed of different lengths – can be configured and overlapped to form a box, the top of which is constructed slightly smaller than the PCB perimeter so the edges of the substrate sit on the frame. The rising table contains a vent, and support pins are placed for stability. Once positioned, the tooling cube creates a semi-sealed environment where the vacuum pulls air through the table vent to create substrate stability during the print cycle. Unlike a conventional vacuum connected to a tooling plate, which uses a sealed technique to generate incredible pull (trust me, don’t get your finger anywhere near the vacuum pipe!), this new approach floods the area with tremendous amounts of air, allows for leakage (unlike dedicated plates) and securely holds the PCB with low vacuum. While there is upfront time to set the plates in the desired location, this system provides the support needed for thin, high-density, heavily routed PCBs without the expense of dedicated plates, and it can be reconfigured for an infinite number of board sizes.

close view of a high-flow vacuum tooling mechanism
Figure 1. High-flow vacuum tooling provides a reconfigurable, lower-cost solution than dedicated tooling plates.
Using this setup with pallets is also effective. For high-volume assembly of smaller PCBs for mobile phones and wearables, many manufacturers have moved toward use of pallets and secure boards in the pallet using tape or clips. Typically, with pallets, no vacuum is employed for substrate stability during the print cycle. As many of the PCBs for mobile products are so intricate, pallets have emerged as a more cost-effective and viable option than dedicated tooling and enable the alignment critical to high yield. One drawback is that without any vacuum securing the pallet, when the stencil separates from the PCB, the board tends to follow the stencil, and the lack of clean separation can negatively affect material transfer efficiency. This situation can be resolved with the new tooling approach. Employing the high-flow vacuum tooling cube, the pallet is framed by the edges of the plates and secured with low vacuum force, which helps encourage clean stencil separation.

To analyze print quality and repeatability of PCBs produced with high-flow vacuum tooling, our company conducted internal testing to evaluate solder paste deposit height, area and volume, the standard stencil printing process KPIs. Means and standard deviations were examined from a Cp/Cpk point of view, with Cpks over 1.33 (4 Sigma) being the baseline acceptable benchmark minimum. Our test board contains an array of technology including 01005s, 0.4mm-pitch CSPs, 0805s and 0.4mm-pitch QFPs, and has 0.55 area ratios that push the boundaries of acceptable limits. To be considered a viable process tool, all tested boards must pass the Cpk threshold. With the novel solution, every board passed and illustrated that the system delivers results comparable to dedicated tooling.

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.