Accommodating for warped boards.

The current consumer electronics manufacturing climate, which dictates thinner boards and stencils, component placements right to the very edge and panelized assemblies with a significant amount of routing, makes it increasingly challenging to ensure board flatness and coplanarity for a good stencil printing outcome. But, as you know from reading this column regularly, one fact is nonnegotiable: Good printing results require a tight stencil-to-board gasket across the whole of the panel. (See “For Successful Printing, Don’t Blow the Gasket,” October 2018.) This requires the PCB to be flat.

Board warpage – or bow and twist, as I like to refer to it – has always been a key consideration for stencil printing. However, in the past, the 2mm-thick boards being processed were more likely to arrive from the fabricator flat and remain that way through topside printing, reflow and bottom-side printing. The occasional bowed panel was easy to rectify with over-the-top clamps and a good tooling vacuum. Today, however, as consumer PCBs have become thinner, with more routing (interspace) around the supporting panel, warpage is a far more common and vexing issue. Smaller, higher-functioning consumer products have moved us toward 0.6mm-thick boards and stencils as thin as 80µm (and thinner). This combination of factors is making PCB bow and twist increasingly likely and its traditional remedy less than ideal in isolation. Any interspace created during the printing process introduces the opportunity for defects not only with board-to-board repeatability, but also within the panelized PCB from corner-to-corner and side-to-side.

How, then, do we determine warpage limits and correct for the problem with today’s thinner profiles and higher-density designs? IPC has several specifications for measuring bow and twist, but I always go back to the rule employed when I worked in manufacturing: If a 100mm square board is placed on a flat surface and a 1mm shim can be slid underneath any part of it, strong over-top clamping must be employed. If any area exceeds the 1mm per 100mm ratio, a lateral snugger system alone isn’t the best solution because the PCB may pop up past the snugger. In this case, an over-the-top clamp approach is the strongest and most reliable way to secure the board; clamp it down flat, apply vacuum and hold it there for processing.

A flat PCB is critical for robust stencil printing for two reasons:

1. Good alignment. Advanced stencil printing platforms align to +/-12µm @2 Cpk, and a board that moves beyond the clamping system after alignment is susceptible to printing defects.
2. On contact printing. The lack of a solid PCB-to-stencil gasket will result in improper aperture filling and nonuniform release, which can present a range of potential defects, not the least of which is bridging.

So, flat is good, and warpage is bad. With thin boards, ultra-thin stencils and edge-to-edge printing, how do we flatten bowed panels, hold them there for alignment and keep them flat for processing? To be sure, over-the-top clamps are the most robust way to flatten a warped PCB, stabilize it and apply a vacuum to keep it secure through processing. But, the clamp itself has a thickness and may actually create an interspace during thin board printing. Lateral snugging, as mentioned, may not be strong enough on its own to do the job. To overcome these challenges, a clamping technology has been developed that offers multidimensional and adaptable PCB clamping to help accommodate for board warpage and thinner dimensions, while also facilitating other types of assemblies. The board enters the printer and is pushed down with over-the-top clamps. The tooling comes up and holds the board in place with a vacuum, after which it is secured with lateral snuggers. Then, the over-the-top clamps retract, leaving a secured board that can be printed all the way to the edge. Stencil printing can subsequently be carried out with a tight gasket, no false separation and repeatable processing board-to-board and within each panel from front-to-back and side-to-side. The adaptability of clamping mechanisms within a single system offers solutions for PCBs of various thicknesses with top-down, lateral clamping or a combination of both modes for maximum support of multiple board types: from super-thin consumer to stout server boards.

Bow and twist is somewhat inevitable in the fast-moving consumer products market, and the ability to adjust for warpage on-the-fly, while also accommodating larger products, is central to a dynamic, successful and profitable operation. In fact, it’s a flat-out necessity.

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.

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