screen printing
Successful Printing, Even When the Walls Close In
For 01005 parts, some apertures are better than others.
Continuous reduction in component size has been at the forefront of electronics product innovation, assembly process development and the industry conversation for years. Readers will no doubt recall the papers presented, tools developed, and processes modified to accommodate the “coming soon” metric 03015 and 0201 components. That preparation is essential. In my opinion, however, it is more likely than not that widespread use of these ultra-small chips is far in the future; it will come, but probably not in the next generation.

Another reality presents, perhaps, a more immediate challenge: increasing component density beyond current norms. Realistically, for next-generation mobile phones and wearables, the primary consumers of the most miniaturized components, board designs will continue to incorporate the 01005 chip (metric 0402). There are a gracious plenty of reasons for this, not the least of which are cost and component availability. The challenge for product designers is how to get the most function from chips that may be larger than they would prefer. What’s the solution? Squeeze the 01005s closer together, of course!

High component density is already demanding. Today, the prevailing dimension for the component to component gap (from the edge of one component to the edge of the adjacent component) is 130µm to 150µm. Assuming a 130µm gap in total, each of the 01005s has a keep-out envelope of 65µm. To prevent bridging defects, that envelope must remain intact through printing, placement and reflow. Paste on pad must be precise during stencil printing. Placement force control is essential to avoid material being pushed out beyond the dimensional envelope. The reflow process cannot result in material hot slump. With tight control, current processes are meeting these conditions in high volume. However, the component envelope is shrinking as handheld designs move to increase function while maintaining use of 01005s, meaning more 01005s on the PCB.

Wide view of ASM 01005 test board
Closeup view of ASM 01005 test board
Figure 1. Wide and closeup views of ASM 01005 test board, component gaps of 75µm.
According to technology roadmaps, the component gap will soon reduce to at least 100µm, with a longer-term goal of 50µm. With the surrounding 01005 envelope effectively decreased to 100µm in the near term, and soon approaching 75µm, any deviation in the printing, placement or reflow process that crosses a dimension smaller than a human hair may result in a bridge. Frightening as this may sound, the reality is that printing with 75µm gaps has been achieved with other processes, so we know it’s possible. Printing, though, is not the end of the story; other processes within the assembly operation can violate this barrier if not in extreme control. To accommodate for placement and reflow tolerances, the logical assumption is to reduce solder paste material volume during printing, which would lessen the likelihood of potential issues downstream. With the appropriate stencil thickness and aperture dimension, print volume can be reduced, but this must be done holistically with process balance in mind so we don’t create more problems than we solve.

Our company has studied approaches to maintaining printing area ratio rules, while successfully reducing paste volume on 01005 pads with component gaps of 75µm – or 37.5µm envelopes per component. Using a 0.60 area ratio rule, which is considered acceptable for fine-feature printing, with a 60µm-thick stencil, the aim was to reduce the aperture size to optimize the paste volume and protect against potential defects downstream. Of course, reducing aperture size may not be enough on its own. As discussed in previous columns, the shape of the aperture also plays a role in volume and, potentially, in paste release efficiency. What our analysis revealed was two aperture architectures are delivering success in maintaining area ratio rules while producing sufficient material volume. Using an ASM test board with 01005s and a 75µm component gap (FIGURE 1), the home plate and obround (FIGURE 2) aperture shapes both resulted in a 2.0 Cpk print process. Placement and reflow evaluation, though still ongoing, have shown equally good results.

While the 01005 component has been part of technology designs for some time, and successfully manufactured in high volume, the ever-shrinking gap between these small chips is a new hurdle. Yes, my friends, the walls do indeed feel like they are closing in. As with most challenges in our exceptionally dynamic industry, however, we are already designing solutions to ensure continued high-yield results.

Rounded corner rectangle illustrating obround aperture
Figure 2. Obround aperture design.
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.