Miniature parts placed adjacent to “castles” are no printing picnic.

The surface mount industry has encountered many challenges since its introduction in the 1980s. One recent challenge, of course, has been Pb-free assembly. And just as the industry is getting a good handle on Pb-free assembly, another significant challenge is surfacing. In this case, it is adoption of assembly processes for miniature components. A few years back, the industry was struggling to assemble 0402 parts and doubted components could get any smaller. We proved ourselves wrong. Miniature components used today include everything from 01005- and 0201-sized passives to 0.3 to 0.4 mm CSPs and BGAs. To make matters more interesting, these miniature components are usually located adjacent to larger, castle-like components. What is a board assembler to do? Get busy, of course!

One of the first steps in the surface mount process is solder paste printing. Printing has been blamed for 50 to 80% of assembly defects. We may argue over the accuracy of that figure, but everyone can agree printing is critical and highly complex. By itself, miniature components have unique printing challenges. When combined with larger components on the same board, the challenges multiply several-fold. The underlying requirement is a process that satisfies solder paste deposition for small and large components. We call this “dynamic” or “broadband” printing. Broadband printing simply means meeting the solder paste need of larger and smaller components on the same board using existing or new material deposition techniques.

Although this is an industry-wide issue, no significant study can be found in the public domain. Nevertheless, both IPC and iNEMI have identified broadband printing as a significant challenge and have programs in place to address it. I have no doubt many OEM and EMS providers are racing the clock to solve this problem, but no simple, readily available solution is on the horizon. Indeed, the approaches are many and include step stencil, overprint of selected components, two-step printing passes, augmented stencil printing, and so on. No single stencil design can meet the needs of all board designs, and each method has advantages and disadvantages. Let’s examine the advantages and disadvantages of each type of approach.

Step stencils. Step stencils are not new to SMT. Applications include step-up stencils for ceramic BGA and through-hole components; relief pockets for raised via, glue printing and additive trace; two-print stencils with relief pockets for low profile components, and so on. This stencil technology, with some modification, can be applied to boards with variable-sized components, with some limitations. One key limitation of step stencils is the keep-out distance as outlined by IPC-7525 standard. If the PCB design meets the recommended keep-out distance between various components, step-up or step-down stencils may provide a solution. Even though the step stencil is a low-cost solution for broadband printing, it cannot handle a PCB above a certain component density. Also, PCBs with metal shields encircling miniature and fine-pitch components are not good candidates for step stencils.

Overprinting. Solder paste overprinting for through-hole intrusive reflow has been practiced for years. In this technique, an oversized aperture is designed for pads that need more solder paste relative to the rest of the board. The overprint size will depend on component type and keep-out distance between various components. Much work is needed to understand the limitation of overprinting larger components in the presence of miniature components.

Two print stencil. When overprinting or step-up/step-down stencils don’t provide enough solder paste for larger components, or are not practical due to a densely populated board, two-print stencils may be the only viable option. In a two-step stencil process, the first printing step uses a thinner stencil (0.003" or 0.004", for example) to deliver paste for miniature components. The second step uses a thicker stencil with relief etch pockets etched on the contact side of the stencil for miniature components. The thicker stencil’s thickness depends on the type of components requiring more paste.

Augmented stencil printing. Augmented stencil printing uses a combination of stencil printing and dispensing. This may be appropriate when the board has few larger components. In this case, a single thickness stencil can be used for printing small parts, followed by dispensing for the few larger components.

The options described work under certain conditions but have severe limitations. Some affect throughput, cost and quality, or may not work for some applications. The industry needs a new answer for material deposition.

The industry thus far has been extremely creative and innovative in solving SMT assembly problems. I have no doubt we will rise to the occasion and develop new technologies that not only solve current problems but open up new, exciting applications to come.

Rita Mohanty, Ph.D., is director advanced development at Speedline Technologies (speedlinetech.com); rmohanty@speedlinetech.com.