Newly developed, dedicated purpose precision milling machinery simplifies rework.

BGAs, CSPs, flip chips and other component packages on handheld devices are commonplace today in consumer, military, and industrial products such as tablets, smartwatches, and handheld computers. These handheld products, along with transmission control modules, cameras and image sensors in the automotive sector, use underfill to withstand mechanical shock or impact that takes place when devices are dropped or struck. Underfilling these component packages creates a compliant layer between the package and printed circuit board that increases the reliability of the interconnections even when subjected to outside forces.

Underfill is a polymeric material used to fill the gap between the PCB and underside of the electronic component package, thereby surrounding and protecting the solder joints. This material boosts the reliability of the component, which is subject to mechanical impacts and shocks by distributing the forces. Thermal stresses caused by the coefficient of thermal expansion (CTE) mismatch between the component and PCB are lessened using underfill. Typically, underfills have a high modulus (E) matched CTE with respect to the solder, as well as a high glass transition temperature (Tg). Underfill dampens and distributes the stress more uniformly on the solder joints, thereby increasing interconnection reliability. As pitch of components such as CSPs becomes tighter, the standoff height between the bottom of the device and PCB lessens, thereby reducing the PCB level reliability.¹

A “rougher” foil may improve stencil performance.

A high-performing stencil printing process deposits the right amount of material volume in the right place, at the right time, and at the lowest cost per print achievable. Every assembly professional strives for this utopia, leaving no solder paste stuck in the apertures or smeared on the underside of the stencil. Naturally, with all the variables, this state is difficult to achieve 100% of the time. A perfect gasket (board to stencil) does not exist in the electronics manufacturing real world. Transfer efficiency is managed through aperture designs to provide the desired material volume on the pad, and solder paste smear (or its potential) is alleviated by cleaning the underside of the stencil between prints to avoid bridging. Cleaning, of course, comes at a cost – both in consumables use and in production time. If more high-quality prints can be achieved between necessary cleans, consumables overhead will be lower and throughput will be higher. 

Additive manufacturing might not be ready for prime time, but it’s making inroads.

Sometimes I find myself walking around the shop floor asking, “Why do we have all this very expensive equipment? There must be a simpler, cheaper way to make a printed circuit board!” And yet, despite phenomenal technological strides, our industry still uses the same basic manufacturing processes since the earliest days of circuit board production: drill – image – plate – press – repeat – then route.

Observing so many different processes, simple logic might make you think printing conductive ink would have replaced plating processes long ago. Yet while printed electronics has advanced considerably, it is not ready for prime time for all applications.

Relying on a single source is a recipe for failure.

“Good, fast and cheap … pick two” is an old maxim that applies – to a degree, anyway – to the printed circuit board industry.

The implications, of course, are that if it’s fast and good, it’s going to be expensive; if it’s good and cheap, it will require lots of time; and if it’s cheap and fast, the quality will be poor.

PCB buyers should keep this in mind when choosing vendors and avoid relying too much on one supplier if they want good quality boards delivered on time and at a reasonable price.