columns | December 19, 2005

Microvia technology inspires the miniaturisation of sub-assemblies

Semiconductor components continue to astound with their ever-finer structures, while maintaining the same high level of performance.
They are essential in setting the pace of electronic developments and the circuit board is under pressure to keep up.

From the automobile industry through to industrial electronics, the importance of microelectronics has risen enormously in recent years. At the same time, it has developed into an essential feature of intelligent devices and systems. The demand for reduced volume and weight, enhanced system performance with shorter signal transit times, increased reliability and minimised system costs have become progressively more important. And as a consequence, this means that heightened demands are placed on developers and layout engineers.

While microvia technology has long since become a telecommunications manufacturing standard, it is now penetrating other market segments. Here microvia technology offers the potential to completely fulfil the demands for technically perfect solutions and rational production. In other words - this technology unites modern technology and economics. Looking at the circuit board industry in the cold light of day, it is apparent that it has a cost-efficient, safe and proven technology at its disposal. With the aid of microvias, the integration of modern components on the boards requires only minor modifications to the multi-layer architecture. Many of the requirements placed on electronic products can be realised without problems as a result. HDI (High Density Interconnect) involves using microvias for high density interconnection of numerous components and functions within a confined space. Microvia circuit boards manage without conventional mechanically drilled through contacts and use the appropriate laser drilling machines as drilling tools.

The drivers for HDI microvia technology are the various component formats, such as COB (Chip on Board), Flip Chip, CSP (Chip Size Packaging) and BGA (Ball Grid Arrays), which are described in terms of "footprint" or pitch. The footprint characterises the overall solder surface, connection surface or landing sites for SMD components. Pitch denotes the separation between the midpoints of the individual solder surfaces. Many new components arrive on the market with a large number of connections and a low pitch which demand a further increase in wiring density on the circuit board. This demonstrates why the challenges facing the technical knowledge of the circuit board designer and the implementation options are so infinitely important for new components. Because even at this early stage, the profitability, as well as the rational technical feasibility and process compatibility of the boards, are decided. This highlights how strongly circuit board development is influenced by the development of components and their geometric design.

In the past, microvias were still staggered relative to one another as a means of achieving contact over several layers. New techniques, with which microvias generate connections across two layers, have become established as particularly cost-effective and efficient in their manufacturing technology. These holes can be produced in one program starting from the outer layer.

Cu-filled microvias represent the latest development ready for series production. Special feature of this technology: The vias can be set directly on top of each other. With this method it is possible to layout components even in very confined geometries.

When are microvias worthwhile?

No textbook specifies where the transition between mechanical drill holes and laser holes is to be found. After all, the application of microvias is not only determined by the technology or the geometry of the components and consequently the circuit board geometry. However, questions concerning profitability can be clearly answered through the application of microvias. In the light of Würth Elektronik's experience from today's perspective, a clear technical boundary can be drawn at a BGA pitch of 0.8 mm. Here conventional technology, with mechanically drilled vias, meets its limitations and the use of microvias (laser drilled blind vias) is necessary.

Naturally however, economic considerations for or against play a significant role. A comparison of variable drilling costs reveals the superiority of microvia technology over mechanical drilling (Ø 0.3 mm) even with a relatively small number of holes. The 100x faster drilling speed and the tool costs approaching ZERO make laser drilling extremely fast and cheap. This effect becomes more pronounced as the number of drill holes increases. The comparison clearly illustrates the cost saving potential via technology has to offer. Experience at Würth Elektronik shows that the proper application of this technology results in savings of between eight and ten percent of the overall costs of "conventional circuits". The advantage of via technology grows beyond measure if smaller drills have to be used for geometrical reasons. The drill unit costs rise dramatically. And the service life of the drills plummets. The cost differential opens up enormously for Ø 0.1 mm mechanically drilled vias compared with Ø 0.1 mm laser drilled microvias. Here the variable costs are in a ratio of around 500:1.

A simple order of magnitude comparison between a conventional drill and a microvia impressively illustrates the relationship between the relative areas and the space requirement.

How is also important

In the case of conventionally drilled "dog bone" structures with a 0.8 mm BGA pitch, the drilling pads are so close to the solder pads that process fluctuations can lead to process problems. A slight shift in the solder-stop mask, excessive movement of the drill or an instable outer layer structure can ultimately mean that drill holes are inadequately protected by the solder mask. These vias can then exert a capillary force on the solder pad. Poor soldering results would then ensue.

Furthermore, as a result of the geometric configuration of the solder and drilling pads, the outer layer is not available as a component layout plane. A similar configuration can also occur with microvias. The solder pads are connected with the vias by means of the established "dog bone" structures mentioned. The aforementioned problems are noticeably reduced. Nevertheless, this configuration may also mean that a component layout on the outer layer is not always practicable.

By far the most elegant solution is the "via in pad". Here the microvias are drilled directly into the pad. This opens up sufficient space between the solder pads for routing conductor tracks.

Comparing the three variants: mechanically drilled vias through the complete circuit board thickness (separation BGA pad to via pad = 110µm; conductor track routing: only in the outer rows), microvias distributed in a dog bone structure (separation BGA pad to via pad = 190µm; conductor track routing: restricted!), microvias drilled directly into the BGA pad (separation BGA pad to via pad = 780µm, conductor track routing: trouble-free

As a result of miniaturisation, the circuit board surface available for component assembly is of major importance. With a conventional component layout using mechanically drilled vias, the drilling density in a BGA precludes assembly on the opposite side of the board. The inside layers, especially the ground and power planes, are also affected by the vertical drill holes. Microvias, on the other hand, simply establish the necessary contact between two layers. As a result, circuit boards can be assembled double-sided within a confined space.

New memory chips, such as the SRAM from Samsung, have often been integrated into existing layouts. In such cases, the design of the component with 0.75 mm BGA pitch predetermines the use of microvias. It is now established practice to analyse the profitability of a redesign when integrating such components. Only the appropriate application of microvia technology opens up the cost-saving potential whose full extent only becomes apparent on close examination and detailed calculation.

If components with an even finer pitch are used, such as the PXA 26x processor family from Intel, single-layer microvias are no longer sufficient for the component layout. This component is equipped with 294 pins in six successive rows and has a pitch of just 0.650 mm. In this case, three levels are required for connector routing.

Laser drilling from layer one to layer two and, at the same time, from layer one to layer three, opens up these three layout planes in a drilling program. As a special feature, the microvias were placed directly on plugged buried vias to achieve the shortest path to the supply voltage and the ground connection.

Today we work with a BGA pitch of 0.5 mm. This dimension is prescribed, for example, by the 10 Gbps XAUI transceiver from TI Texas Instruments. All structural dimensions are reduced to a minimum in this case and are precisely intermatched. The staggering of the solder-stop mask to the solder pad illustrates extremely clearly just how tight the manufacturing tolerances really are. A mid-point displacement of around 30 µm results in the solder mask almost touching the solder pad. From today's perspective, this attains the finest possible BGA pitch that is reproducible under series conditions.

"piggy-back procedure" with the Panasonic Bluetooth module BTZ 4002A

The Panasonic Bluetooth module BTZ 4002A is introduced as the final example of the dynamic nature of miniaturisation. This modular circuit board optimally exploits the advantages of microvia technology in combination with open metalisation of the edges(castellation). Especially remarkable: the modular nature of the board. Complex sub-assemblies are integrated on a fine structured circuit board in a "piggy-back procedure" to ultimately form a simple and cost-effective circuit.

The road to successful integration of modern components increasingly runs towards microvia technology. The cost saving potential offered by microvia technology is of growing importance here - before the technological barrier advanced by the use of microvias is reached. Along with the demand for higher transmission speed and therefore an increase in data rate, other tasks are on the agenda, such as signal integrity. In combination with other techniques, such as heat sink and embedded passives to name just two, microvias open up new options and application areas for which the technology can be optimally deployed.

Roland Schönholz
Würth Elektronik GmbH and Co. KG
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