"Microvias" is the first comprehensive book on microvia technology. It should perhaps be labelled, like software, "Microvias +", or even "Microvias ++." The experienced authorial team of Doctors John Lau and Ricky Lee have produced a comprehensive guide to all aspects of microvia technology. But their book goes beyond just the microvias, to also address related areas in lead-free solder, solder bumping, assembly, and the reliability of low-cost, high-density assemblies using microvias.

The introductory chapter gives an overview of microvia technology and its advantages. The rapid growth in the production of microvia substrates is driven by high volume, hand-held applications such as cell phones, pagers, and laptop computers. More than 75 producers of microvia substrates turned out over $1.5 billion in microvia substrates in 2001. By 2005, worldwide production is forecast to exceed $7.5 billion. The reason is simple: the coming generation of low cost, high performance systems requires microvias to make their chip-on-board (COB) and wafer-level chip-scale packaging (WLCSP) cost-effective.

The following chapter is an 85-page in-depth review of all aspects of "conventional" printed circuit board (PCB). From that base, the narrative marches through the five routes for creating microvias: mechanical drilling, laser drilling, photoimaging, etching, and conductive paste/ink filled microvias. The mechanical drilling chapter includes excellent comparison charts of the capabilities and costs of the wonders to follow. Mechanical drilling itself is quickly rejected, as generally unable to meet the microvia limit of 0.15 mm maximum diameter.

The laser drilling chapter compares the costs, throughputs, and capabilities of the laser tools: the older carbon dioxide laser, for higher productivity with larger holes, and the shorter wavelength UV-YAG and excimer lasers, for small hole size. It looks ahead to more use of dual-laser systems, combining the benefits of both.

The photoimaging chapter describes the strengths and limitations of several types of liquid and dry film dielectrics in photo-forming vias. The mass production approach of fabricating many vias simultaneously gives photovias a throughput advantage over laser drilling. However, neither photoformed nor etched vias can match the small diameter capability of the slower laser technologies.

The etched via chapter includes both wet and dry processes, chemical etching and plasma etching. Chemical etching is the least expensive process for making small holes in dielectrics. Plasma processing is versatile, useful for opening windows and similar structures as well as for forming microvias in the substrate. However, plasma batch processes are suitable for only small volumes or prototypes, and depend on a special resin-coated copper foil.

The conductive paste/ink chapter describes how these materials are used in the new high density interconnect sequential build-up (SBU) technology, to form filled vias, in conjunction with laser drilling, photoimaging, or plasma etching. SBU technology with conductive filling can give densities up to 100 pads per square centimeter.

Lead-free solders get their own chapter, which includes the clearest explanation I’ve seen of how alpha particles cause soft-upset errors in memory IC’s. With lead-free solder in hand, we proceed to the solder-bumping chapter, looking into electroplating, stenciling, and build-up bumping processes. Having bumped, we carry on into the assembly chapter, focused mainly on the virtues and vices of reworkable underfills and of no-flow underfills. Finally, with our microvia devices assembled and underfilled, we reach the last hurdle: 86 pages on the ins and outs of solder joint and microvia reliability for WLCSP.

This book will be a useful reference and guide to all of those in the microelectronics packaging industry, especially when facing the key issues of low cost high density interconnection.

REVIEWED BY: George Riley, FlipChips Dot Com

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