IC packaging technology retrospective—part 2
Rule has been applied regardless of the for electronic circuits using the principals designers must continuously struggle to
component lead configuration, be it two of base grid, street and avenue routing. The work within. However, if one chooses
sided, four sided or area array and ‘therein concept has deep ties to early IBM designs properly components and calls out only
lies the rub.’ of this type, so given the proximity of IBM components which have I/O that fall on
To help appreciate the effect, the ac- to New York City, it is perhaps not surpris- a standard grid, it is possible to wiggle out
companying graphic is offered to show the ing, and it is a highly efficient approach. of that constraint to create easily routed
intrinsic problems and missed opportunity This has a number of compelling designs that will perform better and use
when the 80% rule is applied to area array benefits. For example, there is potential for few layers. It is just a matter of choice.
packages. As can be seen in the graphic on much improved routability as the streets
the left, following the de facto rule leads and avenues used for routing channels are
to greatly increased routing complexity as perfectly defined making the routing much Verdant Electronics founder and president
only two of the five grid pitches derived easier and faster. No less important is all Joseph (Joe) Fjelstad has more than 35 years of
from the concept have ability to conform of the circuit board real estate which can international experience in electronic intercon-
to a common grid based on the smallest be potentially returned to use for compo- nection and packaging technology in a variety
defined lead pitch for component I/O (in nents. Clearly, the mixture of peripheral of capacities from chemist to process engineer
this case 0.5 mm). This might seem a trivial and area array leads of various lead pitches and from international consultant to CEO. Mr.
concern, but it has far reaching impact in results in a much more complex circuit Fjelstad is also a well known author writing on
design. If components of varying grid pitch routing challenge and necessitates the the subject of electronic interconnection technol-
are used in the assembly as seen on the left distancing of components and addition ogies. Prior to founding Verdant, Mr. Fjelstad
side of the figure, the routing of intercon- of many more circuit layers than would co-founded SiliconPipe a leader in the develop-
nections will be more complex. More normally be required to accommodate the ment of high speed interconnection technologies.
sophisticated routing tools are required redistribution wiring required to make all He was also formerly with Tessera Technologies,
and, more importantly, more layers as well. of the required connections. Moreover, a global leader in chip-scale packaging, where he
In contrast, on the right side can be seen such requirements actually reduce circuit was appointed to the first corporate fellowship
the same number of components with performance. for his innovations.
placement based on the use of a common In summary, the 80% rule, which was
grid pitch and wherein it can be seen that imposed by road maps developed with
every component lead falls on grid. ‘Man- peripherally leaded components in mind,
hattan routing’ is the term frequently used has created a ‘straight jacket’ from which
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www.globalsmt.net Global SMT & Packaging – March 2009 – 5
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