Issue link: https://iconnect007.uberflip.com/i/1337116
84 DESIGN007 MAGAZINE I FEBRUARY 2021 less expensive idea—the dual in-line package (DIP) with the die attached and intercon- nected to a lead frame, then protected with an epoxy. Its leads were formed in a manner that allowed them to be soldered into through- holes of a PCB. e DIP package became the preferred format and dominated for most of the early years. It is still in use today, though it would likely be much cheaper to use a more modern format of the era and integrate advancements to bonding such as tab bonding, double bond- ing, and varying wire diameter. However, from a performance perspec- tive, skew, clock speed and frequency require- ments soon exceeded the limitations of sig- nal through wire, through DIP pin, down to the plated holes and through the PCB, to keep up with electronic performance demands. In the 1980s, surface mount technology was identified as the best way to achieve what had become the prime objectives of elec- tronic product developers (i.e., faster, smaller, cheaper, lighter, and better). Surface mount technology offered all these benefits. How- ever, little attention was given to basic geom- etry and the arithmetic relationships between the package dimensions and the trace and via routing implicit in the deployment of these newer package types into a PCBA. To plan for the future, a planning conven- tion was required. is resulted in the argu- ably arbitrary "80% rule" of generational pack- age lead pitch reduction. By its very definition, the "rule" combined both legacy Imperial measurements and metric measurements. e resulting conversion issues triggered an explo- sion in package types: pin grid arrays (PGA), small outline packages (SOP), thin small out- line packages (TSOP), quad flat packages (QFP), land grid arrays (LGA), and ball grid arrays, to name but a few. ese packages had many different lead shapes: flat, straight, gull wing, J-lead, and truncated, among others. us the 80% rule also resulted in an explo- sion in lead pitches, which assured the end of easily designed routing layers with multiple lead pitches employed in a common design. For the reader's consideration, ponder this: During the era dominated by through-hole devices there was fundamentally one lead pitch of 0.1" or 100 mils. If one has ever vis- ited an electronic hobbyist's store, one may have noticed what appeared to be blank epoxy boards with hundreds of holes drilled in them in a gridded fashion. ey are commonly referred to as "bread boards" and they allow hobbyists to more easily assemble their circuit designs using through-hole components such as DIPs, TO cans, and axial leaded discrete devices. e causes for the explosion were pretty much two-fold. First, the global electronic community, including the U.S., agreed to use the metric system for all measurements related to electronic production. is included out- lines and lead pitches. e second was an agree- ment to follow the 80% rule, which held that every subsequent generation of lead pitches should be 80% of the previous lead pitch (or as close as practical). us, legacy pitches such as 100 mil became 2.54 mm, and 50 mil became 1.27 mm. Other finer pitches beginning with 1.25 mm were 1.0 mm. 0.8 mm, 0.65 mm, 0.5 mm, 0.4 mm, 0.3 mm, 0.25 mm, and 0.2 mm. e result is that today, there is a mind-numb- ing number of package options. Earlier generation designers demanded that planning and math come first. In the tape and rubylith era (horse and buggy days, I know), math ruled. e completed schematic was The DIP package became the preferred format and dominated for most of the early years.