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Surface mount part jargon
Talking the talk (big thanks to Bruce Robinson for much of this material)

One of the wonderful things about using surface mount components is that they come with their own, new set of acronyms (mostly related to package types). First off, surface mount components are generally referred to with either the term SMD (Surface Mount Device) or SMT (Surface Mount Technology). Just to keep things interesting, the term SMT can also be used to refer to Surface Mount Transistors (along with SMC for Surface Mount Capacitor, and SMR for Surface Mount Resistor).

Surface mount discrete components (transistors, resistors, diodes, etc.) come in a variety of sizes -- you'll have to be careful when you buy them that you're buying something small enough for your purposes, but not too small to work with by hand. Meanwhile, surface mount ICs come in two main package types, differing just in size:

SOIC (Small Outline IC, a.k.a. SO, or SOJ for ICs with J-shaped leads) -- typical lead spacing ("pin pitch") is 1.27 mm (0.05"), half that of a DIP IC.

TSSOP (Thin Shrink Small Outline Package; a.k.a. TSOP, SSOP) -- typical lead spacing is 0.65 or 0.5 mm (0.026" / 0.02"), roughly half that of SOIC.

A fair number of folks have built PCBs using SOIC ICs; a smaller number can successfully (and reliably) build boards using TSSOP components (note that this is not for the faint of heart, and can be a lesson in both frustration and patience).

The pinouts for surface mount ICs are exactly the same as for the familiar DIP packages (but on a smaller scale, of course).Accordingly, the numbering of IC IDs is also the same, with the exception of the ID suffix (which identifies the package type):

J = ceramic DIP package

N = plastic DIP package (which we usually use)

D = SOIC surface mount package

DT = TSSOP surface mount package

So where you might buy a 74HC240N for through-hole (DIP) mounting, the surface mount equivalents are 74HC240D (which you can easily use) and 74HC240DT (which is small enough to be very challenging to use).

Also note that surface mount ICs have the same voltage specs, same pinouts, and same current ratings as their larger DIP cousins, BUT ...

SOIC has 67% of the power dissipation capability of a DIP IC

TSSOP has 60% of the power dissipation capability of a DIP IC

So running a surface mount IC continuously at its maximum current rating will most likely overheat it. This means that surface mount ICs should work without issues for bicores, but you should do your homework (and "de-rate" the spec current capability) before using one for a motor driver.

For more information...

"Leroy's Engineering Web Site" has listings of surface mount part dimensions for resistors and capacitors; also tips on capacitor placement w.r.t. ICs.

Craig Maynard has a nice page on IC packages (including SMT) here. Manufacturers are also good sources of IC package information -- Oki has an exhaustive page here, while Hitachi has a more-concise one here.

Legalities  Image 
Page author: Eric Seale
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