Product
Brand
Articles
Tools
Package codes are the part of a bill of materials that looks trivial and causes the most avoidable mistakes. A “0603” resistor and a “0603” LED are not the same footprint, and “1608” is the same part as “0603” written the other way round. Getting this wrong costs a re-spin, so it is worth ten minutes to fix the vocabulary.
Two naming systems, one part. Passive components in the 0402/0603/0805 family use imperial codes measured in hundredths of an inch. Divide by four to get the metric code in tenths of a millimetre: 0402 is 1005, 0603 is 1608, 0805 is 2012, 1206 is 3216, 1210 is 3225. Both codes describe the same body, and suppliers list whichever their catalogue prefers, so a BOM that mixes the two systems is normal rather than wrong. What is wrong is assuming the code alone identifies the part: a 0603 ceramic capacitor, a 0603 thick-film resistor and a 0603 LED share a body size and nothing else.
What actually changes as the package shrinks. Moving from 0805 to 0402 is not only a footprint change. The smaller body has less room for electrode layers, which caps the capacitance and voltage you can reach in a given dielectric. It also has worse thermal behaviour: the same dissipated power raises the body temperature more, because there is less surface area to radiate from. Placement tolerance tightens, and the solder joint becomes a larger fraction of the total connection, so tombstoning and cracked joints appear on reflow profiles that were fine for 0805.
Semiconductor packages are a separate language. SOT-23, SOT-223 and SOT-89 are transistor outlines; SOIC, TSSOP and MSOP are leaded IC outlines; QFN, DFN and BGA are area-array packages where the connection is under the body rather than at the edge. Area-array parts cannot be inspected visually after reflow and generally rule out hand assembly, which matters if a design will be built in small batches.
Thermal pad is a package feature, not an accessory. A QFN or a power SOIC is specified with a thermal pad on the underside. Connecting that pad to a copper area is part of using the package at its rated power, not an optional layout nicety. A design that leaves the pad unconnected is not the same part electrically.
A practical way to keep a BOM consistent. Pick one naming system per component family and write it in the same column everywhere. Keep the footprint name from the CAD library next to the manufacturer part number, because that is the field the assembler actually consumes. When a part is second-sourced, confirm the new part uses the same footprint name and not merely the same nominal code.
Where this shows up in sourcing. Two parts can share a package code and still be mechanically incompatible: a 0603 capacitor with a thicker body than the land pattern allows, or a QFN with a different thermal-pad size, will not drop in. When you send a BOM for quotation, the package and footprint fields are as important as the part number.
This article describes general package conventions rather than the specification of any particular part. Always confirm dimensions, land pattern and thermal requirements against the datasheet and the manufacturer’s recommended footprint for the exact part you are designing in.
Need parts for the package you have settled on?
Send the part numbers, package and quantities through our RFQ form, or contact us and we will confirm availability and lead time for the exact part. You can also browse the component catalogue by category.
—
