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We have started to incorporate the IPC7351B standard in our design. We use mils for our layouts. For Example consider part RC0603JR-070RL.

As per IPC7351B,

In mm

RESC1607X60N

and in mils

RESC62992799X23622N

Am I doing it right?

Should we transit from mils to mm?

MightyBeard007
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    NO, Maybe. If you can use both OK, but mm is becoming more standard. but a 0603 in Metric is 1608 ( 1.6 x 0.8 mm) – Tony Stewart EE75 Jan 20 '20 at 05:41
  • @TonyStewartSunnyskyguyEE75 I feel that if we use mils the name will be too long and in mm, the name is more descriptive, by looking at the name we can tell if its 0603 or 0402. – MightyBeard007 Jan 20 '20 at 05:48

2 Answers2

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Digikey only carry YAGEO SMD resistors and none of those are Metric sizes.

Other Mfr's do make both and their part numbers reflect that but it only makes sense to use the part numbers that match the same case size.

These are standard sizes.

enter image description here

They do have smaller sizes.

Tony Stewart EE75
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There are pros and cons for each; certainly the metric names are shorter (and therefore less likely to cause confusion).

For some time, the controlling dimension on many devices has become metric (mm); this is true of parts from connectors to passives to complex integrated circuits and on some devices (Micron memory parts come to mind) the only stated dimensions are in mm (see this part as an example) which tends to push me toward metric units.

Some manufacturers are providing schematic symbol and footprints that use the Ultra Librarian tool which are generally metric in nature. The tool takes a vendor neutral file and can generate the necessary outputs for many different CAD tools.

I would not convert old footprints (they are still perfectly good) and I can only state that I personally use metric formats for new components all the time (for consistency) unless a client specifically requests that I use mils.

Peter Smith
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