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There is an old and bog standard form factor for SMT crystals - HC49/US.enter image description here
The problem is that HC49/US takes a lot of real estate on the board. It's 12mm long.

Another problem with crystals and oscillators in general is that they often become long lead time items. [This is my experience.]

There are lots of small form factor crystals from various manufacturers. But, at a glance, it seems that there is no standardization of form factor. It’s nice when packages are standardized. It alleviates the long lead time problem. Crystals from manufacturer B may be used, if crystals from manufacturer A are out of stock.

Are there standard form factors for compact SMT crystals?

Update

Armed with the insights from Gribo and Gustavo, I did a more focused survey. Found that 12MHz in 3.2x2.5mm package are produced by: Abracon, Citizen, Epson, Fox, ECS, TCS, NDK. To be included in this list, the part had to be in stock at DigiKey. It turns out that 3.2x2.5mm package is common across many manufacturers.

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    \$\begingroup\$ The higher the frequency, the smaller the package can be made \$\endgroup\$ – Gustavo Litovsky Sep 11 '13 at 19:16
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There are few common packages for crystals. For 4 pad variants, there is 5x7mm, 3.2x2.5mm 2.5x1.6mm and few others. They are available from many different manufacturers and share a common pinout.

For 32.768KHz crystal (Very common in microcontroller based designs) there is a common package from Citizen (Watches) that is used by many other manufacturers, but I dont know its dimensions off hand.

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Are there standard form factors for compact SMT crystals?

The answer to this is basically no. However if you start to look closely at the crystals and oscillators offered by various manufacturers you will find that many vendors are offering products in packages that are quite similar to things offered by maybe one or two other vendors.

My practical experience with this is in the realm of packaged SMT oscillators. The least common denominator for crystals still seems to be the SMT version of the HC49U.

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