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Between epoxy resin molded vs hermetically sealed metal can packaging for photodiodes and LEDs are there any relevant functional differences I should be aware of when selecting between otherwise seemingly identical photodiodes or LEDs?

The can offers potential flexibility for lens/filter options while the molded packaging is obviously lower cost. It seems to be a toss up which is actually more mechanically rugged and environmentally stable since one case is entombing/encapsulating in a more permeable material while another is being placed inside an impermeable material. I guess the metal can packaging would also handle higher temperature. Are these the only significant differences?

LED Example: https://dammedia.osram.info/media/resource/hires/osram-dam-5580407/SFH%204550_EN.pdf

https://dammedia.osram.info/media/resource/hires/osram-dam-5723400/SFH%204851_EN.pdf

Photodiode Example (not quite as identical as the LED example):

https://www.marktechopto.com/pdf/products/datasheet/MTD5010N_2011_07_20.pdf

https://dammedia.osram.info/media/resource/hires/osram-dam-5488355/SFH%20213_EN.pdf

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  • \$\begingroup\$ How to handle heat removal, for the more powerful amplifier? \$\endgroup\$
    – analogsystemsrf
    May 19, 2019 at 17:02

3 Answers 3

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The main purpose of the metal housing is to secure a lens used for directing light into the photodiode or out of the "point source" LED, as well as block light from entering or leaving in the wrong direction. Those devices are used mainly for optical sensing and instrumentation applications due to their narrow radiation pattern.

The metal housing could be a good heat dissipator. But considering those devices are orders of magnitude more expensive than their non-canned version, I'm pretty sure a good designer would look for better and cheaper ways of keeping the device cool.

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Hermetic sealing (if it's for real) is, well, hermetic. Epoxy molded packages can allow atmospheric contamination to creep into the package along the leads -- this is why some chips come packaged with desiccant packages; because water can condense (or, more likely, adsorb onto the inner walls) inside the package and cause a steam explosion when soldered.

If it's on a board with a bunch of typical components, then you probably don't care about that part. If it's on the end of some long cable in a harsher environment than the signal processing electronics -- maybe you do.

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  • \$\begingroup\$ Oh really? I definitely did not know that contaminants could creep in where the leads enter the molding. I assumed it would be a hermetic seal similar to the metal cans since it is epoxy which I assumed would have some adhesive properties to the leads. But then what's different about the leads on metal cans that stops things from creeping in? \$\endgroup\$
    – DKNguyen
    May 19, 2019 at 21:30
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It all depends on your application specs.

The metal encapsulation offers shielding immunity for high impedance loops from magnetic fields.

The optical design is most important to achieve the aperture required for reflection at some range and avoid scatter unless required.

Optical clear epoxy comes in different grades and is somewhat hydroscopic on the interface.

For cooling , if required, the cathode lead is the heat conductor to your ground plane.

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