MELF / miniMELF packaged Diodes for PCBA - Good idea/Bad idea? - Page 1
Forum section: electronics / general

author: K3mHtH
author_rank: Regular Contributor
timestamp: December 28, 2019, 03:59:40 am
content: So I'm speccing out parts for boards that are going to be assembled by an assembly house. This is my first board designed for assembly (QTY: ~100-200pc) and I'm speccing out surface mount diodes and came across a lot of parts in the MELF and miniMELF packages.I'm only familiar with them from seeing Dave get geeked out over them, and the little glass cylinders do look pretty cool. Is there any advantage to using these parts? Reliability? Mechanical reliability?I'm guessing a disadvantage might be that the assembly house would have to change nozzles to do the pick n place on those?Anyone specced out MELFs before? If so why?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 28, 2019, 04:38:14 am
content: Right, possible assembly cost increase. They may also have to glue them, or hand solder them.Only way to know is to ask. If this is a Chinese house that doesn't English too well, you may only have one way to find out; on the upside, it's not like it's going to cost you $100, their labor is cheap.I don't know of anything not available in SOD-123 (more or less footprint compatible) or such, not to say there aren't any, I literally just don't know of any.There are very minor side benefits, like plastic packaged parts not being photosensitive. Common diodes are about as leaky as the additional photocurrent in direct sunlight (it's a few uA say on a LL914), so it's not much of a difference for the most part, more of a curiosity. It would make pA diodes unsuitable for MELF.Tim

author: exmadscientist
author_rank: Frequent Contributor
timestamp: December 28, 2019, 09:30:37 am
content: Dear god I hate those things. There's a reason people call them "Mostly End-up Lost on Floor"... do yourself a favor and avoid them! SOD-323/323F is a great package to pick for "any old diode" if you're having trouble picking. It's one of the most popular, most available, and cheapest options. You can go up or down in size from there as needed to meet power dissipation or board size targets.MELF packages used to be popular in the early days of surface mount, as they are pretty much through-hole parts with a different terminal attached. The only modern things I've seen using MELF parts are the very lowest cost Chinese stuff. This is probably because they share enough production steps with through-hole that they continue to be fantastically cheap... but they're still not worth the trouble.

author: Niklas
author_rank: Frequent Contributor
timestamp: December 29, 2019, 10:24:51 pm
content: Board flex and/or thermal expansion mismatch can cause tiny cracks in the glass package. The result is reduced contact pressure between the diode die and the package terminals, in other words open circuit.

author: K3mHtH
author_rank: Regular Contributor
timestamp: December 30, 2019, 07:52:39 am
content: Thanks everyone for their MELF opinions, much appreciated!

author: Rerouter
author_rank: Super Contributor
timestamp: December 30, 2019, 08:46:28 am
content: I would say avoid at all costs, the amount of automotive gear i repair where a melf package has developed an intermittent fault (even fresh out of the box!) is stupid,

author: jmelson
author_rank: Super Contributor
timestamp: December 30, 2019, 09:35:50 pm
content: The whole idea of MELF was for companies that had a whole line making glass-body diodes to be able to continue using the same process, and trim the parts at the end of the lineto be "surface mount". But, many modern P&P machines do not work well with parts that have curved tops. So, your assembler may not be happy with that part. Nowadays, thereis a HUGE selection of diodes in plastic packages supplied on tape for P&P machines.Jon