[sdiy] SMT diodes: MELF vs. SOT-23 chip?

[rsdio at audiobanshee.com]

I don't think it's always a given that MELF packaging is more expensive. It may be true right now from your particular source (Digi-Key?), but I've seen MELF show up as the least expensive diode meeting my needs for MIDI and USB circuits when ordering from Mouser.

Personally, I always choose the least expensive package unless I specifically know of a problem. I do try to make sure that the PCB can accommodate a few options, and I believe that MELF pads are basically the same as other SMD diode pads.

Starting from those motivations, I ended up with some MELF diodes for a particular project and was very surprised when I saw them in person. They really do look like classic axial lead, through hole diodes - except that the leads are missing! I never heard any complaints from my assembly shop. I even had to remove one of the diodes to perform current consumption measurements, and I didn't have any particular problems, although I agree that it seems like they could roll around.

The Bill of Materials for the Soundplane Model A has MELF diodes in the USB section, chosen for lowest price in US$, and there has never been any assembly issue with the three commercial production runs of those boards.

Here in Seattle, we have several local assembly shops that cater to large electronics manufacturers, so perhaps I've been spoiled because they already have MELF capabilities. For your projects, it might be worth interviewing your assembly house(s) to see if they have any recommendations, for or against MELF.

Brian Willoughby
Sound Consulting


On Sep 23, 2015, at 3:06 PM, "Richie Burnett" <rburnett at richieburnett.co.uk> wrote:
> The facts that they're more expensive, and roll all over the place when you try to put them anywhere are good reasons to use rectangular packages instead of MELFs.  Also, the board houses i've dealt with hate MELF packages because they need special vacuum heads for their pick 'n' place machines.
> 
> -----Original Message----- From: Mattias Rickardsson
> Sent: Wednesday, September 23, 2015 10:56 PM
> To: Tom Bugs
> Cc: synthdiy diy
> Subject: Re: [sdiy] SMT diodes: MELF vs. SOT-23 chip?
> 
> Tom,
> 
> no problems so far, I'm just curious if there are any reasons to
> choose MELF before chips. Two MELF diodes are around 50% more
> expensive (each something like €0.005 + €0.003 for mounting it, for
> both = €0.016) compared to one SOT-23 dual diode (€0.008 + €0.003 =
> €0.011). The MELFs also occupy more space on the PCB. But they do look
> much better, with their tiny retro-style glass jar. :-)
> 
> /mr
> 
> On 23 September 2015 at 21:15, Tom Bugs <admin at bugbrand.co.uk> wrote:
>> Interesting question & I hope people will respond 'on list' to share
>> thoughts.
>> 
>> I went to MELF diodes as they seemed so similar to the regular 1N4148s - ie.
>> for common silicon diodes apps
>> Only used SOT-23s for dual schottky between rails (for protection apps) as
>> it seemed such a convenient form.
>> 
>> The only time I actively choose between SOT23 and MELF is for zeners,
>> dependent on current.
>> 
>> What's driving your question Mattias?
>> Are you responding to a strange problem that has cropped up in a design?
>> or...
>> 
>> Cheers, Tom
>> 
>> On 23/09/2015 18:30, Mattias Rickardsson wrote:
>>> I've used surface mount diodes in both MELF (glass style) and SOT-23
>>> duals (chip style), but are there any significant differences between
>>> these two when it comes to audio/synth applications?
>>> 
>>> Sometimes the shape and other physical properties of simple basic
>>> components can affect more than you first might think. :-)
>>> 
>>> Thanks,
>>> 
>>> /mr