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MELF component short togehter

#20334

MELF component short togehter | 12 June, 2002

We had alot of problem with melf diode short together after reflow soldering. The clearance is about 0.8mm & we used "v" shaped opening .

PLease comment !

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JAX

#20336

MELF component short togehter | 13 June, 2002

Is the short apparent prior to reflow?

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#20337

MELF component short togehter | 13 June, 2002

Hi Jax,

After component placement, all melf diodes being checked & found no touching each other. Unfortunately, after reflow those MELFs are shorted.

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JAX

#20340

MELF component short togehter | 13 June, 2002

After reflow, Are the end terminals of the melf packages touching or is it a paste bridge? What type of reflow are you using? IR?... Convection?

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#20345

MELF component short togehter | 13 June, 2002

Help us understand your situation by telling us about: * Is this a terminal-to-terminal short on a single MELF? Or is this a terminal-to-terminal short between or than one MELF? * Clearance of 0.8mm is the distance between what features? * Size MELF are you using. * You say you �used "v" shaped opening�. Are you talking about your stencil? If so, why are you doing that? * What are the dimensions of your stencil thickness and apertures? * Pads, solder mask, and such. * Etc

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#20354

MELF component short togehter | 13 June, 2002

Hi all,

* Defect : termial to terminal touching btw two melfs body that created short circuit. * Clearance : 0.8mm btw two melfs body. * Size melf : diameter=2.4mm to 2.7mm, length=4.8mm to 5.2mm * Stencil opening : "v" shaped to minimise component rolling during reflow & solder beads problem. * Stencil thickness : 6 mil * Aperture :- reduction 0.05mm - 60 degree acute aperture angle - 0.006 inches radius round corners * Oven : FAC type with RTS temp. profile * Pad finish : HASL * Solder mask : Taiyo PSR4000 GREEN UL no: E69262(S)

It is due to self-alignment during reflow soldering ???

Pls comment !

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#20369

MELF component short togehter | 14 June, 2002

Good job clarifying things.

So, your MELF are just rolling off the pads. So much for the rocket scientist that talked you into those �V� shaped apertures, eh? [Or taking it the other way: Wow, just think how bad it would be if they weren�t working so wonderfully.]

Every stinking one of those stupid MELF has a mind of its own, anywho. Feeble though it may be. They just wander around. They end-up here. They end-up there. Not a care in the world. For the two micocents that your buyer saves in material cost, you end-up repaying 200 times over in heartache.

Here�s what�s happening: * Paste melts first at the corners [In your case, the �apexicies� of your triangular paste deposits.] * Edges of your paste deposit melt next. * No wetting action gets going on the MELF teminations to overcome any surface tension with the molten solder, because it�s too early in the process. * Ball bearings on the surface, round component, domed shape of the HASL, and no paste on the outer edge of the pad to slow anything down; you got whoops. [The component does not have to move very far before it is no longer contacting the stickiness of the paste.]

Consider: * Using a �U� shaped aperture. * Turning down the blowers in your oven. * Convincing yourself that your transport is running smoothly. * Determining that you are placing components properly. * Making sure the in-bound and out-bound curtains on your oven are not dragging across the board. [Although, you would have noticed this previously, wouldn�t you? Sorry. Hey, when you do this stream of conscious thing, sometimes it works sometimes it doesn�t.] * Gluing the rotten little @#%## in place. [Although, this may not be the smartest thing you could do, depending on the subsequent processing and product use.] * Using a board fabricator with a flatter HASL process. * Relaying-out the MELF pads to compensate for the doming of the HASL. * 0.8mm clearance between two MELF SB plenty of room.

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