Hi all,
I recently ordered some max2000s and it was written on the package that the parts were moisture sensitive. But the packaging was already was open and the moisture indicator card shows that they should be discarded. Now should I try to use them or its no use wasting time?
One more thing is how can I mount QFN packages myself. I tried by soldering leads directly but spoiled the IC. Any ideas??
Thanx

 

Are these ICs?

I don't know, I've never heard of moisture sensitive ICs before. If they really are moisture sensitive and the packaging has been damaged, you should return them

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All devices absorb moisture to some degree. THey have a shelf life where if they have been on the shelf too long, they absorb too much moisture. Then you have to bake them (like 24-48 hours or weeks sometimes) to get rid of the moisture. The baking temperature and times vary from part to part. But if you ordered the part and that was the case, I'd return it and ask for a replacement.

 

check the data sheet, usually moisture sensitivity is part of some mil-spec compliance ... too much moisture can do a few things

a) corrode the contacts
b) soften plastic or composite pieces
c) cloud optics

many times, the manuf will detail steps that can be taking to re-certify devices, if it is possible, for example Lumileds recommends baking their LED products for something like 6 hours at 150C to drive moisture out of the silicone in the optics

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The moisture sensitive nature of some parts is also a problem when the board is to be soldered in a reflow oven. What happens is the moisture will get into the package where the metal parts (pins, heat sinks, etc.) are molded into the plastic body. When the parts go into the oven to be soldered, the moisture will turn to steam, create pressure and crack the case, damaging or destroying the part. It's very similar to the moisture expanding in popcorn (which literally turns the kernel inside-out), although to a less spectacular degree. I wouldn't worry about it unless you plan to solder the boards in a reflow oven.
JB

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QFN's really need to be done by reflow - one way or another. The toaster oven is the "Standard way" - look around for "toaster oven reflow". If you have money (~$500), buy a "hot-air pencil", preferably one with a self contained air pump. Alternatively a hot-plate will also work - an aluminum topped one meant for chemistry stuff ~$100), or my personal favorite - a hand held paint stripper/hot air gun - on low.

With all these approaches you need to use solder paste and keep an eye on things to make sure you don't overcook things. Just apply a couple small dots on the center pad and thinly spread some around the outside pads. Also be sure to have some flux ready so you can clean up the little solder balls/shorts that will pop out from under the package.

 

soldering leadless qfn without paste or a reflow oven:

http://www.sparkfun.com/commerce/pre...?p=SMD-HowTo-6

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Hot air costs more than a toaster oven though and still heats up the IC packaging. I saw this weird bent-looking solder tip possibly with grooves that I cannot see since it's an image that's supposed to be designed for QFNs...but I have no idea how it might be different froma normal tip.

http://www.howardelectronics.com/jbc/cartridges.html

The QFN iip 2245-009. There is also a PLCC tip 2245-010 and I'm not quite sure how that one is different either. Seems to me that a mini-spoon tips right under those would do the job.

 

i've done leadless before with just an iron, the trick is to extend the pads for the contacts out past edges of the package (use a custom package layout), just enough that you can contact it with your iron ... use copious amounts of flux and a well-wetted iron tip

sure a toaster oven is cheaper than a rework station, but doesn't offer the degree of control or flexibility, plus, it would be mighty clumsy to do actual rework using an oven.

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I should mention: the other way of doing a package with a hidden ground pad (lots of these parts won't function properly unless it is soldered down) is to have a *big* ~150mil+ through hole in the pad itself. That way you can solder the pad down from the backside once you've aligned all the pads on the top. Like justDIY said - be sure to extend the pads a couple mm past the package so you can at least touch the outer pads.

Personally I'd be pretty lost without my hot-air pencil. The smallest nozzle is ~2mm, so it can stay pretty well focused, and you can adjust the airflow so that it doesn't immediately launch the parts when the solder melts. Small surface mount transistors, any chip capacitors/resistors are amazingly easy to work with when you have the proper tools. Doesn't work too well for 100+ pin QFP's, but it'll handle 44MLF's with a little bit of effort.

 

Just got a friend to change a chip at work for me, the sound output chip on a Sony Bravia LCD TV - it's 48 pin and about 3/8th of an inch square. To make it even worse, most of the underside is a heatsink tab, which is soldered to the PCB.

Michael, who did it for me, has a special workstation for changing BGA chips, and he used that - it heats BOTH sides of the board. He even gave me the old chip back in case it didn't cure the fault, but luckily it did!

It was a seriously neat job, you couldn't tell it had been changed at all!.

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Thanx all for those replies. Encouraged, I bought an SMD workstation. Lets see how it goes...