There more to this than first meets the eye. Lets say you design 
something that goes into aviation
equipment, flying at 50000'. Small amounts of pressure trapped inside 
the thin case might crack the
case. High vibration levels might also crack a thin case. The device 
might work fine until the plane
sits on the ground for a few days in a humid place (like central 
America) and moisture becomes
trapped in the cracked case.

The solution is to coat the devices with a semi-hard RTV coating. The 
flexibility of the coating ensures
that moisture can't get it whether the case cracks or not. Hard coatings 
will eventually crack due to
vibration.

--Bob

Bob Axtell wrote:

> In earlier days, the plastic covering of the chip was quite thick, and 
> would provide
> protection from fairly high external pressure. In other words, water 
> pressure would have to be
> very high to get through to the device inside.
>
> But these very thin packages do not have the level of protection that 
> thick packages have.
> If you clean with water, you should bake the PCB to ensure the removal 
> of any trapped water.
> I consider TSSOP to fall into this catagory.
>
> What I do is to clean with paint thinner first, then use hot soapy 
> water, then clean water, then
> a small 125F bakeout in the oven (make sure temp is reached then turn 
> the oven off and let everything
> slowly dry out and cool down at the same time). bakeout lasts for 
> about 1 hr.
>
> If I believe moisture is going to be a problem, I will dipcoat the 
> final PCB with an RTV coating.
>
> F
>
> Philip Stortz wrote:
>
>> so what are people doing about chips that are "moisture sensitive" and
>> what do the different levels mean (i.e. is "level 1" highly sensitive or
>> no problem?).  i've sampled a number of such chips, and while i'm not
>> too worried (wyoming is dry) i'd like to know, and i'd like to know if i
>> really should bake these guys overnight to be safe, particularly on the
>> larger ones.  (fyi, i solder a 28 pin dip faster and better than most,
>> so i'm quick, but i'd hate to pop a chip).  i also may be using a
>> home-brew oven eventually, which i suspect makes it more of an issue
>> given the longer and greater heating that's likely (assuming i don't run
>> into problems doing it by hand, which always has to start a mental timer
>> before i over do things).
>>
>> "John J. McDonough" wrote:
>>  
>
-- 
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