Water in chips

[Tim May]

On Wednesday, July 16, 2003, at 03:28  PM, Bill Frantz wrote:

> At 8:49 AM -0700 7/16/03, Tim May wrote:
>> (By the way, the USB flashdrive (a 256 MB FlashHopper) I have on my
>> keychain--my physical keychain!--is probably waterproof. The USB port
>> has a little plastic cover which slides on snugly. Until I eventually
>> misplace it, I am using it. I expect the thing is showerproof, though 
>> I
>> don't intend to test it. Water resistance can be tested
>> nondestructively with things like Fluorinert, of course. Also, surfers
>> and kayakers often have O-ring sealed gizmos they wear under their wet
>> suits, coming in different sizes. It would be trivial to find one to
>> hold either a USB flashdrive or a Compact Flash card.)
>
> Ever since I heard that manufacturers were cleaning assembled boards 
> with
> soap and water I have wondered just how much you need to protect 
> electronic
> circuits from water.  You obviously don't want to allow them to stay 
> damp
> so they corrode, but immersion for a time (up to weeks) followed by a 
> fresh
> water rinse and drying might not be so bad.  Do any hardware experts 
> have
> an opinion?


DI water (deionized water) is used at various stages to rinse boards, 
wafers, plated devices, etc.

Soap and water is often used for cleaning bare PCBs, but this is long 
before the chips go in.

After wave soldering (PCBs fed on conveyer belt above hot solder bath, 
soldering the devices to the board in one continuous process) the flux 
and dross and gunk is washed off with special soaps (e.g., high acid 
content soaps) and water. But the water is certainly not left on the 
boards, not left to dry and cause spotting, etc.

Alcohol and other rinses are used.

We used to use vapor degreasers a lot, and TCE, before it got 
restricted.

As for the effects of water on packaged chips, they vary. Moisture 
intrusion usually comes when a "driving force" exists for some long 
amount of time, e.g., '85/85" (85 per cent relative humidity, 85 C, for 
some number of hours or days). External corrosion is also possible.

(One of the first things I devised for Intel was dubbed the "water drop 
test." Still in use, 28 years later.)

My point about not wanting to immerse my flash drive is related to why 
I would no immerse any other piece of electronics unless I had a 
compelling reason to do so.


--Tim May, Occupied America
"They that give up essential liberty to obtain a little temporary 
safety deserve neither liberty nor safety." -- Benjamin Franklin, 1759.