AVR-Chat

SMT is more sensitive that through-hole because the body of the part is subject to greater temperature change and much more rapidly. For example, in SMT (in a reflow machine), the whole body if the part is raised to nearly the solder melting temperature. But through hole, even on a wave machine the body does not get nearly so hot. 


The whole issue water is that absorbed into the epoxy resin of the package. Yes, it really happens. If you heat it slowly, the water vaporizes and works its way out. If you heat it quickly, the water does not have a chance to diffuse out, and expands in place, leading to part failures and sometimes ruptures. Some epoxy packages have cavities where the die is; these are even worse because there is an open but enclosed volume in which the water vapor can expand (explosively). 


If you are machine soldering SMT parts, that have a significant part of the device volume as epoxy, and the parts were not packaged in a humidity barrier package, I would prebake no matter what. Without looking it up, which anyone who is concerned should do, I would heat to 50C for a good hour, then solder within a pretty short time. 


One should also point out the obvious: this is much more significant in, say, Louisiana, Alabama, or Florida in the summer, than any time of the year in Colorado or Utah. 


Jim Wagner 
Oregon Research Electronics 

----- Original Message -----
From: "Bob Paddock" <bob.paddock@gmail.com> 
To: AVR-Chat@yahoogroups.com 
Sent: Monday, March 18, 2013 1:34:05 PM 
Subject: Re: [AVR-Chat] moisture-sealed packaging 






On Fri, Mar 15, 2013 at 12:01 PM, Steven Hodge < steve@terrafirma.us > wrote: 
> 
> 
> 
> I realize this is off-AVR topic, but why the sudden(?) proliferation of 
> putting parts in moisture-sealed packaging (e.g., by Digi-Key and Mouser), 
> when they were previously not so packaged? Must the accompanying Threat of 
> Dire Doom be taken seriously if limits are exceeded? Why does it seem 
> limited to just SMT devices and not DIP? 

Don't know why things have change, other than perhaps a better 
awareness of the issue. 

What you want to find is the MSL rating in the datasheet for your 
respective part. 

http://en.wikipedia.org/wiki/Moisture_Sensitivity_Level 

MSL 6 - Mandatory Bake before use 
MSL 5A - 24 hours 
MSL 5 - 48 hours 
MSL 4 - 72 hours 
MSL 3 - 168 hours 
MSL 2A - 4 weeks 
MSL 2 - 1 year 
MSL 1 - Unlimited 

In some cases the MSL is not in the datasheet but in some secondary 
file related to packages and/or quality reports. 
Some manufacturer report it as the empty package so they don't put it 
in a part datasheet, and you have to dig for it. 

-- 
http://blog.softwaresafety.net/ 
http://www.designer-iii.com/ 
http://www.wearablesmartsensors.com/ 



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