In the past I had to qualify MEMS for a production line, to ensure no shock or vibration was seen that could possibly hurt the components. These were found to be very susceptible to failure.
I used an amplified accelerometer to measure the shock and vibration from the manufacturer, on the truck or plane, and through the processes.
We discovered that irregular shock and short vibration exposure did little to harm the MEMS.
What did kill the micromechanical parts was resonant vibration, and we also discovered that certain MEMS were susceptible to their own particular steady-state vibration frequencies and multiples of those frequencies.
Where did these show up?
ONLY IN THE WATER WASH PROCESS!
Weird, eh?
Had nothing to do with moisture ingression to the package itself. The transmitter worked even underwater, and the machine's large pump motors caused a huge cavitation effect when transmitted through the water. The spray sometimes killed the MEMS also, but mostly it is cavitation while fully immersed.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Yuan-chia Joyce Koo
Sent: Monday, December 04, 2017 9:19 AM
To: [log in to unmask]
Subject: Re: [TN] board wash / non-washable modules - the dilemma

if your module is RF MEMS, (since it is susceptible to moisture, it might have opening or movable parts that can change stiffness or sticking when expose to moisture), wash is not good... you never can bake out fully in those micro-opening... sticking due to combination of flux residue (not much, just tiny bit) and moisture will kill you (it might work fine on marginal spec, but when it aging, you see difference out of spec quickly, like within warrantee period of 6 month-ish when use it high humidity condition..).  IMHO.  Best of luck.
jk
On Dec 4, 2017, at 8:48 AM, Graham Collins wrote:

> Thanks for all the (varied) opinions!  No consensus here.
>
> We are a build to print shop, so I'm not responsible to dictate how to 
> put it on - my obligation is to point out the issue and recommend a 
> solution.  I'm going to recommend the "attach last by hand using 
> no-clean flux" method, it will work on this application
> as the module has castellation terminations, accessible by hand.   
> If it was bottom terminations only I'd be facing a bigger problem, in 
> that case I would recommend the "wash and bake".
>
> cheers,
>
>  Graham
>
> regards,
>
> Graham Collins
> Senior Process Engineer
> Sunsel Systems
> (902) 444-7867
>
> On 12/1/2017 4:37 PM, Richard Kraszewski wrote:
>
> Ditto  to Bob's thought. Localized clean if need be.
>
> Rich  Kraszewski
> Senior Staff Process Engineer
> Plexus Engineering Solutions
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Wettermann
> Sent: Friday, December 01, 2017 12:28 PM
> To: [log in to unmask]<mailto:[log in to unmask]>
> Subject: Re: [TN] board wash / non-washable modules - the dilemma
>
> My vote because this is what we have done for small Proto builds is to 
> leave the module off, build and wash w water soluble flux then install 
> the RF module using the no-clean?
>
> Bob Wettermann
> BEST Inc
> Your BEST source for PCB Rework/Repair, Training and Tools
> 847-767-5745
>
>
>
>
> On Dec 1, 2017, at 10:15 AM, Tom Brendlinger 
> <[log in to unmask]><mailto:[log in to unmask]> wrote:
>
> I have heard many times that in high humidity environments with very 
> small amounts of condensing humidity, no clean flux can become 
> re-activated. This paper discusses some options:
> https://link.springer.com/article/10.1007/s11664-014-3609-0
>
> In my opinion, reach out to the part manufacturer and see if you can 
> perform cleaning in a particular way to not risk having issues.
>
> Tom
>
> On Fri, Dec 1, 2017 at 1:05 PM, Robert Kondner 
> <[log in to unmask]><mailto:[log in to unmask]>
> wrote:
>
>
>
> I would go for wash and bake but I worry that washing will "Wash Crud 
> Into"
> the shielded areas and the rinse will not rinse it out.
>
> Are your concerns about no clean flux in a marine environment really 
> justified or is it hog wash from somewhere?
>
> Bob K.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
> Sent: Friday, December 01, 2017 10:09 AM
> To: [log in to unmask]<mailto:[log in to unmask]>
> Subject: [TN] board wash / non-washable modules - the dilemma
>
> Hello Technet!
>
> Wondering what other people are doing with this dilemma.
>
> We have had several customers design in pre-built RF modules on their 
> boards.  I totally get why, it is a pre-built, FCC approved, easy 
> solution
> -
> the fastest way to get a good Wi-Fi or bluetooth solution.
>
> Where things go off the rails for me are where this is used in a high 
> humidity application (e.g. a marine setting).  We strongly prefer to 
> wash boards, we can build no-clean if needed but I'd prefer not to for 
> an on the water use.  But the part a customer has designed in 
> specifically says not to wash it (it has an EMI shield, so they are 
> properly concerned with water entrapment).
>
> So - would you:
>
> - build it no-clean?
> - leave the part off, build and wash as usual, install RF module later 
> using no-clean?
> - wash it (and maybe bake it to dry it out)?
> - ????
>
> --
> regards,
>
> Graham Collins
> Senior Process Engineer
> Sunsel Systems
> (902) 444-7867