I would use the water soluble option and blow it off thoroughly immediately after wash and then bake at 105 Deg. C for 30 minutes.

Why?

1. The blow-off helps by blowing off 95% of the flux (and other) contaminants suspended in the water that remains after going through the rinse cycle. The rinse water is typically relatively clean, but for Bluetooth and other very high frequency applications, the board must be absolutely clean. No-clean flux residues are very problematic for this type of CCA, as they provide both an initial path leading to crosstalk and performance issues, but even if they don't do this initially, they are also hygroscopic and thus over time can cause latent failures or worsening performance.

2. The blow-off is important. The rinse water, even though "relatively" clean, has a much higher surface energy level and does not actually penetrate under tight surfaces as well as the saponified wash water does. While some of the rinse water does mix with the remaining saponified water, and it reduces the amount of flux residues/contaminants in or under the tight spaces, there could still be some leaching out of the not-so-clean water with some suspended flux solids. The blow-off need not take more than one entire minute per CCA, at 30 PSI max.

If you allow your rinse water to evaporate from the circuit card assembly after the wash, those suspended flux particles are then left behind on the surface in the form of a whitish residue. While minimal, it does not take much to provide a path for a UVHF Bluetooth signal. The path across the PWB surface is typically the path of least resistance as opposed to air, so some performance loss is seen.

3. The bake assures that at least any remaining "not so pure" rinse water left under the parts is gone. At worst, you are left with very little residual contaminants only in areas not exposed to air. This greatly reduces any performance-robbing contaminants. This, to me, is the real definition of a clean, pristine, dry assembly. It is never, ever, perfect, but "acceptable" IS achievable.

This is the method of cleaning I use in the companies that I work for when using a WS flux process. I have never seen it fail. 
On the other hand, I have seen many, many failures due to no-clean flux residues. Some went well beyond just Bluetooth performance issues. 

I am not saying never use no-clean; I am simply saying some applications do not lend themselves to no-clean processes.

Odin

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: Friday, December 01, 2017 11:09 AM
To: [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