no clean flux vs washing

[gogoman]

Currently the PCBA are washed after they are assembled,  The assembly house requested to use no clean flux,
comparing the two process what are the disadvantage/ advantages and what types of problems should I look out fore?
thanks

[nsrmagazin]

I don't know myself, but I found this:
https://www.kester.com/Portals/0/Documents/Knowledge%20Base/Publications/International%20Conference%20on%20Soldering%20and%20Reliability%20ICSR%202016_paper_Why%20Clean%20A%20No-Clean%20Flux.pdf?ver=2017-01-26-102146-280

and this:
https://www.assemblymag.com/articles/84286-why-clean-no-clean

[HHaase]

With low profile MLCC's, and low profile high-density BGA/micro-BGA's,  we've seen a BIG shift toward no-clean fluxes as opposed to washable organic flux the past few years.  It's becoming extremely difficult and expensive to wash all the flux residues out from under some of these part types, and since water-soluble fluxes tend to be mildly corrosive, entrapped residues often cause latent failures or odd signals. Heck, you should see what water-soluble fluxes do to wave solder and select solder machines in just a couple of years,  particularly precision ground linear rails.

A good no-clean will usually be a less expensive process (no wash),  and more modern pastes and fluxes leave very little residue compared to even 5 year old formulas.  With high voltage, high frequency, or pico level measurements, sometimes the no-clean residues can cause signal issues.  But we're talking gigahertz frequencies, kilovolts, and picoamps.   

I'd ask them what flux and solder paste they are using.   The latest batch of Kester 'High Reliability' stuff is actually VERY good for having great cleanliness results during testing.  I've become quite fond of Loctite GC-10 solder paste as well, because it's so time and temp stable,  but does require it to be 'worked' a bit straight from the tube before it rolls well.

But if you're going from a water-wash process, to a no-clean, also make sure to get a re-quote.   You should be seeing a cost savings from not having to wash.

[schmitt trigger]

I would add, what type of surface finish does your board have? HASL, ENIG, OSP, inmersion Sn, other?

Make sure that they offer you proof that they already producing other boards with your exact same finish and proposed flux, and have good wetting and yields.

[mrpackethead]

Unless you have a really good reason for using a paste that needs washing, then i'd not go there., Its an additional step, that jsut adds cost and the pastes these days are really good.  I use GC-10 pretty much exclusively,  and while its quite an expensive paste, as a percentage of the cost of assembly its nothing and it just works, and i've had no issues.

[jmelson]

If you have high impedance circuits, the no-clean flux residue can absorb moisture and sometime later become conductive.
Also, if the equipment is operated in a dirty environment, the flux can react with the airborne contaminants and cause similar problems.

But, if you can get away with no-clean, then it saves a whole process after stuffing and reflow.  I make most of my boards with no-clean, and they do fine except in special cases mentioned above.

Jon

[KL27x]

Unless you have a really good reason for using a paste that needs washing

What I have read, and maybe this is dated, is that many of the big operations use the same (no clean) paste on everything. And then they have to use an extra expensive process to clean the no-clean residue when the customer specifies it. Because this is cheaper than using multiple varieties of pastes. I dunno... do you actually specify what kind of paste your assembler uses?

[boB]

Washing is a PITA because you have to use de-ionized water and it's an extra step.  We did that for a very short while.

Gotta be careful with most fluxes only because if they are "active", the flux needs to be de-activated by heating which
happens when the PCB goes through the oven or flow solder.

If you are using no clean liquid flux to touch up your circuit board, you have to be real careful about making sure it's the right
stuff so it doesn't get caught underneath an IC and stay activated because the wrong flux can start shorting out if it needs to be deactivated.

Have seen much trouble with flux and production processes and I have forgotten a lot but at least our production guys have documented the process so things work well now. 


There seems to be a lot to learn to get this all to work right if you aren't just told what flux to use and how to use it.

This is a good reason to have experienced and smart production engineers so that bad stuff doesn't happen. 
It can be pretty scary when you don't know why circuits are failing sometimes but you suspect it might be your soldering and cleaning processes.

[HHaase]

Unless you have a really good reason for using a paste that needs washing

What I have read, and maybe this is dated, is that many of the big operations use the same (no clean) paste on everything. And then they have to use an extra expensive process to clean the no-clean residue when the customer specifies it. Because this is cheaper than using multiple varieties of pastes. I dunno... do you actually specify what kind of paste your assembler uses?

Cleaning residues from no-clean paste can be a pain, particularly older ones.  Either needs to have chemicals in the wash system, or use some kind of spray solvent which varies by the flux type.  IPA only goes so far,  and straight DI water barely touches the stuff.  We try to steer away from that whenever possible. 

But every place I've ever been to has separate no-clean and water soluble pastes.  One big advantage of most organic (water wash) flux is that they tend to be aggressive, and situations that no-cleans can struggle with are often easier with water soluble.  So if we have to wash, we very highly prefer using a paste designed for water wash.   The process gets better results, and is easier to clean.  Heck, I think we currently have 7-8 different pastes as part of our standard library.  Different mesh sizes, flux types, alloys (lead and lead free), customer specified, etc....

As far as specifying the paste, that all depends on your assembler and the agreements you have with them.   Some places only use their standard stuff,  others allow you to specify.  The better your relationship with the supplier, the more they're willing to do for customers. 

-Hans