Well, it depends.
Let's break "cleanliness" down a little and think of it more in terms of
"manufacturing residues". Everything has manufacturing residues (MR) on
them to some degree. MR from components will be different from MR on bare
boards will be different from MR on finished assemblies.
If you have an assembly operation that incorporates cleaning, as we do
here at Rockwell, then I am much less concerned about the MR on items
coming in my door because those big Aquastorms and Microjets are going to
clean them off. So I can tolerate a higher degree of residues on those
items.
If, on the other hand, my assembly operation is no clean, then those MR on
boards and components become much more critical because I have no
opportunity to remove them. I have to be more paranoid about how clean my
bare boards are. I have to be more concerned about those plating residues
on the components. In terms of my assembly operation, I have to be more
concerned about handling residues, storage conditions, incidental
contamination from the environment, etc. All of the manufacturing
residues can be studied by more analytical methods, such as ion
chromatography, which I prefer. These more precise methods allow me to
break down "cleanliness" into what kinds of residues are there and in what
amounts. I will have a different residue picture for a high solids rosin
flux, a different one for water soluble fluxes, a diffferent one for low
solids fluxes, etc.
Being able to specify "cleanliness" for a finished assembly means being
able to look at that mosaic of residues on the finished product,
separating out the different residues, and determining if it is a benign
residue, such as a weak organic acid, or a harmful residue, such as a
chloride. That is what many of the analysts skilled at reading ion
chromatography data try to do. They analyze the residues and give some
guidance as to those ionic fractions likely to cause problems.
Having done this for a number of years, and counseling others in this, the
complicating factor is that almost all assemblies have different
thresholds as to where individual residues go from "benign" to "harmful".
Very fine lines/space assemblies are more sensitive than wide
lines/spaces. High impedance is more sensitive. High voltage is more
sensitive than low. The end use environment will also determine these
thresholds. High humidity tends to drive cleanliness related failures
more than a benign office environment. There are no easy ways to
determine the exact cleanliness thresholds for an assembly, and those of
us who make the assessments can tell you it does not come cheap because
you have to do a lot of correlation testing with product.
So what does this mean for Phil working with his contract manufacturers?
The easiest way to go about this, if you have neither the time nor the
expertise to determine the thresholds for your product, is to use the
general cleanliness recommendations put forth by the experts at Precision
Analytical, Foresite, or Trace Labs. Those levels were developed over
time differentiating "clean" products which showed no problems, and
"dirty" assemblies which did. Will it guarantee that all products are
"scrupulously clean"? No, because you can still get localized
contaminations causing problems, but I will say it is one hell of a lot
better than ROSE testing.
In such a case, your procurement wording becomes easy. When tested per
IPC-TM-650, method 2.3.28, all assemblies shall have a maximum chloride
residue level of xx.xx micrograms per square centimeter, a maximum bromide
level of yy.yy micrograms per square inch.
You can change the requirements to fit the flux technology being used and
which set of criteria you feel most comfortable with.
Doug Pauls
Phil Nutting <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
01/03/2008 12:05 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Phil Nutting <[log in to unmask]>
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Re: [TN] specifying board cleanliness
Graham,
OK, so lets say a board has been built using no-clean. Does that have a
different "acceptable standard of cleanliness" or is it exempt?
I'm going to assume, for now, that your specification listed below is for
boards that have been washed.
Thanks, this is great info.
Phil
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: Thursday, January 03, 2008 10:10 AM
To: [log in to unmask]
Subject: Re: [TN] specifying board cleanliness
Happy New Year Phil!
Well, "it depends"...
First question is how clean do you need them to be? Depends on the
application and environment. Second question is what sort of assembly
process the CM is using - if "no clean" then they will be justifiably
opposed to cleanliness testing.
The generic callout that we most commonly see on drawings is "clean per
J-STD-001 C-22" - which translates into clean the assembly, both sides,
and then do an ionic contamination test with a upper limit of 1.56
micrograms / cm2 NaCl equivalent contamination. I'm not sure why
J-STD-001 gives an option to only clean one side - never seen that
process in use. Anyway, a starting point...
regards,
Graham Collins
Halifax Production Engineering
L-3 communications Electronic Systems
(902) 873-2000 ext. 6215
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Nutting
Sent: Thursday, January 03, 2008 10:45 AM
To: [log in to unmask]
Subject: [TN] specifying board cleanliness
Good Morning and Happy New Year to All,
Let me start by confessing my ignorance relative to cleanliness of
circuit boards. We have no idea what numbers to specify that fit into
the IPC testing methods for board cleanliness. If I tell my CMs to make
the boards clean, it would be good to specify a target value and or test
method.
Can someone give me some guidance on how to chose these specifications?
What are worth while tests and which ones are window dressing?
I've read through some of the IPC tests methods and my head hurts.
Thanks in advance,
Phil Nutting
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