Well said, David!
I might add that if controlled cleaning was unimportant, as has been
implied, then no one would do it. This also answers Reuven's comment.
Brian
David Douthit wrote:
> Ramon,
>
> Now you've done it! You are asking, "How clean is clean?".
>
> As Mr. Doug (where's my Mountain Dew) Paul would say, "It depends!"
>
> Where is the product used?
> How long is the warranty?
> What is the product used for?
> What are the expected environmental conditions?
> What are the contractual requirements?
> How much liability exposure is there?
>
> There are major differences between a CD player or PC
> and a Tactical Weapons System. Each needs to be evaluated on its own merits.
> There are many stories about contamination problems but the vast majority
> are proprietary. After all they cost that company a lot of money and
> why should they help "solve" their competitors problems.
>
> I'm sorry but there is no simple single answer.
>
> David A. Douthit
> Manager
> LoCan LLC
>
>
> "Dehoyos, Ramon" wrote:
>
>
>> David:
>> Our boards are cleaned three times which gives us less than
>>0.3 micro grams per cm sq, so we do not have a problem in this respect. I am
>>only trying to understand more about flux and other residues from other
>>people's perspective. I have talked with people that have specialized in
>>cleaning for years and say that any thing under 1.8 micro grams per cm sq is
>>good. I am sure there are examples such as the ones below of leaking caps
>>and using no clean flux. But ff the flux residue at the top limit of 1.56
>>micro grams per sp in of NaCl is a problem there shoud be a lot more
>>instances at every company, specially with BGAs which are very difficult to
>>clean. Any thoughts?
>> Regards,
>> Ramon
>>
>>
>>
>>>-----Original Message-----
>>>From: David Douthit [SMTP:[log in to unmask]]
>>>Sent: Monday, September 08, 2003 4:05 PM
>>>To: [log in to unmask]
>>>Subject: Re: [TN] Ionograph
>>>
>>>Ramon,
>>>
>>>The answer is YES!!!
>>>
>>>Go to http://residues.com as a starting point.
>>>Virtually all the labs can give you wonderful stories about
>>>contamination induced failures.
>>>
>>>David A. Douthit
>>>Manager
>>>LoCan LLC
>>>
>>>================================
>>>
>>>"Dehoyos, Ramon" wrote:
>>>
>>>
>>>> Brian:
>>>> My appologies for stretching this subject since I know very
>>>
>>>little
>>>
>>>>beyond empirical experience. I was talking about different types of
>>>
>>>boards
>>>
>>>>including boards that performed in the giga frequency. At one time we
>>>
>>>were
>>>
>>>>producing about 20K boards per month and several thousands were of the
>>>
>>>high
>>>
>>>>frequency application type.
>>>> Are there any reports or articles that indicate field failures
>>>>from boards not being properly cleaned?
>>>> Beat Regards,
>>>> Ramon
>>>>
>>>>
>>>>>-----Original Message-----
>>>>>From: Brian Ellis [SMTP:[log in to unmask]]
>>>>>Sent: Monday, September 08, 2003 10:19 AM
>>>>>To: [log in to unmask]
>>>>>Subject: Re: [TN] Ionograph
>>>>>
>>>>>It can depend on the application. Obviously, high-impedance, high
>>>>>frequency applications, used as portable equipment with a high
>>>>>reliability required, cannot be considered in the same breath as the
>>>>
>>>PC
>>>
>>>>>driving the screen you use to read this.
>>>>>
>>>>>However, be aware that salt is not usually the problem but solder flux
>>>>>residues and general handling contamination. This is why the correct
>>>>
>>>way
>>>
>>>>>of expressing the unit is n µg/cm2 eq. NaCl, the "eq." meaning the
>>>>>electrical equivalent, not that there is actually NaCl present.
>>>>>
>>>>>Brian
>>>>>
>>>>>Dehoyos, Ramon wrote:
>>>>>
>>>>>> Brian:
>>>>>> Besides biomedical electronics in which boards need to
>>>>>
>>>be
>>>
>>>>>>super clean, it has been my experience that any thing under 10
>>>>>
>>>micro
>>>
>>>>>grams
>>>>>
>>>>>>of NaCl per square inch is passable. So far I have not heard of a
>>>>>
>>>case,
>>>
>>>>>>while tallking to my co-workers in several companies, where there
>>>>>
>>>has
>>>
>>>>>been
>>>>>
>>>>>>any problems releted to salt contamination. Could you expand on this
>>>>>
>>>>>matter.
>>>>>
>>>>>> Regards,
>>>>>> Ramon
>>>>>>
>>>>>>
>>>>>>
>>>>>>>-----Original Message-----
>>>>>>>From: Brian Ellis [SMTP:[log in to unmask]]
>>>>>>>Sent: Saturday, September 06, 2003 3:29 AM
>>>>>>>To: [log in to unmask]
>>>>>>>Subject: Re: [TN] Ionograph
>>>>>>>
>>>>>>>IMHO, to say 1.56 ug/cm2 eq NaCl or any other figure is useless
>>>>>>
>>>without
>>>
>>>>>>>qualification. Would it be reasonable to use this figure on both a
>>>>>>>through-hole circuit with conductor spacings of 0.5 mm and no
>>>>>>>significant traps under components and a high density interconnect
>>>>>>>structure with shadowing components and gaps under them equal to the
>>>>>>>copper thickness? Of course not, it is ridiculous. The figure quoted
>>>>>>
>>>was
>>>
>>>>>>>derived from conditions in the first case, in the 1970s, before SMDs
>>>>>>>were current. IMHO, the figure, for equal reliability, must be
>>>>>>>proportional to the maximum voltage gradient. In the first case, we
>>>>>>
>>>are
>>>
>>>>>>>talking of e.g. DILs working at 5 V, so we have a voltage gradient
>>>>>>
>>>of 10
>>>
>>>>>>>V/mm. I agree my example is perhaps extreme, so let's say 50 V/mm,
>>>>>>
>>>for
>>>
>>>>>>>the sake of a more practical argument. Your HDIS may be using
>>>>>>>semiconductors working at 3.0 V with minimum track/pad spacings of
>>>>>>
>>>25
>>>
>>>>>>>µm, ie a voltage gradient of 120 V/mm (roughly the maximum advisable
>>>>>>>with FR-4, before dissociation starts), so if 50 V/mm and 1.56
>>>>>>
>>>µg/cm2
>>>
>>>>>>>are OK, then you would want 1.56 * 50/120 = 0.65 µg/cm2, assuming
>>>>>>
>>>equal
>>>
>>>>>>>accessibility of cleaning fluids under the components. As this is
>>>>>>
>>>not
>>>
>>>>>>>the case, I suggest we have to weight the figure to compensate. I
>>>>>>>propose a factor of 3 (from experience, we know that a tight SMA is
>>>>>>
>>>3
>>>
>>>>>>>times more difficult to clean to an identical level than a 1970s
>>>>>>
>>>style
>>>
>>>>>>>assembly). It would therefore seem that 0.2 to 0.25 µg/cm2 eq. NaCl
>>>>>>>would seem the most judicious figure to get an identical level of
>>>>>>>reliability, all other things being equal. Empirically, this
>>>>>>
>>>argument
>>>
>>>>>>>would extend to ~0.5 µg/cm2 eq. NaCl for a tightish non-HDIS SMA.
>>>>>>>
>>>>>>>That having been said, these arguments apply only to cases before
>>>>>>>conformal coating is applied (MIL-P-28809n is specific on this
>>>>>>
>>>point) to
>>>
>>>>>>>avoid vesication and to apply the same arguments to circuits without
>>>>>>>coating is totally unreasonable, because the conditions of operation
>>>>>>
>>>are
>>>
>>>>>>>totally different. Specifications have never considered this and are
>>>>>>>therefore useless. IMHO, the only thing to do is to determine your
>>>>>>>figure empirically. Unfortunately, this cannot be done overnight and
>>>>>>>requires great knowledge of how your products are going to be used
>>>>>>
>>>and
>>>
>>>>>>>under what climatic conditions. You can try accelerated tests but
>>>>>>
>>>they
>>>
>>>>>>>are difficult, even impossible, to correlate with real-life
>>>>>>
>>>conditions,
>>>
>>>>>>>but they may give you a starting point. If products coming back for
>>>>>>>subsequent repair show any signs of environmental damage, then you
>>>>>>
>>>have
>>>
>>>>>>>to tighten the figure. If they come back in a pristine condition or
>>>>>>>don't come back at all, then you may be able to relax your figure
>>>>>>>slightly.
>>>>>>>
>>>>>>>Again, IMHO, no one here can advise you, without knowing a lot more
>>>>>>>about your products, the required reliability over a length of time
>>>>>>
>>>and
>>>
>>>>>>>the conditions of assembly, cleaning and use with a specific figure.
>>>>>>
>>>It
>>>
>>>>>>>may be that your 20 µg/cm2 is OK for you (although I very much doubt
>>>>>>
>>>it,
>>>
>>>>>>>as this figure is outside my knowledge of acceptable figures - and I
>>>>>>
>>>am
>>>
>>>>>>>one of the pioneers of ionic contamination testing, having worked
>>>>>>
>>>for
>>>
>>>>>>>over three decades on this and related subjects).
>>>>>>>
>>>>>>>Please do not assume any figure is correct for you, without
>>>>>>
>>>>>verification.
>>>>>
>>>>>>>Brian
>>>>>>>
>>>>>>>Angela Gregor wrote:
>>>>>>>
>>>>>>>
>>>>>>>>First I wanted to thank you all for the gasket information you
>>>>>>>
>>>gave.
>>>
>>>>>>>>Evidently this forum is very much respected because I didn't get
>>>>>>>
>>>any
>>>
>>>>>>>>arguments from anyone here. Here's another one. At our company we
>>>>>>>
>>>are
>>>
>>>>>>>>currently using Ionograph 500m version 3.02 to test our assembled
>>>>>>>
>>>>>boards
>>>>>
>>>>>>>>after wash. Our pass/fail limit is 20 micro grams of sodium per
>>>>>>>
>>>square
>>>
>>>>>>>>centimeters. I'm not sure our calculation is correct, and I was
>>>>>>>
>>>>>>>wondering
>>>>>>>
>>>>>>>
>>>>>>>>what other companies pass/fail limits are for comparisons. I called
>>>>>>>
>>>>>>>about
>>>>>>>
>>>>>>>
>>>>>>>>three places in my area and got three different answers. If this
>>>>>>>
>>>helps
>>>
>>>>>>>most
>>>>>>>
>>>>>>>
>>>>>>>>of our boards are double sided. I would appreciate any feed-back.
>>>>>>>
>>>>>Thanks
>>>>>
>>>>>>>in
>>>>>>>
>>>>>>>
>>>>>>>>advance.
>>>>>>>>
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>>>>>>
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