[Diy_efi] PCB washing using water

[Mike]

No difference at all Bill,

I accept there are many changes going on in the industry
and water is used in many places but it all relates to the
part of the process, the context and the state of equipment
post manufacture condition etc.

You are describing part of what the industry does and I have
described a different part, here are some background points
which predicated the overall rationale I used to recommend IPA
and specifically exclude water for the posters dilemma:-

a.      Encapsulation epoxies are slightly hygroscopic.

b.      The huge number of digital devices are mostly
        immune to a low level of moisture retention.

c.      In production environments, the vast majority of
        integrated circuits including digital are delivered
        in sealed packages with desiccant gel.

d.      The vast majority of analog circuits (which have any
        sort of high gain property) are delivered in double
        sealed packages with desiccant gel *and* a colour
        coded humidity record card.

e.      Log linear stages both discrete and integrated are often
        used on EFI controllers employing analog devices.

f.      Part of the production process steps in the manufacture
        of EFI controllers includes drying in an oven which is
        also purged of moisture. Leaving to cool in as dry an
        environment as possible to avoid any condensation
        and as soon as possible using conformal coatings.

g.      Water is known to seriously affect analog circuitry on Carbon
        Monoxide fuel cell amplifiers for an industrial 4-20mA
        signalling board, I have direct experience of this.

h.      The poster who originally raised the problem of anti-freeze
        spill on an EFI controller confirmed a conformal coating.

i.      It is unclear if the anti-freeze had dissolved any of the
        conformal coating or any small part of the connectors
        which might allow a path of any fluids to the board or ICs.

Given such a gestalt, would it be sensible to wash thoroughly with
water or use a water free solvent such as Iso-propyl-alcohol and 
further more do we apply a tangential paradigm that "de-ionised water
should be ok" (since its used during integrated circuit manufacture)
or select the most appropriate solvent for circuit board in its
current problematic context for long term reliability  ?

I might add, any solvent has probabilistic causal factors, the
width of the normal curve for IPA in respect of circuit boards
might be viewed as wider than that for water in respect of
freedom from long term effects. Neither is perfect, IPA is known
to be hygroscopic and should be kept sealed.

The posters experience with using water falls within the normal curve in
his specific use and may well confirm the conformal coating
and connectors or other plastic fixtures havent been damaged by
the anti-freeze, one doesnt interpret from this (as one private email has)
that "I told you so, water is ok" <sigh>

It comes down to the luck of the draw at the time and my aim is to
inform as to the wider experience and sensibility of using the best
chemically compatible solvent for the task at hand.

Rgds

Mike












At 10:15 AM 11/2/05, you wrote:
>Mike,
>        I beg to differ on PCB washing, for the last 10 years or so in PCB assembly the fluxes used are water soluble and the PCB wash after assembly is with water to remove whatever residue remains after the reflow process - most of the flux evaporates in the reflow oven which peaks at about 215 Deg C for a few seconds. This change was essentially brought about for environmental reasons.
>        Therefore washing the pcb with water without power applied is safe because tha components are designed to cope with this, and as someone else stated natural drying is preferred to compressed air because oif the static issue.
>        However, that being said, the coolant residues may require a subsequent wash with isopropanol to complete the cleaning - CRC also produce a product called 'Contact Cleaner' which I have used sucessfully on occassion.
>
>        As an aside, there is a rather large change happening now in the electronics industry with the migration away from components and solders containing lead and a number of other substances which are considered 'hazardous'. This is referred to as RoHS (Removal of Hazardous Substances) of which "Lead Free" or "Pb-Free" is a part.
>        This becomes mandatory in most countries during 2006 and has been a big challenge (and expense) for the industry.
>
>        One effect of this is that the melting temperatures of Lead-Free solders will be higher, and Lead Free solders do not flow as well as solders containing lead, therefore the joints will look 'different' and 'dry' joints will be much more difficult to identify visually.....
>
>Regards
>Bill
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Regards from


Mike
Perth, Western Australia
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