Innovative Water Management Solutions
|
Innovative Water Management Solutions
|
|
|
Technical Articles
Water
Management Alternatives for
Aqueous Cleaning (As
appeared in Circuits Assembly, December 2000) Although no-clean process technology has advanced and
gained widespread acceptance, the decline in aqueous cleaning seen in the
mid-1990’s has, to some degree, reversed its trend.
Aqueous cleaning is common in contract assembly operations, the highest
growth segment of the business, and has gained acceptance in high reliability
electronics. The most significant
issue with aqueous cleaning stems from water management. Where do I get all that water, how is it purified, and where
does it go after the cleaning process? And,
how is all of this accomplished within the framework of environmental
responsibility? Fortunately, the biggest problem with water management is
not the lack of technology or equipment, it’s the lack of process knowledge.
Following is a basic overview of aqueous cleaning and associated water
management techniques. Although
emphasis is placed on in-line cleaning of circuit boards, much of the
information transfers to batch cleaning and other processes. Process Overview
· Straight aqueous – water only, no chemical additives ·
Modified aqueous – wash water includes saponifier, detergent, or
some other additive The straight aqueous process is ideal for removing organic
acid, water-soluble fluxes. Water,
especially deionized water, is a powerful polar solvent, and it will remove
polar contamination like the acid residue left behind after soldering.
Water alone, however, will not remove non-polar contamination such as the
sticky rosin in rosin-based flux. Without
adding a non-polar component to water, potentially harmful acids and
particulates will be trapped by the rosin and could eventually degrade the
electrical characteristics of the circuit board.
(Note that even no-clean flux is sometimes cleaned).
The most common additive used is a saponifier, which is an alkaline
detergent with surfactants that will solubolize rosin so it can be rinsed off
with water. This process is
significantly more complex than straight aqueous, and these complexities
translate to water management techniques as well.
Water Management of
the Straight Aqueous Process
An in-line cleaner used in this process typically features
a prewash, recirculating wash (with tank), recirculating rinse(with tank), final
rinse and dry. The purest water
used in the process (from whatever source) enters the system in the final rinse
and cascades to each previous section, finally exiting to drain at the prewash.
The board moves through progressively “cleaner” water until it
reaches the dry section. Ideally, water in the final rinse is pure enough (ie. it has
low conductivity/high resistivity) that any residue evaporated onto the board
during drying will have insignificant ionic characteristics relative to board
cleanliness specifications. Cleaners
typically require 3-5 gallons per minute of incoming water at approximately 140oF.
Incoming water should be preheated to enable the cleaner to maintain
stable process temperatures and facilitate the drying process. Often, incoming water is treated with carbon ion-exchange
to provide a level of deionization commensurate with meeting desired board
cleanliness levels. Water that goes
into a cleaner must come out of it so, in an “open loop” system, 3-5 gpm of
heated water goes down the drain. Naturally,
a number of variables are involved in determining cost of this type of
operation, such as: · Quality of incoming tap water · Municipal water and sewer charges · Cost to heat water (electricity or gas) ·
Frequency and cost of regenerating DI tanks Various cost models exist, but based on 2,000 hours of
operation per year, an open loop DI system (granular activated carbon (GAC),
anion, cation, mixed bed) can cost in the range of $35-40,000.
Water management techniques to make the process more efficient, effective
and environmentally responsible include: · Heat recovery through use of a heat exchange system · Complete recycle without pretreatment of incoming water ·
Complete recycle with pretreatment of incoming water The decision of whether or not to recover thermal energy
from drain-bound water is almost trivial in nature. Sending heated water to drain is equivalent to sending money
down the drain. A heat recovery
system takes hot water exiting the cleaner and runs it through a heat exchanger
to recover much of the thermal energy. In
an open loop system, this energy can be used to heat fresh incoming water, or,
in a recycle system, can heat water returning to the prewash.
Capital investment for a heat recovery system is such that favorable
economic payback can usually be realized in a relatively short period of time. Complete recycling in a straight aqueous system reduces
water usage by approximately a factor of 10 and eliminates the ongoing waste
stream (3-5 gpm). Water usage is
limited to make-up water for evaporative and exhaust losses, and dragout.
Several configurations are available, but most involve a recycle system
(tank, recirculation pump, control system), a media tank set and a booster
heater. Effluent from the
cleaner’s main drain feeds the recycle system, either by gravity or from a
transfer station. The central unit
provides pressure to push effluent through the media tanks, booster heater, and
back to the final rinse. State-of-the-art media sets include the ability to isolate
heavy metals such as lead and copper, and capture them for disposal by a
licensed waste hauler. Deionization
occurs as the water flows through granulated activated carbon (GAC), cation and
anion tanks. This basic system will
typically achieve resistivity in the range of 1-3 meg-ohms. Additional deionization can be obtained by adding a mixed bed
tank (anion and cation in one tank), which will result in water approaching the
highest level of DI at 18.2 meg-ohms. Media tanks need to be regenerated when their ability to deionize water drops below a predetermined level. This will be affected by the amount of flux and contaminants inherent in the process and the quality of incoming make-up water. In many cases, incoming tap water has a high level of total
dissolved solids (TDS), which will increase the burden on media tanks and
increase regeneration frequency and costs. To address this problem, incoming make-up water can be
filtered and purified by a separate reverse osmosis (RO) system.
This process takes the water stream and forces it through membranes.
As the stream splits, a portion goes through the membrane and some is
used to keep the membrane clean. Product
water typically has a resistivity of 25,000 to 500,000 ohm-cm, which is
significantly better than 2,000 to 3,000 ohm-cm of tap water.
The waste stream can be directed to drain, since no contaminants were
added to it. Again, several
cost-payback models exist, but, in general, the addition of RO to the recycle
process makes sense when tap water quality is poor. Water Management of the Modified Aqueous Process
In modified aqueous cleaning, both machine and water
management techniques increase in complexity.
Because saponifier, or any other chemical additive, is costly, it is
desirable to recirculate the chemistry by closing the prewash drain and
directing the flow back to the wash tank. Thus,
the prewash and wash combine to make one larger wash section.
In addition to the cost factor, saponifier is highly ionic in
composition, so it cannot be left on the circuit boards, and it will have an
immediate detrimental effect on the life of media tanks. Different equipment manufacturers have different techniques to minimize dragout of saponifier, but one of the most effective is to utilize an interstage rinse between the wash and recirculating rinse (sometimes called chemical isolation) that floods saponifier off the board, and uses an airknife to squeegee it off. The cleaner will have not one effluent stream, as in the straight aqueous process, but three: · Wash tank - when the tank is drained · Interstage rinse - ongoing, approx. 1 gpm ·
Rinse – ongoing process stream of 3-5 gpm The rinse stream, in effect, becomes a straight aqueous
process loop and can be recycled by conventional means.
The interstage rinse stream will have residual saponifier and, possibly,
some lead content. In some municipalities, the level of contamination is low
enough that this stream can be sent to drain.
Where regulations are more stringent, this stream (and the wash stream)
must be treated by drain media to remove heavy metals.
(Note that some local regulations will require additional pH treatment,
or possibly evaporation to achieve compliance.
Local regulations must always be understood when specifying a water
management system). The same argument for RO pretreatment of incoming water
applies to saponified aqueous cleaning.
Potential benefits of RO are augmented since purified water is supplied
not only to the ongoing rinse process stream, but also to the interstage rinse. Multiple Cleaner and Special Applications
In production situations where dual or multiple cleaners are utilized, the potential cost benefits of heat recovery, recycling and recycling with RO are increased. High capacity systems are now available to support up to four cleaners from one central unit. There are also systems available to aid with lead removal and regulatory compliance in the stencil cleaning process. Keys
to Success with Recycle Systems As
with any production situation, process monitoring, equipment maintenance and
common sense are keys to success with recycle systems.
The most frequent complaint with recycle systems is the length of time
between regeneration of resins. This
period of time depends on a number of variables, including volume of water
processed (directly related to run time), quality of incoming water (which can
be greatly improved by using RO pretreatment), and amount and type of
contaminants introduced to the process water.
Anything ionic in nature is going to burden the resin beds.
Likewise, organic matter can plug up GAC tanks.
A common problem area is the use of water soluble tapes and masks.
The manufacturer of these materials should be consulted for information
regarding impact on the carbon ion exchange process.
Many
times a perceived reduction in media tank life is due to an increase in
production hours or board volume. The
more water and contaminants processed, the faster tanks will need regeneration.
A log of cleaner run time and board volume should be kept after
implementing water management to determine a baseline of operation.
Another
key to success is following the equipment manufacturer’s instructions for
safe, compliant operation. This
is especially important in regard to HMR (heavy metal removal) tanks.
These tanks must be taken off line at the recommended intervals and
disposed of by a licensed waste hauler. Failure
to do so can cause lead break-through into subsequent media tanks. Benefits of Recycling and Water Management Utilizing any or all of the
techniques discussed will yield tangible benefits, such as: · Cost reduction due to: o Reduced water usage o Reduced energy consumption o Reduced (or eliminated) sewer assessments o Improved consistency of the cleaning process · Aid with environmental compliance · Improved company reputation for being “environmentally friendly” ·
Improved operator safety (no exposure to lead in stencil cleaning) Realizing these benefits depends upon a good understanding of water management techniques, strong engineering of equipment, and partnership with a vendor who provides the service and expertise to ensure success.
|
|
Copyright © 2008
REsys, Inc.
|
Before considering water management options, it is best to
divide aqueous cleaning into two categories: