| Overview: |
Micro-abrasive sand blasting
is accomplished by propelling a finely graded abrasive powder into a
stream of compressed air, through an abrasive-resistant hose and out a
miniature nozzle manually or automatically positioned at the workpiece.
The process is used to remove a variety of conformal coatings, including
epoxy, acrylic, urethane, silicone, parylene, and ultraviolet-cured
materials, from printed circuit boards for rework and repair. It replaces
chemical, mechanical, and thermal methods of coating removal. Some
micro-abrasive units also can be used for other functions, such as
deburring, texturing, drilling, and cutting.
There are five functions in micro-abrasive blasting
that control the process: 1) air pressure, 2) nozzle diameter, 3) distance
of the nozzle from the workpiece, 4) powder flow rate, and 5) type of
abrasive powder used. A variety of different abrasives can be used
depending on the application. These may include aluminum oxide, silicon
carbide, glass beads, sodium bicarbonate, walnut shell, or plastic media.
Each is used in a microscopic form usually between 10 and 150 microns.
Several of these media may not be appropriate for conformal coating
removal, so it is important to check the specifications and suggested uses
for each before using them.
Units range from compact, manually operated benchtop
units to fully automated programmable equipment. In either system, clean,
dry air is mixed with a precise quantity of abrasive material and
propelled at 75 to 100 psi through an extremely small nozzle at the end of
a pencil-shaped stylus. The abrasive material is directed at a target area
to accomplish a specific task. Spent material is continuously drawn out of
the work chamber via a vacuum and then sent through a filtration area to a
dust collection system. Some systems collect spent media for reuse, thus
reducing overall operating costs of the system.
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| Compliance Benefit: |
Micro-abrasive sand blasting
operations generate less hazardous waste than chemical stripping since solvents
are not used. The decrease in hazardous waste helps facilities meet the
requirements of waste reduction under RCRA, 40 CFR 262, Appendix,
and may also help facilities reduce their generator status and lessen the
amount of regulations (i.e., recordkeeping, reporting, inspections, transportation,
accumulation time, emergency prevention and preparedness, emergency response)
they are required to comply with under RCRA, 40 CFR 262. In addition,
the decrease in the amount of solvents on site decreases the possibility
that a facility meets any of the reporting thresholds of SARA Title III
for solvents (40 CFR 300, 355, 370, and 372; and EO 12856).
The compliance benefits listed here are only meant to
be used as a general guideline and are not meant to be strictly
interpreted. Actual compliance benefits will vary depending on the factors
involved, e.g. the amount of workload involved
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| Materials Compatibility: |
Abrasive materials must be matched
to the job to avoid inadequate results or damage to the components. For
example, aluminum oxide and silicon carbide will blast through a board in
only a few seconds. Sodium bicarbonate and walnut shells have high electrostatic
discharge readings, which will destroy good components on a board.
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| Safety and Health: |
Consult your local
industrial health specialist, your local health and safety personnel, and
the appropriate MSDS prior to implementing this technology.
|
| Benefits: |
- Reduces use of chemicals in process.
- Improves efficiency of operations, saving on labor,
materials, handling and disposal of hazardous waste.
- Improves personnel health and safety in the work
environment.
- Reduces labor hours for coating removal.
|
| Disadvantages: |
- Possible circuitry degradation may occur when the
abrasive stream hits the coating and generates electrostatic charges.
- Excess removal of plating from traces and leads can
expose the base metal to possible corrosion.
- Removal of solder mask off the printed circuit
board (PCB) may cause possible changes to the insulation value of the
PCB.
- Incorrect choice of abrasive can lead to damage of
workpiece or wasted abrasive medium and labor.
- Nonuniform particle size can result in clogging the
nozzle.
- If the system is not clean and dry, clumping can
result, thus clogging the nozzle.
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| Economic Analysis: |
According to Mr. Carroll Claterbuctz
of NASA, capital costs for all micro-abrasive blast equipment (blaster unit,
dust collector, work chamber, air filter, point ionizer) is approximately
$5,000 to $13,500, depending on options and levels of electrostatic discharge
protection necessary for the job. Check with the manufacturers listed below
for their specifications.
Polyurethane coating is the most common coating
currently in use. Chemical and micro-abrasive methods of coating removal
are the most appropriate methods for this type of coating. This cost
analysis compares the use of chemical and micro-abrasive methods of
coating removal.
Assumptions:
- Labor costs = $45/hour
- Cost of blasting material = $120/15 lbs.
($8/lb.)
- Cost of solvent/chemical = $10/gallon
- Time to remove polyurethane coating from one 1"x1"
component using blast system = 2 minutes
- Time to remove polyurethane coating from one 1"x1"
component using chemical method = 2 hours
- Blast equipment uses up to 50 grams of blast
medium/minute
- Solvent use = 0.125 gallons per job
- Hazardous waste/solvent disposal = $2/lb.
- One gallon of hazardous waste/solvent = 8
lbs.
- Spent blasting media is assumed to be disposed as
hazardous
- 1,500 components are cleaned annually.
Removal of Polyurethane Coating from
Printed Circuit Boards Traditional Chemical Removal vs. Micro-Abrasive
Blast System
| |
Traditional Chemical Removal |
Micro-Abrasive Blast System |
| Capital
Costs: |
$0 |
$8,500 |
| Material
Costs: |
$1,875/yr
(solvent) |
$2,670/yr (blast
material) |
| Labor
Costs: |
|
|
| Removal: |
$135,000 |
$2,247.75 |
| Handling: |
$1,124.55 |
$1,124.55 |
| Cleaning: |
$3,373.65 |
$562.27 |
| Waste
Disposal: |
$12,000 |
$661 |
| Total: |
$153,373.20 |
$7,265.57 |
Economic Analysis Summary:
Most of this
savings is realized in reduced labor costs and reduced hazardous waste
disposal costs. These figures do not factor in worker safety improvements
associated with micro-abrasive blast systems. In addition, the material
costs for micro-abrasive systems would be less if the spent blast material
were reused. Also, the plastic media that is most often reused does not
leave a residue on a printed circuit board, and therefore, eliminates the
need for and the cost of cleaning. All of these factors would decrease the
overall cost of a micro-abrasive blasting system.
Annual Savings for Using Micro Blast System:
$146,108
Capital Cost for Diversion Equipment/Process:
$8,500
Payback Period for Investment in Equipment: Less than one
month
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Approving
Authority: |
Approval is controlled
locally and should be implemented only after engineering approval has been
granted.
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