4.5 Coating Removal System/Process
4.5.1 System Description
The system used for the conformal coating removal study was a SWAM BLASTER Model MV-1 micro blasting machine from Crystal Mark, Inc. The model MV-1 is a manual system. The primary components of the system include a microblast unit, a work chamber, a gas dryer, an ESD point ionizer, a dust collector and a source of dry compressed air or an inert gas. The system design allows easy access and flexibility for pressure regulation, media flow control, nozzle replacements and orifice plate changes.
During manual operation, a mixture of micro abrasive media and compressed gas is propelled through a specially designed hand held nozzle. The abrading process is accomplished by introducing a precise amount of finely graded powder (media) into a stream of compressed gas which can be controlled by a foot switch.
The SWAM BLASTER system features a statically controlled work chamber for the protection of ESD sensitive parts. The system has a point ionizer which generates a balanced flow of positive and negative ions. It has a specially designed nozzle which couples the ions directly into the abrasive stream. By combining ESD neutralization directly into the abrasive pathway, the potential for ESD damage is minimized. Without the point ionizer, ESD voltages in excess of 3000 volts are realized.
The work chamber is constructed of conductive laminate. All elements of the system including inside chamber surfaces, point ionizer, media, PWAs and the operator are connected to a common ground to ensure that they are at the same potential.
The general specifications for the system are the following:
Appendix B shows the Microblaster System and the Probe Assembly.
4.5.2 Electrostatic Discharge Voltage Measurement
4.5.2.1 Equipment
In order to measure ESD voltage generated during micro blasting, a high accuracy, non-contacting electrostatic voltmeter (EVM) manufactured by Monroe Electronics was used. The Monroe Electronics Model 244 Isoprobe EVM measures a surface potential with 99.9% or better accuracy regardless of probe-to-surface spacing on sites as small as 0.1" (2.5mm) diameter. The EVM functions by means of a high voltage amplifier which automatically drives the probe to the same potential as that of the surface under measurement. By monitoring the output of the amplifier, an accurate indication of an unknown ESD potential can be achieved.
With the 244 EVM, a high frequency probe (Model 1017) was used to measure surface voltage. The probe had an end-viewing configuration and superior response speed (<3mS) throughout its ± 3 kV range. During all measurements the probe was held stationary 3 mm distance from the blast sites using a spacer attachment. The probe was also equipped with a gas purging option which improved its performance by preventing the entrance of contaminants and by maintaining a clean atmosphere in the vicinity of the electrode.
A Model 709 static sensor from 3M Corporation was used to monitor the electrostatic field within the work chamber. This portable sensor did not have high accuracy and was used to get a general indication of the electrostatic field present. The sensor was placed within the work chamber approximately 10" from the probe.
4.5.2.2 Equipment Calibration
A Fluke Model 410A high voltage power supply and a Fluke Model 8849A digital multimeter were used to calibrate the EVM. This calibration was performed at the Test Equipment Calibration and Repair (TECR) facility for Goddard operations located at the Unisys facility in Lanham, Maryland.
The calibration procedure involved measurements at 100 volt increments (from 0 to 3000 volts) using 6" x 6" test plate to verify the accuracy of the Monroe EVM and the manufacturer's calibration. The instrument was checked for zero balance using a grounded metal plate prior to each measurement.
4.5.3 Process Description
The conformal coating removal experiments were conducted for two types of coatings and four blast medias. The following are the key variables involved in the conformal coating removal process:
Since the type of blast media is the most important variable, the microblaster parameters such as media flow rate, gas pressure and coating removal time were optimized for each media and type of conformal coating. The nozzle parameters including the size, shape of orifice, distance from the PWA and angle of impingement were kept constant throughout the study.
The microblaster was effective in removing both types of coatings within 30 seconds in most cases. It was observed during the trial runs that as the media blasted through the coating, the surface ESD voltage increased. The ESD voltage peaked when the media hits pad or PWB surface. The primary goal of this study was to monitor the level of surface ESD generated during the coating removal process. Therefore, all measurements were taken instantaneously (within a few seconds) at the end of the coating removal. No attempt was made to monitor the rate of ESD voltage decay after the removal of the coating.
Table 2 shows the optimized system parameters used for the removal of urethane and parylene conformal coatings.
Table 2. Microblaster Parameters
|
Coating Type |
Blast Media |
Gas Pressure (PSI) |
Nozzle Diameter (inch) |
|
|
Type |
Flow Rate |
|||
|
Urethane |
Wheat Starch Sodium Bicarbonate Plastic Bead Glass Bead |
5 4.5 4 2 |
100 95 60 60 |
0.032 0.032 0.032 0.032 |
|
Parylene |
Wheat Starch Sodium Bicarbonate Plastic Bead Glass Bead |
4 5 4 2 |
50 80 60 60 |
0.032 0.032 0.032 0.032 |
At the beginning of each run, all ESD precautions were in effect including grounding the equipment, operator and all elements of the work chamber to a common ground. The point ionization device was used throughout the process except when purposely taking measurements without it. The microblaster reservoir was filled completely with the appropriate media. The blast media was predryed at 100°C for 24 hours before use. The microblaster parameters were set to the optimized values as described in Table 2.
The EVM probe was mounted on a stationary fixture with a spacer assembly so that the PWA could be positioned in front of the probe without surface contact. This ensured that all measurements could be taken at 3mm from the blast site irrespective of the location.
The media flow was activated by a foot switch. In order to achieve high cutting speed and uniform removal of the conformal coating, the tip of the nozzle had to be positioned approximately an inch from the blast site and at an angle between 45 and 90 degrees from the PWB surface.
At each site a 20 - 50 square mm area (5-8 mm diameter) was blasted until the conformal coating was completely removed. The total removal time ranged from 10 - 30 seconds for the pad and trace areas and 30 - 60 seconds for the solder joint areas. At the completion of the conformal coating removal the surface voltage measurements were taken within a few seconds. This method allowed a quick and repeatable measurement of surface voltages before the electrostatic charge could dissipate. All PWAs were microscopically inspected (10-40X) for any surface damage.