WO1990005795A1

WO1990005795A1 - Apparatus and method for applying vapor barrier coating to printed circuit board

Info

Publication number: WO1990005795A1
Application number: PCT/US1988/004064
Authority: WO
Inventors: Takaji Shimada
Original assignee: Nordson Corporation
Priority date: 1988-11-14
Filing date: 1988-11-14
Publication date: 1990-05-31
Also published as: KR920701518A

Abstract

A system for applying a vapor barrier coating to a printed circuit board having circuit components mounted thereon, which includes an airless spray gun having a nozzle (7) configured to provide a flat spray pattern, a source (25) of vapor barrier coating liquid connected to the gun for supplying liquid coating to the gun at a pressure which produces a leaf-shaped flat liquid spray pattern emission from the nozzle, the nozzle having an orifice sufficiently large to provide continuous clog-free flow of liquid therefrom when pressurized liquid is supplied thereto at a pressure which produces the leaf-shaped flat liquid spray pattern, valve means (23) connected between the nozzle and the liquid supply for controlling the supply of pressurized liquid to the nozzle, and actuating means operatively opening and closing said valve for relatively short and long intervals, respectively, to effectively decrease the average flow rate per unit time from the nozzle with respect to that resulting were the valve continuously open, thereby permitting relatively thin liquid coatings to be applied to a printed circuit board when the relative speed between the gun and circuit board is sufficiently slow that an unduly thick coating would be applied were the valve continuously open.

Description

APPARATUS AND METHOD FOR APPLYING VAPOR BARRIER COATING TO PRINTED CIRCUIT BOARD

The present invention relates to an appara¬ tus and method for coating vapor barriers onto fi¬ nished circuit boards using airless spray coating techniques.

In recent years most finished circuit boards having electronic devices mounted thereon are coated with a film of vapor barrier. _.The reason for this is, of course, to protect the finished circuit boards against moisture, electric leakage, and dust. The composition used for the vapor barrier film typically is a synthetic resin such as acrylic resin, polyurethane resin, or epoxy resin, which is dissolved in a volatile solvent. As the barrier layer film coating composition is applied on a clean finished circuit board, the solvent volatilizes to provide a uniform resin film.

The following six methods for coating vapor barriers on finished circuit boards are known:

1) Immersion method,

2) Brushing method,

3) Spray method, 4) Roller method,

5) Film extruding method, and

6) Nozzle flow coating method.

Of these six, those suitable for low production rates, such as with a hand-held applicator, are 1) , 2) and 3) . These three, however, have the following disad¬ vantages:

1) Immersion method a. Masking of circuit components not to be coated is necessary. b. There is a great difference in thickness of the film layer, depending on the configuration of the circuit components and their layout on the circuit board. c. Circuit components can be damaged by the solvent. d. Immediately after applying the coating, the coating drips. e. The organic "solvents incorporated in the coating material pollute the environment.

2) Brushing method a. There is substantial nonuniformity in the thickness of the coated film. b. Hairs from the applicator brush can become embedded in the coated film. c. Considerable effort is required in the brushing operation.

3) Spray method a. Scattering of atomized coating particles in the spray causes waste of coating material, as well as environmental pollution. b. Where it is desired not to coat specific circuit components mounted on the board, masking is necessary. c. Corners present at junctions where circuit components are joined to the circuit board are often left uncoated.

As described above, many faults are present in coating methods adaptable for hand or manual application.

As for automatic coating methods using motor-driven applicators, the roller method 4) has a serious deficiency, namely, uneven parts cannot be coated, while the film extruding method 5) is flawed by the fact that masking is required and the yield rate is low.

The nozzle flow coating method 6) for coating circuit boards with a liquid vapor barrier using an airless spray gun was invented by the present inventor as described in U.S.A. patent application Serial No. 07/206,199 and U.S. Patent 4,753,819 which are incorporated herein in their entirety by reference. This technique has overcome many of the. faults and problems of the other approaches, such as the need for masking, low yield rate, environmental pollution, etc. Basically there are two types of spray coating techniques used in spraying liquid coatings, namely, the air atomization method and the airless atomization method. In liquid coating devices using air atomization, the liquid coating is atomized by means of a high-speed air stream, preferably located in the air nozzle, which impinges the liquid coating stream to disperse or atomize it as it exits the nozzle. Airless liquid spray coating does not use an air stream to atomize the liquid, but hydrostatically pressurizes the liquid to a very considerable degree upstream of the nozzle orifice such that after it exits the nozzle orifice into the air at ambient pressure, it expands and in the process is atomized. In coating applicators using conventional liquid coating atomization methods described above, the atomized liquid coating particles are sprayed toward the circuit board onto the surface to be coated. However, in airless-type spray applicators, atomization of the liquid, particularly when a flat spray pattern nozzle is used, the liquid emitted from the nozzle does not change into an atomized mist immediately upon exiting the nozzle orifice, but for a distance of a few millimeters from the nozzle opening it remains in the form of a flat dovetail-shaped unatomized liquid film. As described in my above-referenced patent applications, I discovered that by lowering the pressure applied to the liquid in -5- an airless-type applicator, the dovetail-shaped unatomized liquid film could be made longer and wider. In particular, I discovered that an airless spray applicator having a flat pattern nozzle wherein the liquid pressure is in the approximate range of 1-4 kg/cm² could be used to effectively apply vapor barriers to circuit boards. At such pressures, which are far below conventional airless spray coating pressures levels, the liquid film pattern takes the form of a leaf which is wider and longer than the dovetail-shaped liquid film patterns achieved in conventional airless coating applicators operating at substantially high pressures. By traversing the applicator longitudinally over the circuit board vapor barrier films in the form of an elongated strip can be applied to the board. By applying this leaf-shaped film in overlapping parallel strips, an entire circuit board can be coated. It is also very easy to inter¬ rupt the leaf-shaped film discharge in the vicinity of circuit components mounted on the circuit board which are not to be coated. For this reason, masking is not needed, and atomization of the coating material does not take place, so the system works effectively and does not pollute the environment, which is highly desirable in the coating industry.

However, my leaf-shaped film coating tech¬ nique described above is not entirely satisfactory in low production rate applications, such as for manual applications using a conventional airless hand-held or manual applicator gun. The reason for this is that the speed of movement of the applicator gun relative to the circuit board is low, resulting in application of a liquid film which is too thick. While it might be expected that overly thick coatings at low gun speeds could be avoided by reducing the diameter of the conventional airless nozzle orifice, which is typically 0.3 mm or larger, I have found that this does not solve the problem. Instead, with a smaller diameter nozzle, the nozzle clogs up easily and an irregular liquid coating discharge occurs. Viewed differently, to avoid clogging at the low pressures of 1-4 kg/cm² necessary to- produce the leaf-shaped liquid coating film, the liquid discharge must be maintained above 20 cc/min. However, unless the applicator speed, relative to the circuit board, is maintained at 100 mm/sec. or more, a 20 cc/min. liquid discharge rate will produce a coating film thicker than desired. Unfortunately, in low production rate circuit board coating applications, particularly using hand-held applicators, the applicator speed is considerably under 100 mm/sec. and thin barrier coatings on the circuit board have been unobtainable.

The objective of this invention is to obtain, using an airless spray applicator operating at pressures in the range of 1-4 kg/cm², thin barrier layer film thicknesses, preferably in the range of 20mm-50mm, under conditions of use wherein the low pressure airless applicator moves at a relatively slow speed over the circuit board, which thin barrier layer coatings are to be produced without reducing the diameter of the nozzle orifice below approximately 0.3mm, which if permitted to occur will result in clogging and irregular coating flow from the nozzle when operated at the unusually low pressures asso¬ ciated with leaf-shaped liquid film spray patterns.

In accordance with the principles of this invention, the foregoing objective is accomplished by rapidly cycling the gun discharge flow control valve on a relatively low duty cycle basis, causing inter¬ mittent discharge of the coating material from an ■ airless application gun having a standard size nozzle, but operating at nonconventional low pressures used to create leaf-shaped liquid coating film patterns. The low duty cycle intermittent discharge is effective to reduce the amount of coating discharged per unit time, without reducing the nozzle orifice diameter which, if permitted to occur, creates clogging and irregular flow when the gun is operated at the unconventionally low pressures required to obtain leaf-shaped liquid film spray patterns. As a consequence, the applicator speed relative to the circuit board may be maintained at relatively low levels, for example, as is desirable for handwork with a manual applicator, while at the same time producing a relatively thin barrier layer film on the circuit board.

These and other objectives and advantages of the invention will be apparent from a detailed de¬ scription thereof taken in conjunction with the drawings in which:

Fig. 1 shows time-correlated plots of trigger actuation, [Fig. 1(a)], and valve actuation [Fig. 1(b)], and time-staggered elevational cross- sections of the circuit board immediately after intermittent application of the coating thereto along the length thereof [Fig. 1(c)] and after a period of time has elapsed and the coating has leveled out to become substantially uniform in thickness [Fig. 1(d)].

Fig. 2 shows a side elevational view of the circuit board immediately in the process of being coated.

Fig. 3 shows a top plan view of the circuit board of Fig. 2 to which coating has been partially applied. •

Fig. 4 is a view of the circuit board of Figures 2 and 3 after leveling has occurred.

Fig. 5 shows a vertical cross-section of the coating applicator handgun to which is connected a pressurized coating source and pulsed power supply.

Fig. 6 is a top plan view of the coating applied to portions of a circuit board having an uncoated circuit component mounted thereon. Fig. 7 is a top plan view of a circuit board covered with overlapped coating strips.

Fig. 8 is an elevational view of an applica¬ tor gun acutely angled relative to the circuit board to provide enhanced coating in corners formed at the point where the base of the circuit components are mounted to the circuit board.

Fig. 9 is an elevational view of an airless spray nozzle and dovetail-shaped liquid film with atomized particles therebelow, which is produced at liquid coating pressures substantially above 4 kg/cm².

Fig. 10 is an elevational view of a leaf-shaped liquid film emitted from the gun nozzle at very low liquid coating pressures useful in this invention.

Fig. 11 is a perspective view of an applica¬ tor gun in the process of coating a single strip on a circuit board having no circuit components thereon.

Fig. 12 is a perspective view of an applica¬ tor gun applying multiple coating strips to a circuit board having no components thereon.

Fig. 13 is a perspective view of a circuit board having components thereon with respect to which coating is applied in selected areas only.

In a preferred embodiment of the invention, the diameter of the discharging nozzle, when the applicator gun is operated at low pressure, e.g. 1-4 kg/cm², should exceed at least approximately 0.3 mm. By intermittently operating the applicator to dis¬ charge coating on only a periodic basis, for example at a duty cycle of 20%, that is, 20% ON and 80% OFF, the discharge flow rate can be reduced per unit time to 20% of that which would result were the discharge continuous and uninterrupted, thereby providing thin coatings at slow gun traversing speeds.

Additionally, in the preferred embodiment the following conditions apply:

1) The relative speed of applicator gun and circuit board is 50 mm/sec;

2) The cycling of the intermittent operation of the flow control discharge valve is 20 cycles/second (50 millisecond per each complete ON/OFF cycle) ;

3) The ON and OFF intervals of the valve are 10 millisecond ON and 40 millisecond OFF, respectively;

4) The relative traversing distance between circuit board and gun per ON/OFF cycle is 2.5 mm (50 mm/sec. gun speed with a cycle interval of 0,025 seconds) .

The state of the deposited film immediately after the coating is deposited onto the circuit board, as shown in Figs. 1(c) , 2, and 3, is an uneven thick¬ ness strip. Since the deposited film is in a liquid state, the uneven thickness strip flattens out after a few seconds because of the self-leveling phenomenon, and finishes hardening after a few minutes to give a uniform thickness thin film, as shown in Figs. 1(d) and 4. In the preferred embodiment, the 20% gun duty cycle (20% ON and 80% OFF) causes the amount of coating discharged per unit time to be one-fifth of a conventional continuous discharge gun which has a 100% duty cycle, in turn producing a coating thickness which is also one-fifth that achieved with a conven¬ tional gun operating at a 100% duty cycle.

The self-leveling action of the» initially uneven thickness deposited coating can differ depend¬ ing on the viscosity and hardening rate of the coating material and the form of the finished circuit board. However, this variation does not present a problem since the film thickness tolerance is +20% for a 30 mm thickness film. While a ±20% tolerance may be thought to be rather high, experience has proven it to be acceptable for vapor barriers applied to circuit boards.

A preferred embodiment of coating applicator handgun useful in practicing this invention, as shown in Fig. 5, iε seen to include a body 2 having a section 2a which serves as a handle. Within handle 2a is a liquid supply passage 3. The upper end 3a of the vertical liquid supply passage 3 is connected to a horizontal liquid passage 5 disposed within the body 2 at a 90 degree angle relative to passage 3. Extending horizontally from the side of the body 2 is an adapter 4 having a bore or passage 4a which is horizontally disposed in line with body passage 5. The outer end of the adapter 4 is threaded into a valve seat 6. A nozzle 7 is threaded onto the valve seat 6. A conical needle valve element 12 located at the end of a horizontal actuator rod 11 is installed axially within the liquid passages 4a and 5. The needle valve element 12 cooperates with the valve seat 6 to close and open the needle valve as the valve actuator rod 11 shifts between left and right positions in a horizon¬ tal direction. A core^' 13 of a solenoid coil 10 is secured to the right end of actuator rod 12 and is slideably received in the bore 9a of a cylinder 9 about which the solenoid coil is wound. The cylinder 9 is sealed at its outer end and is fastened at its inner end to body 2 such that bore 9a communicates with body passage 5. A spring 14 biases the core 13, valve rod 11, and valve element 12 leftwardly to normally close the valve. When coil 10 is energized with a pulse from pulsed power supply 30 in a manner to be described, the core 13 is magnetically pulled rightwardly to open the valve 6, 12.

In the middle of the handle 2a extending from the body 2, a pivotal trigger 15 is installed. Cooperating proximity switch elements 16. and 16B are installed on the trigger 15 and the body handle 2a, respectively. These proximity switch elements 16A and 16B collectively form a switch, which is connected via wires 31 to a source 30 of solenoid-energizing pulses [see Fig. 1(b)] to cause the pulse power supply 30 to energize the solenoid coil 10 with pulses through the electric wires 36, when the trigger 15 is pulled and the proximity switch elements 16A and 16B moved proximate to each other to actuate the trigger switch [see Fig. 1 (a) ] .

The liquid supply passage 3 in the handle portion 2a of the body 2 is connected at its outer end to a vessel or source of pressurized coating liquid 25 via a hose 24 and a flow rate adjusting valve 23. An air compressor 26 supplies pressurized air to the liquid supply source or vessel 25 via air supply line 26a to pressurize the liquid coating contents of the vessel.

In operation, the source of pressurized coating liquid 25 conveys pressurized coating liquid at the desired pressure, generally very low pressure in the approximate range of 1-4 kg/cm² or lower, to the hose 24 and the flow rate adjusting valve 23 to the liquid passage 3 in the lower handle 2a of the handgun body 2. The width W of the leaf-shaped liquid film exiting the nozzle orifice [see Figs. 10 and 11] at the point where it contacts the circuit board is adjusted, as desired for the particular coating appli¬ cation, by the valve 23. The pressurized liquid input to the liquid passage 3 enters the liquid passage 5 where its flow from the nozzle is controlled by the closed/open condition of needle valve element 12 and valve seat 6. When the trigger 15 is pulled [Fig. 1 (a) ] , the proximity switch elements 16A and 16B cooperate to turn ON the pulse supply circuit 30 via lines 31, which in turn causes pulses to energize the solenoid coil 10 via line 36. This, in turn, causes the solenoid coil 10 to be intermittently magnetized at the rate of once per pulse. This intermittent solenoid coil magnetization intermittently attracts the core 13 to the right, momentarily overcoming the bias of spring 14, to open the needle valve 6, 12 [see Fig. 1(b)]. As each pulse terminates, the coil magnetization ends and the'core ^'13 returns to its left position under the bias action of spring 14. The intermittent rightward and^' leftward core motion causes the valve actuator rod 11 to intermittently open and shut the needle valve 6, 12 in synchronism with the pulses, as is apparent from a comparison of Figs 1(a) and 1(b). As the needle valve 6/12 opens and closes, the pressurized liquid coating in the liquid passage 5 flows through the needle valve and exits the flat pattern nozzle orifice 8 on an intermittent basis to deposit the initially uneven film on the circuit board as shown in Figs. 1(c) , 2 and 3.

The following example is illustrative of the parameters of a preferred circuit board coating operation in accordance with the principles of this invention:

1) Nozzle: Cross-cut type with a flat pattern having an orifice of 0.28 mm.

2) Resin content (by volume) : 31%.

3) Pressure of the discharged liquid (resin and solvent) : 2 kg/cm²

4) Type of liquid resin: Urethane

5) Coating viscosity: 80 cps

6) Distance between the nozzle and the article to be coated: 10-14mm

7) Width of the liquid coating spray pattern at the point of contact with the circuit board: 12 mm

8) Coating discharge rate: 4.0 cc/min.

9) Linear speed of the nozzle relative to the circuit board: 60 mm/sec.

10) Valve ON/OFF cycling rate: 20 cycle/sec.

11) Duration of each valve ON/OFF cycle:

ON: 5m sec; OFF: 45m sec; duty cycle: 10%

12) Distance between the nozzle 8 and the circuit board be coated: approximate range of 5 mm. - 50mm.

Spray coating under the foregoing parameters for the purpose of producing an uninterrupted strip initially produced a coating strip having a "wavy" thickness profile along the length of the strip, as shown in Fig. 1(c) . The uninterrupted wavy coating strip initially consisted of numerous successive connected deposits C0-, C1, , C2„, . . . Cn-1.. , Cn each having a generally triangular profile with the maximum height of each deposit initially being 80 microns. As time elapsed, the wavy strip levelled, as shown in Figure 1(d) , to a relatively uniform thickness of approximately 25 microns with a thickness tolerance of ± 20%. The initial deposits C_, C-, C₂, . . . C -, C correspond to valve openings during the intervals P_Q, P-, ₂, . . . P -, P , respectively, as shown in Fig. 1(b) , produced by continuously maintaining the trigger 15 in its ON position during the entire interval T, as shown in Fig. 1(a)

With reference to Fig. 6, which shows use of the principles of this invention to apply overlapped interrupted strips S and S to a circuit board having circuit components X , X», and X which are not to be coated, it is apparent that transversely aligned interruptions in plural adjacent strips S and S can be provided to prevent coating the circuit component X traversing the adjacent strips. Additionally, adjacent individual strips can be interrupted at different points along their respective lengths, such as, the interruptions in strips S. and S„ whereat components X„ and X-. are located.

Interruptions having lengths measured parallel to the direction of the strips, which are provided by releasing the trigger 15 during the interval that the nozzle is traversing the portion of the strip where no coating is desired, can be con¬ veniently provided as short as 15 mm, if desired, with a gun operating with the parameters listed above. Additionally, coating thickness of as low as 30 microns are easily obtained. It was also convenient to apply the coating to the circuit board at an acute angle as shown in Fig. 8, for trigger actuation durations as short as 0.5 seconds, to coat "corners" formed at the base of circuit components mounted to the circuit board.

The principles of this invention are par¬ ticularly useful in applying vapor barrier coatings with hand-held manually-triggered gun applicators which typically move relatively slowly with respect to the circuit board being coated. However, in ^'certain automatic gun coating applications, wherein the gun is moved under control of a robot or motorized reciprocator and triggered under programmed control, circumstances sometimes require that the gun, which is capable of high speed movement, be moved relatively slowly at speeds akin to those found in hand-held gun applications. An example of such a slow speed auto¬ matic gun application is where the circuit boards being coated contain closely spaced tall, or high, circuit components, that is, circuit components which extend a substantial extent in a direction measured perpendicular to the plane of the circuit board. In such case, it is desirable to slow down the speed of the automatic gun very considerably from its normal high speed of operation to enable adequate coating of the circuit board areas located between the circuit components. Accordingly, the principles of this invention apply to gun applicators which move rela¬ tively slowly with respect to the circuit board, whether the gun be hand-held or under automatic programmed robot or reciprocator control.

While the preferred embodiment has been described using a very simple and inexpensive pressure pot as the source of pressurized coating liquid in the relatively low pressure range of 1-4 kg/cm² or lower, it is also possible to use other schemes such as mechanical pumps. Similarly, and while proximity switches are shown as the trigger-actuated switches because they are resistant to dust and do not generate any sparks, other types of switches can be used.

In summary, the method and apparatus of this invention, by discharging on an intermittent low duty cycle basis through a standard size nozzle of an airless gun operating at unconventionally low pres¬ sures necessary to produce leaf-shaped liquid films, can reduce the time-averaged amount of the discharge per unit time during intervals of trigger actuation, which is necessary to avoid unduly thick coating at low gun speeds, without resort to decreasing the nozzle orifice diameter which produces clogging and irregular flow, thereby producing uniform thickness relatively thin film coatings with slow moving guns of the airless applicator ^• type which heretofore was not possible.

What is claimed is:

Claims

CLAIMS 1. In a system for applying a vapor barrier coating to a printed circuit board having circuit components mounted thereon, which includes an airless spray gun having a nozzle configured to provide a flat spray pattern, a source of vapor barrier coating liquid connected to said gun for suppling vapor barrier coating liquid to said gun at a specified pressure which produces a leaf-shaped flat liquid spray pattern emission from said nozzle, said nozzle having an orifice sufficiently large to provide continuous clog-free flow of vapor barrier coating liquid therefrom when pressurized liquid is supplied thereto at said specified pressure which produces said leaf-shaped flat liquid spray pattern emission, and valve means connected between said nozzle and said liquid supply for controlling the supply of said pressurized liquid to said nozzle, the improvement comprising: actuating means operatively connected to said valve for repeatedly intermittently opening and closing said valve for relatively short and long intervals, respectively, to effectively decrease the average flow rate per unit time from the nozzle with respect to that resulting were the valve continuously open, thereby permitting relatively thin liquid coatings to be applied to a printed circuit board when the relative speed between the gun and circuit board is sufficiently slow that an unduly thick coating would be applied were the valve continuously open.

2. The system of claim 1 wherein the diameter of the nozzle orifice is not less than approximately 0.3 mm.

3. The system of claim 1 wherein the ratio of the closed interval to the open intervals is approximately 4.

4. The system of claim 1 wherein the open interval is in the approximate range of 7-12 milli¬ seconds, and the^' closed interval is in the approximate range of 15-60 milliseconds.

5. The system of claim 1 wherein the open interval is approximately 10 milliseconds and the closed interval is approximately 40 milliseconds.

6. The system of claim 1 wherein the valve opens and closes approximately 20 times per second.

7. The apparatus of claim 1 wherein the pressure of the coating liquid supplied to the gun is in the approximate range of 0.3-4 g/cm^:

8. The apparatus of claim 1 wherein the gun includes a handle provided with a manually-operated switch means for triggering said actuating means to cause repeatedly intermittently opening and closing of said valve when said switch means is manually oper¬ ated.

-23-

9. A method for applying a vapor barrier coating to a printed circuit board having circuit components mounted thereon by means of an airless spray gun having a nozzle configured to provide a flat spray pattern, comprising the steps of: supplying vapor barrier coating liquid to the gun at a specified pressure to produce a leaf-shaped flat liquid spray pattern emission from the nozzle, the nozzle having an orifice sufficiently large to provide continuous clog-free flow of vapor barrier coating liquid therefrom when pressurized liquid is supplied thereto at the specified pressure, and intermittently opening and closing a valve connected between the nozzle and pressurized liquid supply for relatively short and long intervals, respectively, to effectively decrease the average flow rate per unit time from the nozzle with respect to that resulting were the valve continuously open, thereby permitting relatively thin liquid coatings to be applied to a printed circuit board when the rela¬ tive speed between the gun and circuit board is sufficiently slow that an unduly thick coating would be applied were the valve continuously open.

10. The method of claim 9 wherein the liquid supplying step includes supplying liquid to the gun at a pressure in the approximate range of 0.3-4

2 kg/cm for emission from a nozzle orifice of not less than approximately 0.3 mm.

11. The method of claim 9 wherein the ratio of the closed interval to the open interval is approximately 4.

12. The method of claim 9 wherein the open interval is in the approximate range of 7-12 millise¬ conds, and the closed interval is in the approximate range of 15-60 milliseconds.

13. The method of claim 1 wherein the open interval is approximately 10 milliseconds and the closed interval is approximately 40 milliseconds.

14. The method of claim 1 wherein the valve opens and closes approximately 20 times per second.