CA2088851A1

CA2088851A1 - Method of discrete conformal coating

Info

Publication number: CA2088851A1
Application number: CA002088851A
Authority: CA
Inventors: Hendrik F. Bok
Original assignee: Specialty Coating Systems Inc
Current assignee: Specialty Coating Systems Inc
Priority date: 1991-02-25
Filing date: 1993-02-05
Publication date: 1994-08-06
Also published as: JPH06233962A; FI930518A0; NO930407L; EP0609478A1; NO930407D0; US5266349A; FI930518A

Abstract

ABSTRACT OF THE DISCLOSURE:
Spray coating, particularly flat spray coating of circuit boards. Applicant's method of discrete conformal coating eliminates "railroading" at the edges of the flat spray web and assures precise control of the amount of coating material placed on the circuit board surface, as well as on the varying and complex circuit board components. The method includes pressurizing the coating; longitudinally advancing a surface to be coated, such as a circuit board, beneath the coating, while simultaneously feeding the coating onto the advancing surface as a plural series of aligned droplets extending transversely across the advancing surface. The feeding of coating is triggered "ON/OFF" so as to define longitudinally the series of droplets feeding onto the advancing surface.

Description

20888~1 CRQ i~RE~ERENCES_TO RELATED APPl.ICATlONS:
An improvement upon applicant's METHOD OF CONFORMAL COAT~JG
tSerial No. 07/205,8B9), filed June 13, 1988, now Patent No. _ ~ACKGROUND C)F THE INVENTION:
1. Pield of the_Invention:
Spray coating, particularly conformal coating of printed wiring boards. The method of applying conformal coatings onto the component or arcuit side of a printed wiring board by applying a film of coating ma~erial which is extruded through a pressurized spray nozzle.
¦ 2. The Prior Art:
TURNER 3,461,840 ' NORD et al. 3,556,441 ¦ NORD et al. 3,843,055 I ! WILLLAMS et al. 3,858,812 ~ ~ -l I STEWART 3,923,253 I WAUGH 4,207,3~6 i ROOD 4,346,~49 ROOD 4,349,947 ' KOMON et al. 4,5n,290 ! TAMURA 4,600,601 ¦ ONO et al. 4,698,240 SHIMAD.4 EPO 230 782 Being discussed in a forthcoming INFORMATlON DISCLOSURE
STATEMENT.
8ACKGROUND OP THE INV~NTlON:
llle need to conformally coat assembled printed wiring boards evolved in the early 1960's, as it was recognized that the electronic boards needed protection from the environment, hu nidity, gasses, rapid temperature changes, fungus, and the like. lnitially, a tw~component epoxide coating was used which, after D-l6620 - 2 -~:

2~8~ )1 ~11 .

the like. Initially, a two-component epoxide coa~ing Wl15 wed which, ~fter application, formed a 10 mils thick film on the planar areas of the printed wiring board and much thicker fillets at the base of the individual components.
Eventually Military Specification MIL-I-46058 was developed, defining coating thicknesses ranging from one to three rnils. From that time on, all military boards have been subject to specifications that require coating rnaterials to be applied within this range. Industrial printed wiring boards also experienced an increase in i demand for a protective coating, as line widths and spacings and solder pads and I l lead spacing of lC's and hybrid packages were restricted.
l,l A typical conformal coating process includes cleaning, masking, coating, ! I curing, demasking and touch-up of the printed wiring board.
Masking is sometimes tedious and requires a high degree of manual skill.
j Materials used include paper, vinyl and sther plastic tapes, peelable latex ccatings, ¦I silicone caps and the like. In most cases, this masking effort adds considerable ,¦ expense in manufacturing the printed wiring boards.
After the printed wiring boards have been masked. the coating material is applied either by spraying, which can be performed either by machine or manuallyusing a multiple, four directional spray pattern or by dip coa'dng.
Once the coating is cured, the masking material is manually removed.
During the demasking process, the adjacent coating film may be inadvertently lifted, thus requiring repair. Also, if ~e masking material has covered areas which require coating, additional touch-up is required. Thus the industry has continually been looking either to automate masking or to selectively apply the coating and, thereby, eliminate the need for masldng.

D-16620 i~ _ 3 _ - ~ - 2~8~

Il .
¦ A method for selectively coating printed wiring boards was introduoed a few years ago and has been defined in NORDSON/SHIMADA ~Patent No. 4,753,819).
The technique utilizes a film-forming fluid nozzle to form a web much like a miniature curtain, as illustrated herewith in FIG. 1. As is the case with any kind of S a curtain pattern, the web can only be sustained by two heavy side streams, each ¦ stream defirung ~e edges of the web. In general, a web width of between 0.4 to 0.6 I ¦ incll is maintained ~y ~hanges in the fluid pressure.
!I The web is moved back and forth across the length of the board being I automatically stopped and started in order to deposit the coating in selective areas.
, After the first pass the web is indexed, and during the second pass another web is ¦ i applied adjacent to the previously applied web. The two webs are designed to meet ! I one another or to have a slight overlap, thus creating an unintersupted film.
~I Thus, the entire board is coated with overlapping ~ibbons of coating ! material. The needle valve controlling the nOw of coating through a no771e is 1~ 11 activated automatically. One of the problems with this process is due to the relatively high speed of the traversing filming-nozzle. The output of the nozzle is approximately 100 ml/minute (1.66 ml per second), thus necessitating a linear no771e velocity of approximately 20 inches per second in order to deposit the desired coating thickness on the stationary band. This high application speed causes shadow effects on the taller components of the band, resulting in a lack of c~mpiete coa~dng coverage. "ON/OFF" positions, indexed according to the patte~n to be deposited, need to anticipate early needle closing and opening, as a considerable amount of coating material is "in transit" at the time of the needle , D-16620 - 4 - .

2~888~1 1 cl~sing and, of ~urse, ehe web needs to be establiched sequentially of opening the ¦
needle.
As illustrated herewi~ in FlG. 1, a "railroad track" pattern 30, 32 formed by the NORDSON/SHIMADA web 36 will flow together and will aeate a relatively 5even film. However, when web 36 is placed adjacent to a web previously applied, the heavy edges 30, 32 of the web will foroe small bubbles into the wet film. These I air bubbles then might not be able to break through the surface of the coating, thus ¦ ! causing voids in an otherwise continuous film.
In any case, 80~o of the suspended film in web 36 is contained in the two side 1~i streams or tracks 30, 32. A web 36 ranging from 0.4 to 0.6 incll wide, which after flow out and leveling measures 0.6 to 0.8 inch, cannot be used to coat to very I ¦ narrow tolerances. For these areas, simply rotating nozzle 34 ninety degrees only ! I increases the coating deposition because the same coating flow rate is now applied ! I in a very narrow web.
151, In gene~al, it is not very difficult to apply solvent based coatings below dle maximum allowable thicknesses as speafied in Military Specification MIL-1-46058.'1I Coatings of 100% solids, however, present a real challenge to maintain control of ¦I thickness, because of lack ~f s~lvent evaporation and associatet coating thickness ¦I reduc~ion. Also, solutions for application to small size orifices and the faster nozzle velocities, both present difficult problems to solve.
Consequently, applicant's discrete conformal coating method has been developed to eliminate the problerns associated with the we~Blming or conforrnalcoating prooesses.
. I . ,., D-16620 i~ _ 5 _ ~
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SUMMARY OF THE lI~VEl~llON
Method for conformal coating of printed wiring boards so as to control discretely the amount of ~oating material applied in a spray pattern to the circuit board, so as to prevent "railroading" or "fishtailing" at the sides of ~e pattern and S so as to enable discrete masking of circuit board components which are to be protected from the coating. Applicant press lrizes the coating while longihldinally ¦¦ advancing the printed circuit board beneath the coating and simultaneously ¦ feeding the coating onto the advancing surhce of the circuit board as a plural series I 11 of aligned droplets extending transversely across the advanang board surfaoe. The l 1 feeding of coating is triggered ON and OFF so as to space and define longitudinally the series of droplets feeding onto the advancing surface.
il DESCRIPTION OF THE DRAWINGS:
FIG. 1 is a schematic front ele~lational view of the flat spray pattern or curtain pattern of coating wherein fishtailing or railroading forms at the edges of ',, the pattern as in NORDSON/SHIMADA 4,7~3,819.
FIG. 2 is a schematic view according to applicant's method of discrete conformal coating, wherein the pressurized coating is fed onto an advancing surhce of the circuit board as plural series of aligned droplets extending ~ ¦ transversely across the advanang surface.
l FIG. 3 is a schematic view of the NORDSON/SHIMADA technique 4,753,819 as applied o a printed circuit 'board having regular components 52, 54, 56, 58 and ,. 60, wherein a shadowing effect 62 occurs as the flat spray pattern hits vertical ~ I co~ I onent 52.

D-16620 I . - 6 -il ~1 203~8~:~

FIG. 4 }s a schema~ic side elevatil)ral view l;howing the ¦
NORDSON/SHIMADA 4,753,819 shadow effect 74, 77 of coating 76 as applied to individual circuit board components 64, 66, 68, 70 and 72.
I FIG. 5 is a schematic view of applicant~s method of discrete conformal ¦ coating wherein the feeding of the coating material as individual droplets 42 eliminates the shadow effect and uniformly coats both the components 52~, 54f aswell as the intervening spaces and solder pad.
FIG. 6 is an enlarged fragrnentary schema~dc, showing applicant's feeding of ~ drople~ 42 verticalIy downwardly onto circuit board 48 comp~nent 54 and its 1 soldered connections, according to applicant's method.
il FIG. 7 is a bottorn plan of applicant's coating feeding manifold 82 employingjI pumps at either side for feeding coating material through channels engraved in~
j the manifold so as to interconnect the individual dispense needles or feeding hlbes ¦ 38, 40 in the aligned series 78, 80. ~ ~:
I FIG. 8 is a side elevation of the FIG. 7 manifold 84 supporting 128 dispensing, needles 38, 40 and employing pump blocks 86, 88 at either side. ~ I
I PIG. 9 is an enlarged side schematic of the thirty-two coating sections ~2 i being fed by coating tubes 92, each section supporting four needles 38, 40.
I FlG. 10 is a fragmentary transverse section through eoating section 90 and ¦ feeding tube 92.
I FIG. 11 is a bottom plan showing the individual droplets pattern 42 after ¦ flowing together on the surface of the advancing board.
FlG. 12 is a front eleva~don of a selective conformal coating machine using a solvent based, heat curable coating material.

D-16620 I . - 7-208~8~

FlG. 13 is a front elevation of a discrete conforrnal coating machine using a solvent based, W curable coating material.
FlG. 14 is a front elevation of a disete c~nformal coating machine using 100% solid UV curing material.
FIG. 15 is a showing of a modified coating machine involving a pre-warm station, a ~ating station, a flowout and leveling station and unload station.
FIG. 16 is a side elevation of the machine illustrated in FIG. 15.
FIG. 17 is a schematic showing of a discrete conformal coating machine jl embodying a supply manifold with individual valves feeding coating through 1¦ sapphire orifice 150, so as to develop droplets 42.
ll FIG. 18 is a front elevation, of a coating machine, such as illustrated in FIG.
¦ 1 14 and employlng an optical scanner. -~IG. 19 is a side elevation of the machine illustrated in FIG. 18.
i ! FIG. 20 is an enlarged end elevation of the machine illustrated in FIG. 17 and ~ ~ -1 l showing an array of coating valves 146 feeding the nozzles 38, 40.
I! FIG. 21 is an enlarged schematic view showing the ooating applicator of the !¦ device illustrated in FIG. 18.
. I FIC. "2 i~ a~l~ a tran6parent film 116 ambod~
- y~ led ci.~t bo~rd irnagc wit~ non coat~ ~rc~ mJs~ccd in bl~ck.
DESCRIPIION OF TE~ PREFERRED EMBODIMENTS:
Applicant's coating applicator corlsists of parallel transversely aligned seriesof indivitual orifices 38, 40 spaced 0.10 inches apart, each ~eries with an overall typical length of 10 inches, as illus~ated in FIG. 2. In-line to each orifioe may be a solenoid controlled valve 146, or pump 86, 88, which feeds tiny droplets i D-l6620 ¦ --8 -. .
ll ~ .

20~8~

42 of coating material at a rate between 5 to 30 droplets per ~econd. The size of droplets 42 is controlled by the orifice size, feed pressure, viscosity of coating material, pump or valve cyding and duty cyde. As illustraled in FIG. 2, droplets42 are larger than those created by conventional air atomized spray coating, which S after striking the surface of the advancing wiring board 48, droplets 42 quickly flow together as at 44 and after levelling aeate a continuous film 46. But, unlike spray coating, droplets 42 fall straight down from orifices 38, 40, positioned 1.0 to 1.5 inches above the planar board surhce. E~sentially, applicant eliminates air borne I overspray which is typical of conventional spray coating. Since the volume left in 1, the supply tube 92 and orifices 38, 40 is zero (æro dead volume), and each droplet ¦ 42 leaves the orifice with some velocity, the "ON/OFF" flow of the stream of I droplets is instantaneous and precise. Also, because the small droplets 42 areplaced side-by-side and are not splashed into one another, air bubbles are not I created in the resultant wet fi1m 46.
l When all the pulsing solenoid valves 146 are energized collectively (or the pumps 86, 88 are activated~, board 48 is advanoed beneath ~e applicator at a speed typically of 1" per second. As a result, a large number of evenly spaced droplets 44 which almost touch one another are formed on board 48 surface; subsequently, thedroplets flow together to form a ~ontinuous film 46. l~e speed of advancing board 48 may be varied from .5 to 5 inches per second.
Sinoe the diameter of the orifioes 38, 40 is approximately .0155 inch, a single orifice can coat to tolerances as narrow as 0.10 inch. Accordingly, open areas of 0.10 to 0.10 inch - such as test points, meta~ shielding partitior~, etc., ran be effectively "masked" or avoided.
. .

D-16620 _ 9 _ I' 20~53831 ¦ As illustrated in ~IG. 1~, a program can easily be created using a series of ¦¦ optical sensors, positioned at ~e same oenter lines as the coating orifices, to scan a ¦ positive film of the board, having uncoated areas opaqued. Such optical sensing I will be completed at the siame speed ias the board is advanced. A coating pattern is S ¦ thus created without tedious programming, and the program may ~e stored on a i disk with an identity typical of the particular board layout. The coating is ~en fed onto the board in one sweep as board 48 is advanced, while ~e open or "masked"
areas as seen by ~e optical scanner are left uncoated.
Because coating materials are generally supplied pre-filtered and do not 1, contain any appreciable solid particles, the coating materials are easily filtered to 40 ¦ microns. The coating materials pass through a filter inside the applicator's enclosure just prior to being dispersed to each solenoid-driven valve or the manifold pumps. Both filter and internal supply manifold have been designed to I hold only a small volume of liquid.
I In order to clean the applicator of any coating material, a 3-way selector I valve 134 allows fast flushing of the filter, manifold, valves and orifices as for-¦ 1 example in FIG. 17.
In FIG. 1 there is illustrated the conventional flat spray pattern as in ~ NORDSON/SHIMADA 4,753,819 wherein web 36 of coating material is discharged through nozzle 38 and onto printed circuit board 50. Web 36 is formed by means of side streams 32, 30 which form "railroad tracks" on the surhoe of the printed eircuit board. This "railroad traclc" effect is further illustrated in FIG. 3 wherein the web 36 is advanced in a series of contiguous or overlaping patters over ~e surfaoe of the circuit boart, embodying individualized eomponents 52, 54, 56, 58, 60. A
I ,.......................................................... : ,' I
D-16620 -10- ~ ~

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¦ shadowing or miss effect 62 is developed, wherein the ge~metry of the component 52 precludes deposition of the coating.
According to applicant's method of conformal coating, the coating is pressurized then fed as a plural series 78, 130 of aligned droplets 42 extendingtransversely across the surface of printed circuit board 48 being advanced underneath. As illustrated in FIG. 2, applicant's Phase I ernbodies the feeding downwardly of droplets 42 from individual nozzles 38, 40. The nozzles or needles38, 40 in each series are approximately .0155 diameter and spaced apart approximately .106 inches.
,'1 In Phase II, droplets 42 begin flowing together as at 44 ~n the advan~ng I board surface, and in Phase III, there is a leveling of droplets as a continuous i, coating 46.
1l As illustrated in FIGS. 4 and 6, individual needles 38, 40, which may have an I ' outer diameter of .035 and an inner diameter of .0155 inches, discharge droplets 42 15 11 in a vertical downward direction, there is no shadowing effect with respect to the ¦ geometry of circuit board 48 components 52', 5~', 56', 58' 60'.
i The conventional shadowing effect is illustrated in FIG. 3, as well as FiG. 5, wherein web 36 is applied to circuit board 50 individualized component 64, 66, 68, 70 and 72. The heavy coating 76 appears on the sides and tops of the circuit board components; however, there is a shadowing effect as at 74, 76 where the spray pattern is essentially blocked.
In contrast and according to applicanYs method illustrated in PIG. 6, the ~ coating being fed as individual droplets 42 is shown addressing component 54 as ¦ well as the solder pad connection 55.
1~ .

.. 20~g8~1 ~

In FIGS. 7 and 23, there is illustrated a manifold 82 embodying aligned ~eries 78, 80 of nozzles 38, 40, extending transversely across the field of advanang of ~e , circuit board. Nozzles 3&, 40 are .106 inches apart in each series and staggered such ¦¦ that there is a .212 distanoe between adjacent noziles in each series.
S ¦~ Manifold 82, 84 is illustrated in FIG. 8 as embodying ~irty-two coating ¦i sections 90, each coating section 90 having four dispensing nozzles 38. Thus,!! thirty-two coating sections each embodying four needles 38 may be empl~yed to¦ provide a series of 128 dispensing needles to form an eight inch manifold extending transversely across the field of advancing of the arcuit board. Pump 1 blocks 86, 88 may communicate with channels etched within manifold û2, 84, or to feed coating material via tubes 92 to the individual needles as îllustrated in FIGS. 9 and 10. A bleeding screw 91 may be used to de-air dispensing tube 92 and individual sections 90. The pattern of droplets 42 flowing together cn the board as I I at 44 is illustrated in FIG. 11.
15 1~ In FIG. 12 there is illustrated a conformal coating machine using a solvent ¦, based, heat curable coating material. The machine embodies 3 housing 94 with I exhaust 96 supported upon frame 100, so as to embody load station 106, coa~ingi, application station 108 with coating pressure pot 110 and solvent pressure pot 112;
I infrared/convection preheating station 112 with embodying infrared heating I component 118 and insulative elements 120. As unload station 114 supports DCmotor conveyor may drive 104. As will be apparent, ~he nozzle manifolts 98 may be supported transversely of the circuit boards 48 being advanced on endless ............... conveyor 102.

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In I IG. 13, a similar installation is illustra~ed for typial discrete c~nfonnalcoating using a solvent based, W curable coating material and embodying W
station 115' having W lamp 122 and exhaust 116.
In FIG. 14 there is illustrated a similar installation for discrete confonnal S ¦ coating utilizing 100% solid, UV coating material. A IR/convection preheat shtion 124 is illustrated together with an inspection station 126. Hot plates 127,129 may be ~ utilized for heating and during coating of the printed circuit board.
I In FlGS. 15 and 16, there is illustrated a further modification wherein a fine¦1, mesh stainless steel belt 17n may be driven by fisre phase step motor drive sprocket 1l, 171 beneath identical infrared heaters 201, 203. The housing 204 may support¦ nozzle manifold 202, exhaust 198 and control panel 200. As illus~aated in FIG. 16, the step drive motor sprocket 171 may be driven by module and stepper driver 173.
I A warm air blower 20~ may be employed to preheat coating material within ¦ I manifold 202.
I In FIG. 17 there is illustrated a modified arrangement wherein pr~filtered I coating material 136 is advanoed by air pressurization 138 int~ supply manifold 130 i having individual feeding tubes 142 directed to valves 146 by mear s of ~ree way valve control 134 and in-line filter 132. Enclosure 128 may be heated by hot air 140 and the coating material may be advanced by valves 146 through sapphire orifioe 20 150, having approximate diameter of .009 and an air exhaust 144 may be providet.
Also, a three-way valve 154 and air supply 156 may be employed in connection with pre-filtered flush or solid material which extends to ~e thre~way valve 134.
A As illustrated in FIG. ~, the coating station will supp~rt one hundred solenoid valves 146 in a five by twenty array and activated by electrical connection D-l6620 - 13 -I

.. , , ~ , . .

20~88~1 1 ¦!

145. Coating material input will be through either one of two feed manifolds.
Each valve output will be connected to a common orifioe manifold 145. llle manifold 145 will contain two rows, fifty per row, of orifice assemblies consisting of a synthetic sapphire orifioe 40 and connecting tube 41. The spacing between each¦ assembly will be .106 inches and .212 inches between each row.
¦ This arrangement is further illustrated in FIG. 20 wherein thermocouple 170 ¦ is employed to regulate the hot air supply. An electrical connection 145 may be l employed to regulate ~he individual coating valves 146 which feed into coatingil manifold 143.
10A il In FIG. 18~rl19 a~, there is illustrated a unitary coating machine embodying load station, preheating station, selective coating station, flow-out and i I leveling shtion and unloading station with an optical scanning system 164 guided by PCB image 166 with non-coated areas masked sr marked in black, conveyor 1 drive 170 controlled by compu~er 158 having hard drive 160 component and 1' individual diskettes 162, programmed according to the optical scan. The i' preheating zone may be controlled by panel 182 extending to infrared heater 181 !~ and panel 184 extending to infrared heater 174. A coating control panel 168 may control the coating station 176.
! In the FIG. 14,100% solids coating machine illustrated in PIG. 4 in cure zone ¦ 126 will enhance leveling of the coating after application. All three zones 124,10B' ¦1 and 126 will be independently temperature controlled. .
As illustrated in FIG. 18, the ~ptical scanner may be comprised of infrared emitting diodes and photo transistor receivers may be employed. The narrow I `'~'' ! ~
D-16620 ~ . -14-. . . . .,. . .. .: ,, . , . ,:.: .:., . , - . .. . .. . - - . .

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infrared beams are spaced equally to the droplet ejecting orlfices 38, 40 of the applicator.
The scanner m chanism will accept a transparent film with the PCB image of the non-coated areas blacked out; a sample format being illustrated in FIG. 22. A
Stransparency can be produoed using a CAD program or a cut and paste method.
As illustrated in FlG. 18, a custom built printed circuit board 166 will control¦ I both the scanner and the coating machine. The image sensors affixed to board 166 will pass data to the CPU via a standard I/0 board plugged into the mother board.
,I Board 166 will also contain the required driver circuits for the scanner motor, l01I conveyor motor and the coating valves. Provisions for adjusting the pulse frequency and the duty cyc:le may also be included on the printed arcuit board.
On start up, the computer will display a MAIN MENU. The operator may ¦ choose to SCAN a film or COAT a printed circuit board.
I If SCAN is selected, the operator will be prompted to enter a name for the 15I file and insert the film into the scanner. The board will drive the scanner's stepper motsr and scan the film. The scanned data will then be passed to the CPU.
The CPU will assemble the scanned data into a file and store the file on the computer's hard drive. Once stored, the computer will return the operator to the MAn~1 MENU.
20Upon selection of COAT, the operator will be prompted to enter the narne of the coating file. After entering the file's name, the operator will be asked to enter the coating pulse frequency (~30 HZ), duty cycle and conveyor speed (.5 to 5 inches/second). Ihis will shrt ~e machine's conveyor.

D-16620 ¦ -15 -1.

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20B~8~i Placing a board 48 on the conveyor in the proper orientat on is the next sbep.
Pressing the spaice bar on the c~mputer will toggle the conveyor on and off. This will allow additional time in the post bake (Zone 1) and the after bake (Zone 3)should it become necessary. l his toggling is defeated turing the coating operation.
I A sensor looks for the leading edge of the printed arcuit board and starts ~e ¦ valve pulsing routing at the correct time. By placing additional boards on theconveyor, the system will automatically seek and coat. Once a single board or batch ¦~ has been coated and removed from the conveyor, simply press ENTER on the ¦ I computer; the conveyor will stop and the screen will retuln to the MAIN MEN~J.
10 I Manifestly, advantages of the present method for discrete conforrnal coating ¦l over conventional film-forming techniques include:
~etter control of coating thickness is obtained and even 100% solids -coating materials can be applied within military specification thickness ' requirements.
1 i Coating materials need not be circulated.
!I Very narrow coating/masking tolerances can be achieved.
No shadow effect occurs. 1, One sweep application is easily obtained.
l No bubbles are left in ~e cured film.
The method gives 100% transfer efficiency.
Simplified prograrnming and program storage is facilitated.
,1~
,.,

Claims

1. Method of discrete conformal coating of a surface comprising:
a) pressurizing the coating;
b) longitudinally advancing a surface to be coated beneath said.
coating; while c) simultaneously feeding the coating onto said advancing surface as plural series of aligned droplets extending transversely across said ad-vancing surface, and d) triggering "ON/OFF" said feeding so as to define longitudinally the series of droplets feeding onto said advancing surface.

2. Method of discrete conformal coating of a surface as in claim 1, wherein said feeding is such that said droplets upon striking said advancing surface flow together as a coating film upon said surface.

3. Method of discrete conformal coating of a surface as in claim 2 including "masking" a portion of said advancing surface by triggering "ON/OFF" said feeding of the plural series.

4. Method of discrete conformal coating of a surface as in claim 3, wherein said feeding of droplets is in a substantially vertical trajectory downwardly onto said advancing surface.

5. Method of discrete conformal coating of a surface as in claim 4, including advancing said surface to be coated at a rate of .5 to 5 inches per second.

6. Method of discrete conformal coating of a surface as in claim 4, including guiding said triggering "ON/OFF" by means of optical sensors.

7. Method of discrete conformal coating of a surface as in claim 4, including varying the rate of feeding said coating as a function of droplet dimensions.

8. Method of discrete conformal coating of a surface as in claim 3, including varying the rate of said pressurizing as a function of coating dimensions upon said advancing surface.

9. Method of discrete conformal coating of a surface as in claim 4, further including staggering the series of aligned droplets transversely with respect toadjacent series of aligned droplets, so as to avoid bridging of coating between droplets prior to feeding onto said advancing surface.

10. Method of discrete conformal coating of a surface as in claim 10, wherein each series of aligned droplets is approximately .212 inches apart from each adjacent series.

11. Method of discrete conformal coating of a surface as in claim 10, wherein said staggering of adjacent series of droplets is approximately .106 inches.

12. Method of discrete conformal coating of a surface as in claim 3, wherein said advancing of said surface is in the range .5 to 5 inches per second.

13. Method of discrete conformal coating of a surface as in claim 3, wherein said feeding of coating droplets is at a rate of 5 to 30 droplets per second.

14. Method of discrete conformal coating of a surface as in claim 1, including pre-heating the coating.

15. Method of discrete conformal coating of a surface as in claim 1, including pre-heating said advancing surface, so as to promote flowing together of droplets upon the advancing surface.

16. Method of discrete conformal coating of a surface as in claim 7, wherein said varying the rate of feeding includes programming a duty cycle of feeding according to the characteristics of said advancing surface.

17. Method of discrete conformal coating of a surface as in claim 1, including controlling said feeding by varying the lateral spacing between aligned droplets.

18. Method of discrete conformal coating of a surface as in claim 1, including controlling said feeding by varying the longitudinal spacing between said pluralseries of aligned droplets.

19. Method of discrete conformal coating of a surface as in claim 3, wherein said masking of said advancing surface includes triggering ON/OFF selected lateral portions of individual series.

20. Method of discrete conformal coating of a surface as in claim 16, wherein said programming includes optical sensing of a printed wiring board master.

21. Method of discrete conformal coating of a surface as in claim 15, including post-heating the advancing surface.