Polymers for High Technology - ACS Publications - American

Chapter 24

Downloaded by CORNELL UNIV on May 23, 2017 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch024

A d h e s i o n a n d Yield o f P o l y a c r y l a t e - B a s e d P h o t o r e s i s t L a m i n a t i o n in Printed-Circuit F a b r i c a t i o n Influence of Substrate Thickness and Preheat Treatment Eric S. W. Kong Hewlett-Packard Laboratories, Palo Alto, CA 94304 During fuser-roll resist lamination processes, the copper-clad FR-4 substrate surface temperature was found to be inversely proportional to the substrate thickness. This temperature fluctuation has resulted in changes of adhesive forces in the copper/resist interface which in turn can affect the yield in the printed circuit manufacturing processes. By using infrared preheat treatment on the substrate prior to fuser-roll lamination, the adhesion was found to be improved in the copper/resist interface. This adhesion improvement was found to be reflected in yield increase in fine line printed circuit fabrication. As a standard procedure during image transfer processes in printed circuit fabrication, dry film photoresist is normally applied to a cleaned copper substrate using heat and pressure from a fuser roll. *^ Resist lamination using a filled silicone-rubber coated fuser roll is a common practice Schematics showing the dry film lamination process are shown in Figures 1 and 2. The resist lamination must be carried out in a yellow safe light environment. Control of airborne contamination (Class 1000 or better clean rooms) as well as control of the temperature (ca. 25 °C or lower) and humidity (50% or lower RH) are important factors for fine line image transfer at high yields. 1

Ripsom and Wopschall suggested a linear relationship between logarithmic Riston 3600 resist viscosity and temperature^. At 25 °C, the resist viscosity was reported to be 10^ poises. At 100 °C, the viscosity would decrease to 10^·* poises. Hence, there is over 3 orders of magnitude drop in resin viscosity as the temperature is raised from room temperature to 100 °C, which is 5 degrees below the typical fuser roll lamination

0097-6156/87/0346-0279S06.00/0 © 1987 American Chemical Society Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

POLYMERS FOR HIGH T E C H N O L O G Y


280

F i g u r e 1. T h r e e - d i m e n s i o n a l s c h e m a t i c o f t h e h o t r o l l l a m i n a t i o n process: PET i s p o l y ( e t h y l e n e t e r e p h t h a l a t e ) ; PE i s p o l y e t h y l e n e . ,v^«

TAKE-UP ROLL

SUPPLY ROLL/,

Hot R o l l

Lamination

^

V - ^ .

Ù

HOT ROLL PRINTED CIRCUIT BOARD < φ

Dry-Film

Configuration

l l l l l l U H L — ^PET

PHOTORESIST

Figure 2. Two-dimensional schematic of the hot r o l l lamination process: PET i s p o l y ( e t h y l e n e t e r e p h t h a l a t e ) ; PE i s p o l y e t h y l e n e .

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

24.

KONG

Polyacrylate-Based

Photoresist

Lamination


temperature. Ripsom and Wopschall^ have also reported the glass transition temperature (Tg) to be between 46 and 64 ° C . H e n c e , heating the resist to temperatures near 100 ° C while applying a pressure of 40 PSIg can e f f e c t i v e l y force the softened resist to bond firmly onto the copper surface of the F R - 4 substrate. It has been reported, however, that " i n t e r f a c i a l voids" between the copper/resist interface are rather difficult to avoid after the resist l a m i n a t i o n . ^ A survey of the literature suggested that preheating the F R - 4 substrates prior to resist lamination could improve the adhesion substantially. Specifically, a positive correlation was reported to exist between good resist adhesion and copper surface temperature^ at lamination. Secondly, moisture can often cause resist lifting and resist breakdown during plating, M hence it is recommended that moisture are to be removed by preheating the panels just prior to l a m i n a t i o n . One obvious motivation behind industrial research is to improve the yield or connectivity of the printed c i r c u i t r y . In this investigation, two main variables were studied concerning their influence on the yield of fine line c i r c u i t r y : substrate thickness and preheat conditions. C o m p a r a t i v e data from the literature as well as from private communications w i l l also be c r i t i c a l l y reviewed. Experimental Materials. The photoresist was acrylate-based Riston 3600 series supplied by E.I. du Pont de Nemour and C o m p a n y , Wilmington, D e l a w a r e . This is a resist which can be developed in aqueous m e d i u m . This resist is negative working in the sense that it would predominantly undergo crosslinking polymerization upon ultraviolet light i r r a d i a t i o n . The copper clad Fiberglass-reinforced epoxy F R - 4 laminates were supplied by Nelco Products, F u l l e r t o n , C a l i f o r n i a . The copper clad thickness was 35 microns (1.4 mils = 0.0014 inch) which is equivalent to 1.0 o z . of copper per square inch. Laminates of the thicknesses 10, 21, 31, 59, and 93 mils were utilized in this investigation. The Adhesion Tests The pull tests were performed in order to c h a r a c t e r i z e the adhesive forces between the copper/photoresist i n t e r f a c e . The tensile tests were measured using a screw-driven U N I T E Ο M A T I C tester manufactured by United C a l i b r a t i o n C o r p o r a t i o n , Garden G r o v e , C a l i f o r n i a . The tensile loading forces were measured using a 10 lb. load c e l l manufactured by Interface C o m p a n y , Scottsdale, A r i z o n a . Calibrations were done using standard weights of 5.0 and 10.0 lbs. supplied by the National Bureau of Standards. Tensile deformation was d e t e c t e d by a linear variable differential transformer ( L V D T ) . The pull-test specimens were prepared adhering studs of area 0.07 square inch (0.45 sq. cm) d i r e c t l y onto the photoresist (0.0015 inch or 0.038 m m thick) by cyanoacrylate adhesives. The cyanoacrylate adhesive was supplied by 3M C o m p a n y , Minneapolis, Minnesota, with the Trade Name C A - 8 S c o t c h - W e l d . Prior to applying the adhesive onto the stud, the latter was degreased and surface roughened by


281

282


the following procedure: grinding by sandpaper; degreasing by isopropanol; treatment by Hydrohone process; and a final degreasing by trichloroethylene.


Surface T r e a t m e n t A pumice scrubber was used prior to lamination to remove cupric oxide from the copper surface of F R - 4 substrates. The scrubber was manufactured by Resco Equipment North A m e r i c a , M o n t r e a l , C a n a d a . Abrasive jets at 26 PSIg of pumice and water cleansed the malleable copper surface. (Pumice is 73% silicone dioxide and 12% aluminum oxide, plus small amount of other minerals). The m e c h a n i c a l action of the pumice also c r e a t e d a roughened, mushroom-like morphology on the copper (evidence from scanning e l e c t r o n micrographs) which enhanced the adhesion with the resist. The copper "surface roughness" right after the scrubbing had a root-mean-square value of 3.0 ± 1.0 microns and the c o n t a c t angle was 70 ± 10 degrees while interacting with deionized water. The pumice treated copper surface held a continuous film of distilled water for 30 seconds. As a F R - 4 substrate passed through the scrubber, it was subjected to 8% sulfuric a c i d cleansing (to rid of cupric oxide); soft water rinse; pumice jet scrubbing; air knife t r e a t m e n t ; deionized water rinse; and finally a drying process at 45 ° C . Scanning E l e c t r o n M i c r o s c o p y : A l l pumice treated copper surface morphology were examined under a scanning e l e c t r o n microscope equipped with an x-ray a n a l y z e r . The e l e c t r o n microscope was Model J S M 840 supplied by J E O L L i m i t e d , T o k y o , Japan and the x-ray analyzer was Model TN-5500 manufactured by T r a c o r Northern, Middleton, Wisconsin. This technique was used extensively at many stages of the printed c i r c u i t fabrication, especially at the point right after resist lamination and development. Profilometer and C o n t r a c t Angle Measurements: The copper surface after Pumice treatment was also examined by profilometry and c h a r a c t e r i z e d by c o n t a c t angle measurement. The profilometer was supplied by Sheffield Measurement Division of D a y t o n , O h i o . The c o n t a c t angle measurements were made using a microscope device supplied by G i l m o n t Instruments, Great Neck, New Y o r k . Dry F i l m L a m i n a t i o n : The resist lamination was performed using a Model 712 Hot R o l l L a m i n a t o r supplied by M a c D e r m i d , Inc., Waterbury, C o n n e c t i c u t . The heart of the lamination system is an a l u m i n u m - c o r e d fuser roll which is about 2.5 inches in diameter and is c o a t e d by 0.1 inch thick additives-filled silicon rubber. Figure 3 shows a s c h e m a t i c of the fuser r o l l . A l l dry film lamination experiments were performed using a conveyor speed of 20 inches per minute; an air-assisted pressure of 40 PSIg and a fuser roll temperature of 105 ° C . The hold time between surface treatment and resist lamination was kept constant at 60 minutes. In the t h e r m a l profile measurement experiment, a fine-wire thermocouple is soldered onto the center of an 8 inch by 11 inch copper clad F R - 4 printed c i r c u i t board (The solder was an alloy of 63% tin and 37% lead which melts at 183 ° C ) . The thermocouple used was an A m e r i c a n Wire Gage 30 c h r o m e l - a l u m e l wire which had a maximum use temperature of


24.

KONG

Polyacrylate-Based

482°C. The supplier of Stamford, C o n n e c t i c u t .

Photoresist the

283

Lamination

thermocouples

was

Omega

Engineering,


C o m p u t e r Simulation: The e m p i r i c a l thermal profiles were simulated using a Model 9236 computer supplied by Hewlett P a c k a r d , Palo A l t o , California. U l t r a v i o l e t Curing/Printing: The laminated boards were subjected to ultraviolet light irradiation using an exposure unit (Model OB-1600 F P D ) supplied by O p t i c a l Radiation C o r p o r a t i o n , A z u s a , C a l i f o r n i a . The exposure unit provides a c o l l i m a t e d light source. A l l of its specifications are summarized in Table 1. Figure 4 shows a schematic of the optics in this system. In order to measure the amount of radiant energy per unit area (milliJoules per square centimeter) impinging onto the photoresist, a radiometer (Model IL740A) was u t i l i z e d . The photoresist research radiometer was supplied by International Light, Newburyport, Massachusetts. A l l resists were subjected to 75 m J / c m ^ of radiant energy. A hold-time of 10 minutes was allowed to elapse before the developing process began. The artwork used was IPC pattern A - 3 8 . The pattern has conductor widths between 2.5 and 4.5 mils. The developer: The exposed dry film was developed using 1% sodium carbonate solution in a v e r t i c a l processing developer supplied by C i r c u i t Services, Minneapolis. The p H of the developing solution was kept at 11.0 ± 0.2 at a l l times during the development process as monitored by a i n line p H meter/controller (Model 31171-00) manufactured by Hach C o m p a n y , Loveland, C o l o r a d o . The conveyor speed was 5.0 feet per minute. The developing sump temperature was kept at 85 ± 2 ° F (ca. 29 ° C ) ; while the rinse temperature was kept at 80 ± 2 ° F (ca. 27 ° C ) . The developing chamber spraying pressure was 29 PSIg and the two stages of rinse chamber deionized water pressure was 12 PSIg and 18 PSIg respectively. O p t i c a l Microscopy: The resist pattern was examined under an o p t i c a l microscope normally at the stage shortly after the development. The Nikon Optiphot microscope was supplied by Nikon Instrument Division of Garden C i t y , New Y o r k . E t c h i n g : E t c h i n g of the copper was performed in a C h e m c u t etcher at a temperature of 130 ± 5 ° C (ca. 54 ° C ) . The etcher was Model 547-20, supplied by C h e m c u t C o r p o r a t i o n , State C o l l e g e , Pennsylvania. The chemistry of the etching solution was that of c u p r i c chloride/hydrochloric acid and hydrogen peroxide. The conveyor speed was operated at 4.0 feet per minute. Stripping: The resist was stripped or removed using a C h e m c u t stripper placed in tandem with the C h e m c u t e t c h e r . The Model number was also 547-20. Potassium hydroxide solution was used as the stripping medium at a temperature of 150 ° F (ca. 66 ° C ) . The conveyor speed was 7.0 feet per minute. The e l e c t r i c a l tests: In order to test the yield of the print and e t c h process, e l e c t r i c a l tests were performed on the printed circuits looking for both open and short connections. D e d i c a t e d fixtures for the e l e c t r i c a l tests were supplied by E v e r e t t / C h a r l e s Test Equipment, Santa C l a r a , California.


284


0.100 INCH THICK ADDITIVES-FILLED THERMALLY CONDUCTIVE SILICON RUBBER


ALUMINUM CORE (ca2.5INCHES IN DIAMETER)

"FILLED" (SiR 0) RUBBER HAS A THERMAL CONDUCTIVITY OF ca. 3.0 TO 4.0 BTU-INCH/HOUR Ft °F AND A DUROMETER HARDNESS VALUE OF 50 TO 60 2

n

2

Figure

Table I .

3.

Configuration

Specifications

of a "hot r o l l " .

of the u l t r a v i o l e t

light

exposure

• LIGHT SOURCE:

1600 WATT PRESSURIZED SHORTARC MERCURY/XENON LAMP

• LIGHT INTENSITY:

> 20 mW/cm AT EMISSION OF 330 to 440 nm AT LEVEL OF PHOTOTOOL

• LIGHT COLLIMATION:

< 2° HALF ANGLE

• LIGHT DECLINATION:

< 1.5°

• LIGHT UNIFORMITY:

± 10% FROM POINT TO POINT OVER AREA OF 1 2 X 1 2 in (304.8 Χ 304.8 mm )

2

2

2


system.

24.

KONG

Polyacrylate-Based

Photoresist

Lamination

285


Results and Discussion. A l l F R - 4 substrates were examined under the electron microscope after the pumice surface treatment to analyze the morphology as well as determine whether there were any residual pumice particles left on the substrate. A " m u s h r o o m - l i k e " , surface-roughened morphological relief was observed on the copper surface. Scanning electron micrographs of a pumice treated copper surface indicated striations that reflect the conveyorized nature of pumice t r e a t m e n t . No trace of the elements silicon or aluminum were observed by the x-ray analyzer, hence indicating a total removal of pumice from the substrate. The copper surface roughness was observed to have a root-mean-square value of 3.0 ± 1.0 microns and a c o n t a c t angle of about 70 degrees when that surface interacted with distilled water. The fact that this copper surface held a continuous water film for at least 30 seconds indicated that very little organic contaminants were left on the copper right after the pumice/sulfuric acid/water rinse t r e a t m e n t . Figure 5 shows the t i m e - t e m p e r a t u r e profiles for substrates of various thicknesses interacting with the fuser/hot roll. In this specific experiment, no resist was l a m i n a t e d : only the direct thermal interaction between the bare board and the fuser roll was studied. The data clearly indicate an inverse relationship between board thickness and surface temperature. For example, a 10 m i l thick ( 1 m i l is c a . 25.4 microns) substrate shows a peak temperature of 93.3 ± 1.5 ° C (From this point on the error band of plus and minus 1.5 degrees will not be reiterated). As a thicker substrate, for example, 31 m i l thick panel was fed through the laminator, a much lower peak temperature of 74.0 ° C was registered. This result is undoubtedly due to the much higher thermal mass of the 31 m i l board compared to that of the 10 m i l substrate: both have the same amount of copper cladding materials (35 microns thick) but the former have about three times the amount of Fiberglas/epoxy composite. Further increasing the board thickness to 93 mils resulted in a further decrease in surface temperature to 68.7 ° C . G r a v i m e t r i c a l l y , a panel size of 11 inches by 8 inches would weigh 51.9g for a 10-mil board; 84.8g for a 21-mil board; 114.7g for a 3 1 - m i l board; and 278.9g for a 9 3 - m i l board. As the data in the following discussion w i l l demonstrate, this surface temperature fluctuations have a direct influence on the adhesion properties with the resist and even the fabrication yield of the printed circuits. Figure 6 shows the thermal profiles during Riston 3615 photoresist lamination. Since the resist served as a thermal energy absorber/insulator, the surface temperature of the substrate became lower compared to the first series of experiments shown in Figure 5. In general, the surface temperature with the resist lamination is about 5 to 6 degrees C e l c i u s below that of the situation in which the board is in d i r e c t c o n t a c t with the fuser r o l l . A g a i n , it was found that the resist/copper i n t e r f a c i a l temperature decreased as the board thickness increased. F o r the thinnest board of 10 mils, the i n t e r f a c i a l peak temperature was 87.3 ° C . For board thickness of 22 mils, that temperature decreased to 74.5 ° C . F o r a board thickness of 31 mils, the i n t e r f a c i a l peak temperature further dropped to 69.9 ° C . For a board thickness of 59 m i l s , that peak temperature further dropped to 62.5 ° C . F r o m this point however, the temperature decrease became more and more a s y m p t o t i c .



286

EXHAUST COLLIMATING MIRROR

HEAT SINKC


DICHROIC MIRROR

FOLDING MIRROR

-SHUTTER -QUARTZ INTEGRATOR

COLLECTOR Figure

100 °c

4.

Optics

of anu l t r a v i o l e t

93.3°C

80 °c

EXPOSURE PLANE

unit.

HOT ROLL TEMP = 105°C CONVEYOR SPEED = 207MIN

74.0 °C » L 68.7 °C *

Uj 60 C

93 MILS 31 MILS 10 MILS

oc D

Polymers for High Technology - ACS Publications - American

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