This invention relates to the techniques for forming a solder mask on the surface of a printed circuit board based on the lamination of a dry composite film onto the surface of a printed circuit board.

A solder mask is a hard, permanent layer of a nonconductive material which covers the surface of a printed circuit board, encapsulating the traces of the printed circuit while leaving exposed holes and pads for soldering. Of course the mask may be formed on both faces of the board.

The article entitled «Solder mask, liquid or dry-film? by Michael Weinhold, PCB Magazine No. 6, September 1990, pages 36- 50, contains a description of the various techniques used for forming solder masks.

Photolithographic techniques for defining the solder mask have in most cases replaced the more traditional serigraphic technique, because of the much improved definition limits. More lately, the photolithographic technique has been coupled to the use of dry-film photoimageable material (dry-film solder mask or DFSM), commonly a negative or positive resist film with an acrylic/epoxidic base, supported on a polyester cover sheet, for example of Mylar or of a polyethylene tereftalate (PET), which, at the end of the lamination of the film over the surface of the printed circuit board and before or also after exposing the resist film through a certain artwork or master mask, may be peeled off, before developing and finally hardening the so defined solder mask. Techniques based on the use of composite dry films have several advantages as compared with the use of liquid resists. Beside ensuring an improved isolation reliability of the conductive traces and other features covered by the mask (that is without leaving exposed sharp edges or insufficiently

protecting them as in the case of masks prepared with liquid resists having definitely lower conformability characteristics) permit the so called tenting or curtaining of interconnections holes or vias, thus preventing the need of subsequent working steps, as when using liquid resists.

Of course, also the techniques based on the use of composite dry-films have a number of peculiar problems, deriving from the difficulties of handling and applying a relatively thin film on a relatively large surface without air entrapments that would produce so-called puddling defects upon lamination and from a marked criticality of the control of the plastic flow of the masking film during lamination, to prevent the permanence of undesirable cavities (because of insufficient plastic flow or air entrapment) or points wherein the thickness of the masking film is excessively thin or even absent (because of an excessive plastic flow during lamination).

Commonly, lamination of a supported film on one or more often both the faces of a printed circuit board is performed according to a process conducted substantially under vacuum and which includes a period of several seconds (up to 10-20 seconds) of permanence under vacuum for extracting and discharging all the air entrapped between the film and the surface (typically having an irregular profile) of the printed circuit board while heating the assembly to a temperature sufficient to make the resist film fluid. Finally a mechanical pressing (slap down) is commonly effected by a flexible diaphragm or a siliconic rubber bellow acting inside the evacuated mold cavity which is urged against the polyester cover sheet.

Because of the relative rigidity or poor pliability of the polyester cover sheet, conformation of the resist film to the irregular surface of the printed circuit board, may occur in practice by a plastic flow of the resist material to fill completely the valleys between adjacent conductive traces under the lamination pressure.

According to this technique, the thickness of the resist supported on the polyester cover sheet must be sufficient (substantially greater than the height of the conductive traces) in order for the top of the traces to remain sufficiently submerged (protected) by a layer of resist of sufficient thickness. This technique is substantially a planarizing technique, that is it tends to produce a mask having a substantially planar surface (that is with a reduced difference of the level of peaks and valleys). Therefore the thickness of the mask is relatively high, thus creating relatively high steps around the photolithographically defined areas, and left without resist coverage (soldering pads, etc.). This may create problems during surface mounting of components on the printed circuit board. In order to obviate to these drawbacks of the above described dfs technique, a modified technique has been developed which is based upon the use of a supported photoimageable film wherein the thickness of the resist is relatively thin and may in practice be less than the height of the conductive traces defined on the printed circuit board and on a vacuum, heat lamination technique that does not deliberately produce a filling of the evacuated spaces between adjacent traces. Upon extracting the hot laminated board from the evacuated chamber, the polyester cover sheet is immediately peeled off. This determines a further plastic flow of the resist film, still sufficiently fluid, once the relatively rigid polyester cover sheet is removed that brings the resist to conform to the irregular profile of the surface of the board, thus filling the evacuated spaces between adjacent traces.

Such a technique is described in the article entitled:<<Highly Conformable conformask-dry-film solder maskj)by A. Candore, PCB No. 6, September 1990, pages 52-56 (Conformask is a registered trademark of Morton Int.). This technique is also described in the European patent application No. 89301228.6, filed on February 9, 1989, and in a corresponding US patent No. 4,992,354, assigned

to Morton Thiokol Inc.. The pertinent descriptions contained in said prior documents are to be intended herein incorporated.

Such a technique has the potential advantage of producing a relatively thin solder mask, highly conformed to the surface of the printed circuit board. Beside facilitating the surface mounting operations of components on the board, the thinness of the resist layer would reduce exposition, development and hardening times and, as result, fabrication costs.

However, this technique of applying a thin photoimageable dry-film does not overcome and rather makes even more critical the heat lamination conducted under vacuum in terms of defects due to the squashing of trapped air bubbles caused by an imperfect or insufficient evacuation of all the air trapped between the film and the face of the board, and because of the increased thinness of the film.

Generally, the application of the lamination pressure (slap- down) on the heated sandwich composed of the board and of the films (or of the film in case of masking a single face of the board), is customarily performed inside the evacuated chamber for ensuring evacuation of all the air trapped through an expansion of air bubbles, lifting of the film and eventual release of the air followed by a collapse of the film back onto the face of the board.

The use of thin dry-films of resist according to the above- noted lamination technique for obtaining thin highly conformed solder mask, makes extremely critical or impossible the tenting of metallized interconnection holes (vias) of the board, because upon the peeling off of the cover sheet while the film is still hot (that is immediately upon extracting the laminated board from the heated evacuated chamber), the plastic flow of the thin resist layer under the effect of the vacuum present inside the holes, eventually sealed on both sides by the solder mask film, may easily cause the break of the continuity of the tenting film. In order to obviate to this difficulty, after few seconds from the

peeling off of the cover sheet, the sandwich must be rapidly quenched by jets of chilled air or other means of fast refrigeration. Notwithstanding this, in many situations, it is nevertheless necessary to proceed with successive tenting operations, performed according to traditional techniques used in liquid resist processing. This represents the loss of one of the main potential advantages of the techniques based on the use Qf photoimageable dry-film resist and jeopardizes some of the considerations made when making a comparative cost/effectiveness assessment of alternative processes.

There is therefore the need and/or utility for a method of applying a photoimageable and developable dry-film solder mask on the face of a printed circuit board capable of ensuring the tenting of interconnection holes and a substantial reduction of the criticality of the lamination process, thus avoiding the occurrence of defects due to the lamination of entrapped air bubbles, not completely evacuated before pressing. Of course, the above mentioned first requisite assumes a substantial importance when using thin resist dry-films for enhanced conformability that remain potentially fully adapted (because of their dry-film nature) to produce a reliable tenting of holes.

These objectives and advantages are reached by the method of the present invention which is based on effecting an air evacuation followed by pressing under vacuum while maintaining the sandwich composed of the printed circuit board and the supported film or films, to a temperature that remains essentially lower than the temperature of substantial fluidification of the photoimageable resist film. That is in this phase of the lamination process no substantial filling of the evacuated spaces is obtained.

The sandwich thus prelaminated under vacuum at a relatively cold or insufficiently heated condition, is thereafter heated under atmospheric pressure up to the fluidification temperature of the resist film thus allowing, under the effect of the

atmospheric pressure a conformation of the layer of resist to the board's surface.

In case tenting of holes is required, the sandwich, evacuated from the air and pressed under vacuum at room temperature or at a higher temperature which remains essentially lower than the temperature of fluidification of the resist layer, is cooled back to room temperature and, while being still protected by the cover sheet, may be exposed through a purposely designed exposure mask to a UV flash (for about 50mJ/cm), to define only the tenting zones. Such pre-exposition for defining the tenting zones confers locally to the resist layer the ability of withstanding softening during the subsequent heating step of the sandwich to carry out the conclusive lamination step by conforming the unexposed part of the resist layer that becomes fully flowable under the effect of atmospheric pressure. The pre- exposed portions of the resist film (over the tenting zones about the holes) do not soften to the extent of plastically following because a certain degree of polymerization has occurred in the exposed zones. The peeling off of the polyester cover sheet may take place immediately before the heating or immediately after the heating (while the resist is still hot).

Of course, the choice of effecting the peeling off of the cover sheet before heating or after the sandwich has been heated, will depend from specific working conditions. When treating boards having relatively high traces, a peeling off of the cover sheet before heating favors a greater conformability of the resist film following its fluidification and generally a better degree of conformity of the mask. On the contrary, where the conformability is not a critical requisite, as in case of boards with traces of reduced height, effecting the heating while retaining in place the polyester cover sheet may help in retaining a perfect sealing of the evacuated spaces and offer a

continued protection of the sandwich also during the heat treatment.

Especially when using a dry-film having a relatively thick resist layer, the cover sheet may be retained on the sandwich until development of the exposed solder mask is initiated.

The different aspects and advantages of the process of the invention will become even more evident through the following description of several important embodiments and by referring to the annexed drawings, wherein: Figure 1 is a partial cross section of a generic printed circuit board;

Figure 2 is a partial schematic cross section of a composite dry-film of a photoimageable resist for forming a solder mask on the face of a printed circuit board; Figures 3-6 show a lamination process of a dry resist film on the face of a printed circuit board, according to the invention;

Figures 7-10 show an alternative embodiment of the lamination process of the invention;

Figure 11 is a flow chart of a process of the invention for forming a solder mask on the face of a printed circuit board.

With reference to Fig. 1, a printed circuit board (PCB) is generically composed of a fiberglass reinforced resin board 1 on the faces of which conductive traces or tracks 2, commonly of copper, are defined. A supported dry-film used for forming a solder mask on the face of a printed circuit board is schematically depicted in Fig. 2.

The cover sheet, which represents the support 3 of the composite film may be a sheet of polyethylene tereftalate (PET) or other commercial polyester film, as for example Mylar. The photoimageable resist layer 4 (which may be of negative or positive type) may have a thickness of about 50 μm (in case of dry-film designed for forming a highly conformal solder mask) or between about 75 μm and lOOμm (in case of dry-film designed for

a planarizing lamination process). The film of resist 4 may be a negative or positive resist having an acrylic and/or an epoxidic base.

Between the cover sheet of polyester 3 and the resist layer 4 may also be present an intermediate layer of few micrometers of a release and oxygen impermeable resin suitable to provide a sufficiently prolonged protection of the resist layer 4 against inhibiting effects that may be caused by absorbed atmospheric oxygen following the peeling off of the cover sheet 3. Finally, a thin protection foil 5, commonly of a polyolefine, having a thickness of about 50 μm, protects the photoimageable resist layer 4 during handling, allowing storing of the composite film in rolls and to perform the cutting operations and other handling steps up to the moment of placing the dry-film on the face of the printed circuit board which is effected obviously after peeling-off the protection foil 5 from the normally tacky surface of the resist layer 4.

Figures 3 to 6 show in a partial and schematic way the process of the invention according to a first embodiment thereof. Application of the film 4, still supported by the cover sheet 3 after having peeled-off the protection foil 5, on the face of a printed circuit board (pcb) is depicted in Fig. 3. The surface of the photoimageable resin layer (resist) 4, freed of the protection foil 5, is typically sticky at room temperature and suitable to couple with sufficient tenacity to the surface of the pcb. Normally, is not necessary in this connection to apply any pressure. A slight finger pressure exerted on the four corners may be recommended in order to ensure adhesion. It is evident that all the spaces between adjacent conductive traces are filled with air. Of course, the film may be applied over both faces of the printed board.

The board carrying the film or films on both faces, is placed on the bottom, preferably covered with a blanket of siliconic

rubber, of an evacuable mold, held at room temperature, eventually by providing an appropriate cooling.

The mold is closed by a countermold provided with a mobile pressing diaphragm, which may be in the form of bellows of silicon rubber. The countermold, that is the pressing diaphragm does not come into contact with the sandwich until the end of the vacuum treatment, when the so-called slap-down is effected. Therefore the countermold and the pressing diaphragm may be heated to a higher than room temperature (for example between 50 and 70°C), depending on the environment conditions and other parameters (such as the type of film, the type of printed board being treated, etc.).

According to a main aspect of the process of the invention, the chamber is evacuated down to about 2mbar. This vacuum level is commonly reached in about 4-8 seconds and may be maintained for a similar interval of time in order to allow a completed extraction of the air, through a swelling, lifting of the film until releasing the interstitial air, followed by a collapsing back of the film on the evacuated surface of the board. This process of air evacuation may last overall about 12- 18 seconds from the moment of closing the mold and applying the vacuum. During this time of air evacuation, the sandwich rests essentially on the lower portion of the mold, kept at a normal room temperature and the presence of the overlying pressing diaphragm, which, depending on the ambient temperature and on the peculiar rheological characteristics of the resist layer of the dry-film employed, may also be heated at a process temperature higher than room temperature, does not determine a substantial increase of the temperature of the sandwich, during the entire phase of evacuation of air from the interstices between the film and the board.

Therefore, air evacuation may take place while the film (that is the layer of photoimageable resist) still substantially in a nonflowing state.

This condition eliminates practically completely the occurrence of defects due to an incomplete evacuation of the interstitial air. The limited tackiness of the photoimageable layer (which naturally tends to increase upon heating up toward a substantial flowing condition of the resist), probably represents a determining factor in allowing the lifting of the film because of the vacuum created in the chamber and a complete release through the perimeter of the sandwich of any residue of entrapped air. Maintaining the photoimageable layer in a substantially solid state here intended as opposed to a flowing state whereby the material is capable of flowing under its own weight or by application of relatively small pressure, for example the same atmospheric pressure, during the air evacuation step (swelling, lifting and collapsing of the film preserves the uniformity of the film by preventing an anticipated plastic flow of the photoimageable resin layer, thus retaining perfectly uniform thickness and conformal characteristics of the finished solder mask. Conversely, in known processes, the simultaneousness of heating that is of the softening of the photoimageable resin layer to a flowing condition with the creation and maintainment of vacuum in the chamber is responsible of a large portion of so-called puddling defects due to an incomplete air evacuation. Even the attempts of reducing the incidence of defects by prolonging the time of evacuation of the heated sandwich, beside increasing the working times, tends to enhance the occurrence of marked disuniformities of the thickness of the finished solder mask up to cause the occurrence of spots without coverage because of a decreased conformal characteristics of the photoimageable resin layer (caused by an excessive thinning out of the resin layer in swelled areas during the prolonged vacuum treatment).

Reverting to the process of the invention depicted in the figures, at the end of the air evacuation period, a slap-down of the mobile pressing diaphragm is activated normally by letting air at atmospheric pressure into an evacuated thrust chamber of the diaphragm (bellows), which abuts on the evacuated sandwich. The pressure may be maintained for an interval comprised between 4 and 10 seconds. During this pressing step under vacuum, the pressing diaphragm, eventually heated at a temperature higher than room temperature, may transfer heat to the sandwich in an amount sufficient to increase the temperature of the sandwich from room temperature kept during the evacuation period up to a moderately higher temperature which may be comprised between 35 and 45 °C, depending on the type of dry-film used. This temperature and time limited heating is essentially insufficient to completely make flowable the resist layer but may be useful, depending on the particular characteristics of the resist layer, in order to make it sufficiently plastic to produce a perfect sealing of the evacuated spaces. An eventual increase of temperature over room temperature during pressing is in any way such that the conformation of the photoimageable resin layer 4 to the surface of the printed circuit board remains decisively incomplete or only partially realized as depicted in Fig. 4, according to a fundamental aspect of the invention. In practice, the pressing conducted under vacuum and effected in conditions of substantial absence or only incipient state of flowability of the photoimageable resin layer 4, has the effect of sealing the evacuated cavities 6 that remain between the film and the face of the board. The sandwich may then be extracted from the pressing chamber and left cool back to room temperature eventually. In case a particularly thin film for higher conformability is being used, and interconnection holes of the board must be tented, the pressed sandwich may be exposed through a dedicated mask for

illuminating only the zones of tenting with UV light for few millijoules/cm in order to cause a sufficient degree of polymerization of the photosensitive resin in the zones of tenting. This has been found to be sufficient to prevent the resist layer to become flowable in these exposed zones during the successive heat treatment conducted at atmospheric pressure. The sandwich so evacuated and pressed and eventually exposed to UV only in the tenting zones, may be subjected to the final step of the lamination process of the invention according to two different embodiments.

According to a first embodiment, the sandwich is freed of the cover sheets, as schematically depicted in Fig. 5, and heated by IR irradiation (preferably using IR radiation of relatively long wavelength that is preferentially absorbed by shiny surfaces) for a time sufficient to raise the temperature of the layer of photoimageable resin 4 above the temperature of plastic flow, that is to make the resin fluid, thus producing a complete conformation of the photoresist layer 4 to the surface of the board, under the effect of the atmospheric pressure, as schematically depicted in Fig. 6. The IR heating treatment may be carried out by passing the sandwich over transfer rollers through a chamber equipped with opposed IR sources (below and above the moving board). The speed of board and the intensity of the IR radiation may be adjusted to achieve a most appropriate temperature vs. time profile in the heating of the photoresist layer to produce a complete conformation of the resin film to the board surface. Normally, the resist is heated up to a temperature that may be comprised between 60 and 80 ^β C depending on its characteristics. In case highly conformal thin films are used, a rapid quench of the board upon exiting the oven by chilled air jets may be necessary in order to lower the temperature of the superficial layer down to about 5-10 °C in a practically instantaneous manner. Such a rapid quenching may not be necessary if a dry-

film of a relatively large thickness (of the planarizing type) is used.

According to an alternative embodiment of the process of the invention, it may be preferable to retain the cover sheet until the heating is completed. This alternative sequence of steps is schematically depicted in Figures 7-10. According to this alternative embodiment of the process, the sandwich, still with the cover sheet or sheets 3, is passed through the ir oven and the cover sheet 3 is peeled off immediately after exiting of the heating chamber. Upon the peeling off of the cover sheet 3, the flowable photosensitive resin 4, no longer sustained by the cover sheet 3, flows plastically under the effect of the atmospheric pressure, thus conforming itself to the irregular surface of the printed circuit board, as depicted in Fig. 10. The layer of photoimageable resin thus laminated on the face of the printed circuit board, may then be exposed to UV through a master mask to cause the polymerization of the exposed resin (positive resist) or of the nonexposed resin (negative resist). In case a dry-film with a relatively thick resist layer (planarizing type) is used, it is possible to retain the transparent cover sheet 3 until exposition of the resist is completed. Of course, the cover sheet will eventually need to be peeled off before developing the exposed film in order to remove the resin from the areas defined by the exposition through the master mask. A final drying treatment at a temperature of 140 °C for about an hour, followed by a treatment with UV , may be finally performed for completing the hardening of the solder mask thus formed and defined. It has been found that the UV treatment should be performed after a prolonged thermal treatment for favoring a complete drying of the resin before its external surface is hardened by exposition to UV.