Temporary solder masks provide a coating material used to mask or to protect selected areas of a printed circuit board from the action of an etchant, solder, or plating. Specifically, a temporary solder mask is a coating which masks off a printed circuit board surface and prevents those areas from accepting any solder during vapor phase or wave soldering processing. A prime function of a temporary solder mask is to restrict the molten solder pick up or flow to those areas of the printed circuit board, holes, pads and conductor lines that are not covered by a permanent solder mask.
A distinction is made between permanent and temporary solder masks. Permanent solder mask compositions, which consist of tri- or poly-functional monomers, are highly cross-linked upon curing via ultraviolet light or electron beam. This cross-linking prevents permanent solder masks from being soluble in organic solvents or water.
On the other hand, temporary solder mask compositions do not contain tri- or poly-functional monomers and therefore do not cross-link upon curing. Rather, temporary solder mask compositions, upon curing, undergo a linear polymerization reaction, thus allowing the temporary mask to be soluble in organic solvents, water or even to be peeled off of a circuit board. The solubility or lack thereof of temporary and permanent solder masks is crucial to the utilization of these masks in the electronic industry. Temporary masks are applied to a selected or limited area of a printed circuit board to protect certain holes or features such as connector fingers from accepting solder. The temporary mask keeps solder out of the selected holes and thus allows for certain process or temperature sensitive components to be added manually at a later time. In contrast, permanent solder masks are not removed and thus become an integral part of the printed circuit board from the very beginning of its application to the board.
The more commonly seen solvent soluble mask is the inorganic extenders and/or organometallic salts. These formulas may consist of low molecular weight acrylic homopolymers extended with aluminum oxide, calcium carbonate or titanium dioxide along with plasticizers and carried in a chlorinated, aromatic or low molecular weight aliphatic solvent. Some formulations have substituted calcium octoate, zinc octoate, aluminum octoate or various inorganic stearate salts for the acrylic polymer.
Although inorganically extended solder masks apply, cure, and wash fairly well, a major problem exists with respect to contamination. The inorganic extenders, being insoluble in the cleaning solvent, tend to plug filters, collect in boiling sumps and generally harass cleaning systems. Being partially solvent soluble, the organometallic components (e.g., calcium octoate) tend to form micro-gels when present in high enough concentrations. When these gelled-aggregates become large enough, they can also plug solvent filters and will tend to settle out and collect in sumps.
However, the worst aspect of this type of mask with respect to contaminations is the deleterious effects on flux and solder. Flux is an acidic compound that removes oxides from metals and prepares the surface of a printed circuit board for soldering. Being ionic in nature, this type of mask has the potential to reek havoc on flux with activators through simple ion exchange at elevated temperatures. Potential post solder/wash contamination of printed circuit boards can also present problems.
A further problem is that the current technology uses solvent borne thermal cure masks which require evaporation of the carrier solvent to occur before the mask is cured and ready for use. This process can take anywhere from 1-24 hours. Also, these carrier solvents pose a high environmental risk upon evaporation. With respect to the previous existing thermal cure masks, they possess material which is only 30-60% active, the remaining being water or solvent carrier. Thus, with these thermal systems, the solvent or water must be evaporated. With solvent systems, this evaporation creates health and environmental hazards because the solvents used for polymer solubilization are toxic (e.g., acetone, toluene, xylene, etc.). In fact, many states within the United States have passed legislation prohibiting direct evaporation of such toxic solvents into the air.
On the other hand, in the present invention, the UV cure materials are 100% active, that is, the materials are not carried in a solvent and, all of the material reacts or polymerizes, leaving minimal or no volatile organic compounds.
Accordingly, it is an object of this invention to provide a UV curable temporary solder mask that, after application to a surface to be protected, may be cured in approximately. 5-45 seconds, depending upon the user's needs, using an ultraviolet radiation source having a wave length in the A range. This temporary solder mask may be applied robotically, manually or by silkscreen/template methods. This unique solder mask is ready for soldering operation immediately following the cure procedure and provides complete protection to desired areas, will withstand temperatures of up to 650.degree. F. and can be removed without leaving any residue. Furthermore, the present invention is not carried in a solvent and is 100% active. Therefore there are minimal health and environmental concerns. Also, this invention may be removed from the surface to be protected by either dissolving this temporary mask in an aqueous nonalkaline solution, an organic solvent or peeling the mask off of the protected surface.
These and other objects and features of the invention will be made apparent from the following description of the invention.