Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Patent
1980-12-08
1983-07-19
Brown, J. Travis
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
430278, 430313, 430314, 430315, 430324, 430325, 430326, 430905, 430908, 430909, 430910, G03C 168
Patent
active
043944345
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to improved plating resists containing finely divided fillers.
BACKGROUND ART
In printed circuit board manufacture, a plating resist pattern controls where electrolytic or electroless deposition of metal takes place. Therefore, a plating resist should not take up undesired metal deposits on its surface during the plating step. Such undesired metal deposits will be referred to herein as "extraneous metal". Extraneous metal deposition is generally not a problem with state of the art electrolytic plating resists. However, due to the severity of electroless bath conditions and the longer immersion times generally employed, state of the art electroless plating resists are susceptible to the deposition of extraneous metal. These resists may also be physically attacked by the electroless plating bath, leading to bath contamination. In electroless copper plating, extraneous copper deposition is a particularly severe problem, due to the extended immersion times required to produce the desired thick copper layers and the highly alkaline conditions (e.g. 10 hours at pH 12-13 and 61.degree. C. (142.degree. F.) for a 0.0025 cm (1.0 mil) coating). The extraneous copper so deposited may range from a loosely adhering dust to tightly bonded overplating. Accordingly, individual inspection is required to detect such extraneous copper. If possible, the unwanted particles are removed by brushing. A substantial percentage of printed circuit boards (PCB's) cannot be so corrected and must be etched bare and reworked, an expensive and time consuming process.
The susceptibility of plating resists to extraneous metal deposition is frequently aggravated due to the presence of fillers commonly used in such resists. Two types of fillers are typically used in plating resists; namely, thixotropic fillers, herein referred to as thixotropes; and extender fillers, herein referred to as extenders. Thixotropes are finely divided substances which, when dispersed into a resist, cause the resist to exhibit shear thinning, e.g. low viscosity when forced through a screen but high viscosity when no force is applied. This improves screen printing performance by allowing an intricate pattern to be printed without deterioration of the pattern before it has cured. Thixotropic screen printable resists are usually cured by methods other than solvent drying (e.g., actinic radiation and visible light cure). Thixotropes which have been used in such resists include untreated fumed silicas (e.g. "Cab-O-Sil M5", commercially available from Cabot Corp.); treated fumed silicas (e.g. "Aerosil R-972", commercially available from DeGussa); and organic materials (e.g. "Thixcen R", commercially available from Baker Castor Oil Co.).
Extenders are frequently added to solvent-dried screen printable resists and to photoresists in order to increase the viscosity of the plating resist (which may be desired to improve the handling characteristics ) and reduce the cost of the product by diluting it. Talc, calcium carbonate and precipitated silicas have all been used as extenders.
Unfortunately, the addition of the fillers discussed above (whether thixotropes or extenders) to plating resists increases their tendency to attract extraneous metal. Thixotropic screen printable resists subjected to electroless copper plating are especially susceptible to extraneous copper deposition. In order to obtain good resistance to extraneous copper deposition, the screen printing resists of the prior art have employed heat cured two part resists. These resists must be mixed before application and used within a few hours of mixing. The heat curing step necessitates a lengthy baking operation and a large processing area. Heat cured resists tend to thin out and flow onto uncoated areas of the PCB during the heat curing step, thereby limiting attainable pattern definition. Actinic radiation cured screen printing resists would be preferred to these heat cured resists since they need not be mixed before use, are cured with a faster and more energy effici
REFERENCES:
patent: 3600330 (1971-08-01), Schneble et al.
patent: 3743537 (1973-07-01), Honjo et al.
patent: 3754920 (1973-08-01), Kuchta
patent: 3801427 (1974-04-01), Morishita et al.
patent: 3958996 (1976-03-01), Inskip
patent: 4308312 (1981-12-01), Urban
Degussa Technical Bulletin No. 56
Alexander Cruzan
Brown J. Travis
Cleveland David R.
Minnesota Mining and Manufacturing Company
Sell Donald M.
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