|GETTING THE LEAD OUT
Proposed deadlines for eliminating lead in electronics manufacturing really began to impact the industry in the early 1990s. My years of experience with RFI/EMC filter manufacturing taught me how heavily the industry relied on using a lead-tin combination to facilitate soldering as well as to prevent rusting. Many manufacturers used outside suppliers for tin plating but increasingly heavy environmental fines put many of these suppliers out of business. Manufacturers have needed to find an environmentally friendly substitute for the lead-tin coating that would perform as well and be cost effective.
The product, called AC-78, is a multipurpose conductive and electroplate-able nickel based polymer that was initially designed as a viable alternative to lead-tin coating. The goal was to find a coating that could be used for protection against rusting and that could easily be soldered in the assembly process. This product not only met those requirements but also demonstrated properties valuable for numerous other applications. It easily adhered to nearly every substrate including titanium, stainless steel, glass, ceramic, many plastics, rubber, wood, paper and textiles and transformed these substrates into electrically conductive materials. Further investigation showed that non-metallic substrates exhibited RFI/EMC shielding properties.
Initial tests showed that this new polymer product performed well in solder-ability and especially in rust protection, exceeding 1000 hours (ASTM B117) salt-spray chamber testing. Component attachment using Eutectic solder with this coating produced good solder contacts and joints that passed pull tests. Solder-less attachments can also be achieved by marrying components with the coated substrate while curing them simultaneously.
This product has many applications in the printed circuit board industry where it is being evaluated for multi-layering, solder-less bumping, screen printing and through hole design techniques.
Resistivity is a function of both curing conditions (cure temperature and cure time) and the substrate used. Under ideal conditions, the volume resistivity can be as low as 0.65 ohms/square/mil.
The enclosed charts show decreasing resistivity over cure time with curing at different temperatures (125°C, 150°C, 175°C and 200°C) for four substrates (glass microscope slides, 96% alumina, glass fiber epoxy board, anodized aluminum metal sheet). However the decrease in resistivity over curing time is small.
With higher cure temperatures, the plateau of constant resistivity is reached more quickly. Increased cure temperatures result in a lower level for the final plateau of constant resistivity.
Different substrates demonstrate different resistivity readings. For lower cure temperatures the resistivity differences are significant; whereas for higher cure temperatures, such as 200°C, different substrates show similar resistivity.
125°C 30 60 90 120 150 180 (min.)
Glass slide 1.51 1.48 1.4 1.39 1.37 1.29
Alumina 1.698 1.63 1.55 1.5 1.495 1.23
Glass/epoxy 1.4 1.33 1.27 1.25 1.23 1.15
Anodized Al 1.3 1.26 1.21 1.18 1.15 1.15
150°C 30 60 120 180 (min.)
Glass slide 1.34 1.33 1.3 1.25
Alumina 1.25 1.22 1.17 1.17
Glass/epoxy 1.25 1.24 1.24 1.14
Anodized Al 0.97 0.96 0.94 0.93
175°C 30 60 90 120 150 (min.)
Glass slide 0.84 0.82 0.83 0.83 0.82
Alumina 0.87 0.81 0.82 0.82 0.79
Glass/epoxy 1.07 1.06 1.05 1.03 1
Anodized Al 1.1 1.1 1.09 1.03 1.02
200°C 30 60 90 (min.)
Glass slide 0.8 0.76 0.76
Alumina 0.77 0.73 0.73
Glass/epoxy 0.89 0.71 0.69
Anodized Al 0.75 0.75 0.73
222°C-225°C 15 30 (min.)
Glass slide 0.936 0.8
Alumina 0.77 0.76
Glass/epoxy 0.76 0.75
Anodized Al 0.74 0.74
In test environment the coating has shown good to excellent solder-ability with the use of Kester flux 1588. Other fluxes show less or no solder wetting at all. No solder leaching has been observed with the use of the product.
Wetting was also extremely good at 100% for the flexible polyurethane.
Test parameter: Tests have been conducted with 10 x 10 second dips with examining at each stage for de-wetting. As the number of dips rose the solder appearance got better with no sign of leaching after the 10 dips into 62/36/2 (tin/lead/silver) bath at 220°C.2
Applied on a steel substrate with a thickness of 1-2 mils, the polymer coating passed a fog test (ASTM B117) for 1,000 hours in a salt spray chamber (5% salt solution, 95% humidity at 75°F) with no sign of rusting.
The polymer coating has shown very good adhesion with actual wire and glass substrates breaking in the adhesion tests.
Test parameters: The solder strips from the solder-ability tests (approx. 3 to 4 mm in width) were used. Tinned copper wire was butt soldered using a hand held soldering iron. No additional flux was needed to get a good solder joint. The pull tests were then conducted with a Chattilion tester at 3 inches/minute. All failures were from the conductor occurred where the conductor was seen on the bottom of the solder joint attached to the copper wire. The exception was some of the glass slides where glass broke at >20 lbs and the conductor remained attached to the wire.
Though it is made of a ferrous metallic component nickel, the polymer coating acts as an effective shield to electric and magnetic fields. The attenuation is comparable to that of conductive nickel paint. If applied at 3 mils thickness and measured between the frequency of 3 MHz to 1 GHz the attenuation is approximately 40 dB (measured in a dual chamber test set-up) or 50 dB (transmission line test set-up) respectively.
Applying The Coating
1. Substrate Preparation:
Cleanliness of the substrate is of extreme importance for the successful application of a conformal coating. Surfaces must be free of moisture, dirt, wax, grease and all other contaminating materials. Use solvent to thoroughly clean the surface of the material. Contamination under the coating will cause problems that may lead to assembly failures. For some substrates, additional surface preparation steps, such as sandblasting might be necessary to ensure quality adhesion.
Mix thoroughly before applying. The formula contains metal in suspension in a solvent, which must be mixed thoroughly before applying. Also, during application, formula needs to be constantly stirred to avoid settling of the components.
•Spraying: In its original formula, the polymer is very viscous, but can be sprayed on with special high viscosity spraying equipment. If the formula is too viscous for a particular spray equipment, it can be diluted with additional solvents. The amount of solvent and spray pressure will depend on the specific type of spray equipment used. If diluted, it will be necessary to spray on additional layers to reach the original properties of the formula.
•Screen-Printing: Formula can also be applied through screen-printing. However, the metal particle size might be the limiting factor for fine screen-printing. Milling may be necessary. For the original formula, a mesh size of 200 may be used. Product tests with milling have shown easy use of mesh 400 sieves.
The formula can also be applied by brushing or dipping.
The curing temperature and cure time depend on the desired resistivity and the substrate used.
After applying formula, carefully place the coated material into a preheated oven to cure at
ca. 100°C-120°C for five minutes (to remove moisture without generating bubbles), then at
ca. 220°C for 10 minutes and finally at 260°C for 5 minutes (ideal cure cycle). Don1t touch the surface before and during the curing procedure. Remove from oven and allow to cool to room temperature. The coated material is now conductive and ready to be soldered or electroplated.
First test results indicate that an alternative infrared curing method can be used to avoid potential damage to the substrate, caused by exposure to an elevated curing temperature.
Infrared curing is recommended for low-temperature substrates, such as plastics. Infrared cure may also be used when the curing time needs to be significantly reduced (<5 minutes).
For the cleaning of the spray gun, regular cleaners like MEK should be sufficient. The recommended solder fluxes are organic and water-soluble. Use water to clean surface residue on coated substrate.
Note that solder without a flux core must be used. We recommend SN 63 -PB 37 solder wire. Apply a soldering iron (e.g. 60 Watt) to the coated surface and allow it to reach a temperature that will melt the solder (ca. 185°C for recommended solder). Apply flux and then the solder wire.
For large metal substrates, which act as a heat sink, it is of paramount importance to preheat the entire substrate to at least 100°C in order to activate the flux.
Printed Circuit Boards
Electroplating non-conductive materials
This multipurpose conductive and electroplate-able nickel based polymer is a viable alternative to lead-tin coating. It1s suitability to a variety of surfaces and applications make it possible to incorporate revolutionary materials into electronics, providing vast new opportunities for the engineer, designer, and manufacturer.