Patent Application
20090283305
The present invention generally relates to printed circuit boards, and more specifically to a coating for circuit boards
Printed circuit boards include traces of a highly conductive material, such as copper, disposed on the surface of a laminate to electrically connect electronic components. Copper, however has a relatively high melting point, and thus is difficult to connect with other electrical components directly. Additionally, copper oxidizes quickly and the connection can fail if the oxygen is left exposed to air. Accordingly, the exposed conductive portions on the circuit board, e.g., traces, along with the contact pads and vias of a circuit board, are typically covered with a conductive solderable coating. The conductive solderable coating is used to bond the contact pad to a lead of the electronic component.
Conventionally, tin-lead compounds have been used to create the solderable coating material. Tin-lead compounds are advantageous because they have a melting temperature which can be adjusted by varying the relative amounts of tin and lead in the compound. For example, a tin-lead compound having 63% tin and 37% lead is eutectic, meaning that it has the lowest possible melting point for the mixture of the two components, melting at 183° C. By varying the relative amounts of lead and tin, the melt temperature could be raised to a higher melting temperature as needed by the application. Nevertheless, despite this versatility, a call for a cleaner environment has led to the elimination of the use of lead as a doping agent. Consequently, the electronics industry has been replacing tin-lead compounds in component coatings and soldering materials with other alternative replacements.
Several materials have been used for printed circuit board (PCB) coatings as a replacement to tin-lead compounds. These include gold, tin, silver and/or silver combined with a tarnish inhibitor. The coatings can be applied to the conductive surfaces on the circuit board by electroplating, electroless plating or an immersion process. In the immersion process the copper atoms near the surface are replaced by atoms of the coating material. However, each of these material coatings has disadvantages. Gold is too costly to consider as a practical alternative to tin-lead. Although, a better value, tin coatings have a tendency to produce whisker growth. Tin whiskers are electrically conductive structures of tin that grow from the surface of the pure tin coating due to mechanical stress. These thin strands of tin have been observed to grow to lengths up to 10 mm. Thus, a PCB having closely spaced circuit elements or traces with a pure tin coating is susceptible to short circuit failure caused by tin whiskers bridging gaps between electrical components. Silver also has a number of disadvantages. First, the cost of silver is significantly higher than the tin-lead compounds it is being used to replace. Second, the silver coating typically needs a tarnish inhibitor to prevent tarnishing. Third, pure silver coatings are susceptible to dendrites, crystalline structures that grow from the surface and, which similar to tin whiskers, may cause short circuit failure. Lastly, pure silver is also susceptible to electromigration—a phenomenon in which the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms causes transport of the silver material. Although this effect is usually negligible, the high direct current densities used in circuit boards and the small cross section of the silver coating can result in gaps being formed in the surface coating over time.
Accordingly, there is a need for a coating for printed circuit boards that is inexpensive, is not highly susceptible to whisker growth, dendrite growth or electromigration, and does not require a tarnish inhibitor.
The present invention in one embodiment is directed to a coating for circuit boards that is made of a tin-silver compound. In one embodiment, a printed circuit board or a printed circuit card includes a conductive circuit with an exposed surface disposed on a substrate. A tin-silver coating covers the exposed surface of the conductive circuit. The conductive circuit can include electrical traces, contact pads and vias, each of which may include or be formed of copper. In a specific embodiment, the tin-silver coating can include a tin weight percentage between 85 and 99.5, while the silver weight percentage can be between 0.5 and 15%. In one embodiment the tin-silver coating can be between 5 and 60 millionths of an inch. A barrier plate may also be included between the conductive circuit and the tin-silver coating.
The tin-silver compound is provided as a coating to prevent the formation of silver dendrites or tin whiskers which appear most frequently in pure tin coated electrical components under mechanical stress. The relative proportion of tin and silver may be modified to thereby modify the temperature characteristics, such as melt temperature, to suit particular applications.
The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of illustrative embodiments of the invention in which:
The conductive circuit 12 can be placed on the laminate 4 by a variety of known methods. In an exemplary embodiment, the non-conductive substrate 4 includes a conductive layer bonded to its entire top surface 8 and possibly also its bottom surface 10. In the described embodiment, the conductive layer is made of copper. Unnecessary portions of the copper layer are then etched away leaving a pattern that provides the conductive circuit 12. Any vias in the PCB may then be drilled through the laminate 4 and the remaining conductive circuit pattern. The walls of the via 6 can then be plated with a conductive material, such as copper, to provide the conductive layer 16 on the outer surface of the via 6 as shown in
Covering the exposed surfaces 20 of the remaining conductive circuit 12 is a coating 18 made of a tin-silver compound. The tin-silver coating can be applied over the conductive circuit by electroless plating or an immersion process. Electroless plating is a known chemical process in which the plated atoms are adhered to the desired surface. The immersion process is also chemical, but in contrast, atoms on the surface of the underlayer, in this case the conductive circuit 12, are replaced by the applied material. In one embodiment, the tin-silver compound is made up of between 85 and 99.5 weight of tin and between 0.5 and 15% weight of silver and is applied at a thickness range between 5 and 60 microinches. The relative proportion of tin and silver may be modified to thereby modify the temperature characteristics, such as melt temperature, to Suit particular applications.
Tin-silver coatings are described in U.S. Pat. Nos. 6,924,044 and 7,147,933, which are incorporated herein by reference. Other materials or doping agents, such as bismuth, silicon, magnesium, iron, manganese, zinc or antimony, may be added to the compound as desired to contribute properties, such as hardness, as required by a particular application. If one or more such additions are added to the compound, these additions will preferably make up less than 10% weight of the compound.
The tin-silver coating is advantageous compared to pure tin or pure silver coatings. In comparison to pure tin coatings, the tin-silver coating does not produce whiskers which may lead to short circuit failure. In comparison to pure silver coatings, the tin-silver coating is less expensive, has much lower electromigration and does not produce dendrites which may result in short circuit failure. Further, the tin-silver coating does not require a tarnish inhibitor, which would otherwise be required with a pure silver coating. Nor is an organic solder preservant necessary which would otherwise be needed for an exposed copper layer.
In the embodiment shown in
Once the tin-silver coating 18 is applied to cover the conductive circuit, including any traces 14, vias 6 and contact pads therein, a solder mask 24 may be applied on top of the conductive coating 18, as shown in
While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/053,514, filed on May 15, 2008, the contents of which are hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61053514 | May 2008 | US |
