Today, printed circuits are one of the most widely used means of interconnecting electronic components. These circuits are used in virtually every industry that requires the use of electronic components. The wide-spread acceptance of these circuits has led to their use in an expanded range of applications. However, this expanded use has placed greater demands on the physical properties of the circuits. These increased demands have also caused several problems related to the use of printed circuits. Some of these problems, such as the complexity of the required circuit interconnections, the increased demands for circuit miniaturization (decreasing the circuit size), increased circuit densities (more components in the circuit space) and solder compatibility problems associated with mass-production soldering equipment have been addressed by a series of improvements in printed circuit manufacturing processes. These improvements have led to printed circuits with higher densities and greater reliability. However, some demands placed on the circuits, such as use of circuits in high temperature environments, the weight of printed circuits and the use of Lead components in these circuits have created other problems that have not been adequately addressed.
First, the use of these printed circuits has expanded to high temperature (greater than 130.degree. C.) applications. Typically, maximum temperatures for current applications using printed circuits do not exceed 130.degree. C. Exposure to higher temperatures can affect the physical properties of the circuits, especially the solder, and sometimes the operating capability of the electrical components.
The need for printed circuits with increased reliability during operations in high temperature environments is becoming more important especially in the automotive industry and energy industry applications. For example, in oil and gas exploration, printed circuits are housed in logging tools that are lowered into boreholes drilled in the earth's subsurface. Most printed circuit boards can affectively operate at maximum temperatures of approximately 130.degree. C. However, the temperatures at certain borehole depths may reach 185.degree. C. Therefore, if logging tools contain printed circuits, these circuits must be capable of reliably performing during exposure to these high temperatures. Also, in motor vehicles, printed circuits are used in instruments that monitor engine equipment and are located in environments that also reach temperatures above 150.degree. C.
A second problem related to the exposure of printed circuits to high temperatures is the effect of solders with high melting point temperatures on the electronic components during the printed circuit manufacturing process. During the manufacturing process of printed circuits, electronic components are attached to a printed wiring board to create the printed circuit. Solder is used to attach the electronic components to the printed wiring board. In the soldering process, solder is melted to its reflow temperature. This is the temperature at which the solder readily bonds with the wiring board to which it attaches an electronic component. During the melting process, solder changes from a solid to a pasty state and then to a liquid. A better solder joint is created when the solder is in a completely liquid state. If the solder is not heated to the reflow temperature, the solder joint would be created with a pasty solder. This form of solder does not have the best bond and could lead to a broken solder joint. The solder reflow temperature is generally 30.degree. C. to 40.degree. C. above the initial melting point temperature of the solder and 80.degree. C. above the application temperature. High temperature applications require solders with melting point temperatures much higher than the current application temperatures, therefore during the manufacturing process, electronic components will be exposed to very high solder reflow temperatures. This exposure and contact with the melted solder at these extremely high temperatures can weaken and damage the electronic components and even the printed circuit board. This weakening of components effectively shortens their life and decreases circuit reliability.
Another problem that affects the use of printed circuits is the physical weight of the circuits. The weight of the equipment is a major concern in the aerospace industry. This concern applies to all portions of the equipment including printed circuits. Therefore, there is a need to reduce the weight of the equipment including the printed circuits without reducing the equipment reliability or quality.
Still another problem with printed circuits is the amount of Lead contained in the solder that is used in the printed circuits. Because of environmental concerns and the increasing number of government regulations limiting the use of Lead, the future use of Lead materials in electronic components is uncertain. Therefore, there is a desire to produce printed circuits with reduced amounts of Lead.
As indicated earlier, the major factor that affects the use of printed circuits in high temperature environments is the solder used in these printed circuits. As the temperature of an environment rises, the melting point of the solder in the printed circuit is of more concern. The initial melting point of the solder is the temperature at which the solder begins to change from a solid to a liquid. If applications reach the initial melting point temperature of the solder, the strength of the solder and solder joint would weaken and thereby lead to circuit failure. One solution is to have solder materials with higher melting point temperatures. However, as the initial melting point of solder increases, the reflow temperature of the solder used in manufacturing the circuit also increases. The increase in the reflow temperature can damage electronic components during the manufacturing process.
The possibility of component damage is especially true for printed circuits with active surface mount devices. Because of the high lead counts of some surface mount devices and the fact that surface mount device leads do not extend through the printed wiring board, during the normal manufacturing process for circuits with surface mount devices, a "Mass Reflow" type of solder process is required to reliably attach the surface mount devices to the printed circuit board. In a Mass Reflow solder process, the entire board assembly, including electronic components, is exposed to the reflow temperature of the solder being used. The reflow temperature of the solder is high enough to melt the solder and attach an electrical component to the circuit board. However, the temperature is also low enough that the electronic components are not damaged from exposure to the heat. This reflow process is adequate for manufacturing printed circuits used in current temperature environments but, high temperature applications would require manufacturing printed circuits at reflow temperatures much higher than those presently used. Exposure of the components to these reflow temperatures, which are much higher than the application temperatures, could lead to component damage and failure.
Currently some high temperature applications use throughhole electronic components mounted on printed circuit boards. However, during the manufacturing process, these components are wave-soldered to the printed circuit boards. During wave-soldering, the bottom of the printed circuit board comes in contact with a stream of solder. This solder is usually Multicore HMP solder with a very high melting point temperature of approximately 330.degree. C. However, since only the component pins on the bottom of the board are exposed to the high temperature solder, the electrical components are protected from the heat and no real damage is done to the board or components. However, for mixed technology printed circuit boards having both active surface mount and through hole devices, this wave soldering process would not be sufficient to adequately attach the surface mount components to the board. Therefore, another process is needed for manufacturing mixed technology circuits for use in high temperature environments.
Another factor that impacts the operation and manufacturing of printed circuits, is the percentage of Lead in the solder material. Current estimates indicate that Lead is included in approximately 99 percent of all printed circuits. In addition, because pure Lead has a high melting point, the general practice is to increase the amount of Lead in the solder of the circuits as the temperature of the circuit applications increase. As stated earlier, Lead is not a desired material to be included in these circuits. There are several reasons to use non-Lead components in printed circuits. It is known that combining Lead with other materials can result in solder compounds with lower than desired melting point temperatures. These lower melting point solders are not desirable in high temperature applications. In addition, Lead is a heavy material that adds weight to the solder and to the printed circuit. There is also growing support for government regulations to prohibit the use of Lead in electronic components. These reasons encourage a movement away from use of Lead component materials in printed circuits.
Because most applications of printed circuits are in environments where the maximum temperatures are approximately 130.degree. C., concerns about solder melting points and solder reflow temperatures for high temperature environment applications are not a major concern. Furthermore, printed circuits that currently operate at temperatures above 130.degree. C. use large amounts of Lead in their solder. In addition, these circuits do not contain active surface mount devices. Therefore, the printed circuits that are currently manufactured and used are not reliable in or desirable for high temperature applications.
For printed circuits to operate in high temperature environments, in particular those printed circuits that contain active surface mount devices, the circuits should have a solder that has a high enough melting point to maintain its mechanical properties in the high temperature application environments. However, this solder should have a low enough reflow temperature so that exposure of the active surface mount devices to the reflow temperatures during the manufacturing process do not harm the circuit devices or the printed circuit board. In addition, the solder should have minimal Lead content. Consequently, further improvements are needed especially in the solder used in these printed circuits to enable the circuits to perform in a reliable manner in high temperature environments.