Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1997-12-18
2001-08-07
Paladini, Albert W. (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S262000, C257S676000, C257S690000, C361S772000, C361S773000
Reexamination Certificate
active
06271479
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and a device for connecting an electrical component to a circuit board. More closely, it relates to the electrical and mechanical connection of a high-power component which transfers high frequency electrical signals to electrically conducting conductor paths on a circuit board.
STATE OF THE ART
The connecting of electrical components to conductor paths on circuit boards normally takes place through soldering. The components can have projecting connections, so-called legs, or some other type of connection which are soldered to some kind of contact surface which is connected to the conductor paths on the circuit board.
The electrical components can, during use, heat up and be subject to stresses, such as temperature changes or mechanical influences. The component, the component legs and the the circuit board have different coefficients of expansion, whereby the soldered joint is subject to stresses during heating and cooling. Therefore electrical components are normally designed so that they rest on vertically bent legs which permit the components to move. When the component is heated up and expands, the forces produced then can partially be absorbed by the resilient legs.
Patent documents U.S. Pat. Nos. 4,777,564 A, 5,241,134 A and 4,991,059 A describe different embodiments of vertically mounted and bent legs for electrical components.
Not all electrical components can, however, be designed with resilient legs. High-power components, such as high-power transistors, develop such high power losses that the temperature increase becomes considerable during operation. Such components are therefore normally mounted sunken in a hole in the circuit board in contact with a cooling flange in order that sufficient cooling is obtained. An example of such components are high-power transistors which amplify high-frequency electrical signals in base stations in a mobile communication system.
The design of the legs influences the signal quality of the high-frequency signals to a great extent. If the legs are made too long or too bent, the power losses increase in the transistor stage which lowers the amplification of the transistor. The longer and the nearer the bends are, the more narrow-band the transistor stage becomes, at the same time as its tolerance sensitivity increases. If the losses become so great that they encrouch on the amplification, it means that the signal strength is attenuated. In order to avoid this, the legs should be straight, relatively short and arranged projecting horizontally from the transistor.
A disadvantage of today's soldered joints on high-power components is that the soldered joint fatigues and gradually cracks when it is subject to extreme stresses during repeated temperature changes. These temperature changes give rise to cyclic loading of the legs, so-called thermal fatigue.
For practical reasons such component legs are furthermore normally soldered manually, wherefore the quality of the soldered joints can vary.
A soldering material which is normally used during soldering of high-power components is the alloy 62%Sn+36%Pb+2%Ag.
Yet another disadvantage when using soldering materials during the fastening of electrical components is that soldering materials containing lead are environmentally dangerous.
It has become more and more popular, in particular from the environmental point of view, to use electrically conducting adhesives for connecting electronic components to conducting foundations.
Patent documents EP 0 708 582 A1 and EP 0 549 159 A2 describe electrically conducting epoxy adhesives containing conducting particles. The adhesives have improved characteristics in relation to strength and electrical conductivity compared to normal silver-filled epoxy adhesives.
The patent document U.S. Pat. No. 4,904,414 A describes an electrically conducting adhesive which is designed to retain its elastic properties in a large temperature interval (−60° C. to +180° C.).
The patent documents U.S. Pat. Nos. 5,136,365 A, 5,395,876 A, 5,362,421 A and 4,729,809 A describe different electrically conducting adhesives which can be used during the electrical connection of different electrically conducting surfaces.
DESCRIPTION OF THE INVENTION
The present invention tackles the problem of how high-power components which transfer high-frequency electrical signals are to be connected electrically and mechanically to conductor paths on a circuit board.
A further problem is how a connection arrangement between high-power components and conductor paths on the circuit board should be designed in order to hold when subject to large, repeated temperature changes.
An object of the present invention is consequently to connect high-power components, which amplify high-frequency electrical signals, to conductor paths on circuit boards, and to design the connection arrangement between high-power components and conductor paths on the circuit board, so that it does not break when it is subjected to large, repeated temperature changes.
The problems are solved by using electrically conducting adhesives for connecting component legs to conductor surfaces on the circuit board, in combination with selecting the length and shape of the component legs depending on a predetermined threshold value of the signal attenuation for the electrical high-frequency signal.
In more detail, the problem is solved by electrical connections, so-called legs, which project horizontally from the component, being fastened by means of electrically conducting adhesive onto the contact surfaces. The contact surfaces form a part of the conductor paths on the circuit board. The electrically conducting adhesive should have a good pull-off strength to the foundation so that good adhesion to the foundation is obtained. A threshold value for the highest acceptable signal attenuation of the signal strength of the high-frequency signal is determined. The component legs are shaped, according to a preferential embodiment, so that they are as long as possible without the resulting signal attenuation surpassing the threshold level. The component legs can also be made with a number of cavities, whereby the adhesive provides a better grip as it penetrates up into the cavities.
An advantage of the invention is that the length of life for the connection between the legs and the contact surface increases, which results in a lower risk of electrical breakdown.
Yet another advantage of the invention is that the use of electrically conducting adhesive is more environmentally friendly than the use of soldering materials containing lead.
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Burns Doane Swecker & Mathis L.L.P.
Paladini Albert W.
Telefonaktiebolaget LM Ericsson (publ)
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