Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1998-12-02
2001-06-12
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S034000
Reexamination Certificate
active
06246011
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for improving electrical equipment reliability by use of improved solder jointing techniques.
BACKGROUND TO THE INVENTION
Solder joints serve three functions: physical attachment of electronic components (for example peripheral packages) to a circuit board, completion of an electrically conductive path from the circuit component to the board, and provision of a thermally conductive path between the electronic component and the board. These functions may have conflicting requirements.
For physical attachment of electronic components it is typically the case that longer fatigue life may be achieved by use of larger solder joints. Such fatigue is caused by thermal expansion mismatch between the component and the circuit board. Similarly, larger solder joints typically improve thermal conductivity between component and board.
Use of smaller solder joints, whilst advantageous with respect to electrical conductivity for high frequency signals, has the disadvantage that resistance to solder joint fatigue failure may be reduced. It is also difficult to align small solder joints during re-flow in manufacture.
A known approach to improving package reliability with respect to solder joint fatigue is to underfill components with an epoxy resin (or other substance having similar adhesive, non-electrically-conductive properties) in order to constrain movement of a package relative to a board. This results in additional production processes—underfill dispensing and curing, either by heating or by ultra-violet irradiation—adding to time and cost of manufacture. The use of epoxy underfill also makes subsequent re-work more difficult and, in particular, precludes simple re-soldering.
It is known to provide strain relief for corner joints between an electronic component and a board by enlarging the solder joints at those corner joints.
U.S. Pat. No. 5,639,013 discloses a method for improving reliability of solder joints by forming convex solder joints having increased pedestal height.
OBJECT TO THE INVENTION
The invention seeks to provide a method for improving the reliability of solder joints for electrical components having high frequency interconnections and for improving the alignment of such joints during manufacture.
The invention also seeks to provide more reliable electrical apparatus.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an electrical circuit arrangement comprising a circuit board and a surface-mount electrical component attached thereto by a plurality of electrically-conductive joints, wherein one dimension of one of said joints is predetermined from a characteristic of a signal to be conducted through said joint.
Preferably also, a dimension of a number of joints is predetermined from a characteristic of respective signals to be conducted through said joints.
Preferably, said characteristic of a signal is the frequency of said signal.
Preferably, the size of said one dimension of said joint is determined to be inversely proportional to the frequency of an electrical signal to be conducted through said joint.
Preferably, the dimension is the length of the joint.
Preferably, the arrangement is such that a first joint is positioned adjacent to at least one larger joint whereby to afford strain relief to said first joint.
More preferably, the arrangement is such that said first joint is positioned adjacent at least two larger joints.
Preferably, said first joint is arranged, in operation, to conduct high frequency electrical signals. Said high-frequency electrical signals may be radio frequency signals.
Said larger joint may be arranged, in operation, to form part of an electrical grounding or shielding member.
Advantageously, the benefits of small joint sizes for high-frequency signals are realised.
Advantageously, the benefits of large joint sizes for ground connections are realised.
Advantageously, expected fatigue life of relatively smaller joints is increased.
Advantageously, the presence of larger solder joints improves thermal conductivity between the components for improved heat dissipation in operation.
The joints may be of solder or of electrically-conductive adhesive.
The invention also provides for a system for the purposes of telecommunication (including wireless telecommunication) which comprises one or more instances of apparatus embodying the present invention, together with other additional apparatus.
According to a further aspect of the present invention, there is provided a method for improving the reliability of solder joints comprising the steps of: providing first and second components to be soldered together; providing solder; applying predetermined volumes of said solder to each of a plurality of connection points on at least one of said components; juxtaposing the components so as to locate the solder between and in contact with both components; re-flowing the solder; allowing the solder to cool whereby to form solid solder joints each providing at least one of physical, thermal, and electrical connection, between the components; and wherein a dimension of one of said joints is predetermined from a characteristic of a signal to be conducted through said one of said joints.
Preferably, the volume of solder at each connection point is inversely proportional to the frequency of electrical signals to be carried.
Preferably, the solder is applied in paste form.
The solder paste may be applied by screen printing, plating, jetting, direct dispensing, or stamping.
The solder may be applied to one component only.
One of said components may comprise a circuit board.
Preferably, the solder is applied to the circuit board.
One of said components may comprise one of a peripheral package, a ball-grid, and a pin-mount package.
The solder may be applied to the component.
Preferably, the solder, when re-flowed, exerts a net force such that the components are buoyant with respect to each other.
Advantageously, the use of larger solder joints to constrain smaller critical signal connections improves the manufacturing process, by increasing the degree of self-alignment brought about by solder surface tension during re-flow, thus facilitating the difficult task of aligning small solder joints consistently.
Advantageously, assembly and materials costs are reduced compared with traditional underfill techniques.
According to a further aspect of the present invention, there is provided a method for improving the reliability of conductive adhesive joints comprising the steps of: providing first and second components to be glued together; providing electrically-conductive adhesive; applying predetermined volumes of said adhesive to each of a plurality of connection points on at least one of said components; juxtaposing the components so as to locate the adhesive between and in contact with both components; allowing the adhesive to set whereby to form solid joints each providing at least one of a physical, thermal, and electrical connection, between the components; and wherein a dimension of one of said joints is predetermined from a characteristic of a signal to be conducted through said one of said joints.
In a preferred embodiment, joints are arranged across the rim of the package radially relative to the centre of the package. However in some situations a grid layout of joints oriented orthogonally with respect to the package edge is more convenient for circuit board layout.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.
REFERENCES:
patent: 5484964 (1996-01-01), Dawson et al.
patent: 5639013 (1997-06-01), Jairazbhoy et al.
patent: 5790377 (1998-08-01), Schreiber et al.
patent: 5874780 (1999-02-01), Murakami
patent: 6101099 (2000-08-01), Olsson
Abraham Ian Andrew
Liu Andrew Siu Hing
Gaffin Jeffrey
Lee Mann Smith McWilliams Sweeney & Ohlson
Norris Jeremy
Nortel Networks Limited
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