Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Utility Patent
1999-06-23
2001-01-02
Talbot, Brian K. (Department: 1762)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C427S096400, C427S313000, C427S372200, C428S901000, C174S250000
Utility Patent
active
06168854
ABSTRACT:
TECHNICAL FIELD
This invention relates to printed circuit boards and in particular relates to methods of manufacturing high density printed circuit boards; assembled printed circuit boards; methods of operating electronic systems comprising assembled boards; and methods of assembling high density printed circuit boards.
BACKGROUND TO THE INVENTION
The fabrication of printed circuit boards is a multi-stage process. Typically, many chemicals and materials are employed for the various stages of fabrication.
One such material is referred to as solder resist. This material is traditionally applied to the printed board to prevent solder, when liquid, from bridging between components and forming short circuits. The solder resist has a secondary function as a protective coating and can help prevent corrosion and mechanical abrasion of the conductive board tracks.
FIG. 1
is a representation of a typical prior art printed circuit board. It contains pads
1
which are connected to lands
2
surrounding via holes
3
by a short strip of conducting track
5
. The location of a typical surface mounted component
4
is represented in outline. Via holes are used to form electrical connections between the various layers of double sided and multi-layer boards.
Prior art processes coat the connecting track with solder resist. The solder resist operates to impede the flow of liquid solder during the soldering process, so that liquid solder does not flow away from the pad and into the via hole. Should such a flow of solder occur, then the remaining fillet of solder between the surface mounted component and the pad would most likely have insufficient mechanical strength for a reliable joint to be formed.
The minimum distance over which this solder resist may be applied is approximately 0.2 mm. This is because the solder resist tends to break over shorter distance. Because of manufacturing tolerances connecting tracks between pads and lands are typically specified as 0.5 mm.
Circuit layouts using fine pitch surface mounted components with spacing between pads and lands of approximately 0.65 mm or less are referred to as high density layouts.
After the initial surface coating of solder resist is applied, selected areas of the board are exposed to UV light. The UV light operates to set the solder resist so that it does not react with etching chemicals. The board then undergoes a further etching process resulting in exposed areas of copper on the surface of the board. Typically these areas will be the surface mounted component pads and any other areas that are to be soldered. Accurate registration is required for this UV process, particularly on boards using fine pitch and small outline components.
Areas of copper exposed when the solder resist is removed are vulnerable to oxidisation during storage. These exposed areas are therefore coated with a protective coating, such as an antioxidant, that is stable for typical storage periods. The anti-oxidant must also be suitable for the component assembly soldering process. One such surface finish is a eutectic or near eutectic composition of tin/lead (Sn/Pb).
When Sn/Pb is used as a surface finish, it is applied by a Hot Air Solder Level process (HASL). Typically the surface of the Sn/Pb will be nonuniform. Such a surface is undesirable for the application of solder paste during the assembly process for fine pitched surface mounted components.
Due to the problems associated with the Hot Air Surface Levelling process, other flat types of material finishes are gaining popularity e.g. Electroless Ni, Immersion Au; Organo Ag; Electroless Pd; organic surface protectants; etc. These material finishes are more expensive than electroplate Sn/Pb surface finishes.
Printed Circuit Boards assembled with surface mounted components use a soldering technique referred to as a reflow soldering. The process may be described as the screen printing of solder paste onto the component pads of the board.
Reflow soldering involves heating the board above the eutectic temperature of the solder. The resultant liquid solder simultaneously wets onto the terminations of the surface mounted components and the pads, forming mechanical and electrical connections when the solder cools below its eutectic temperature.
SUMMARY OF INVENTION
The present invention seeks to reduce at least one disadvantage present in the prior art.
According to a first aspect, the present invention provides a method of manufacturing a printed circuit board comprising a high density conductive pattern comprising at least one pad suitable for forming a solder connection with at least one surface mount component; the method comprising the steps of:
1) forming the pattern by applying etch resist to discrete areas of a conductive surface of a circuit board blank and etching to form the conductive pattern having an etch resist coating; and
2) forming a protective coating on the pattern by heating the etch resist, which protective coating forms a solderable surface for receiving solder to solder the surface mount component.
The use of etch resist as a protective coating has the advantage of avoiding the conventional manufacturing steps of removing the etch resist and applying a protective coating to the exposed pattern.
The step of forming the protective coating by heating the etch resist can have the effect of liquefying the etch resist to form an alloy.
Preferably the alloy comprises a eutectic or near eutectic alloy such as a tin/lead alloy.
Preferably the etch resist layer that is applied at step
1
is thicker than the etch resist layer that would normally be applied in pcb manufacturing processes, and preferably has a thickness in the range of 5 to 15 micrometers.
In some circumstances, such as where high abrasion or insulation properties are required, it may still be desirable to apply solder resist, and solder resist can be applied over the etch resist. When a eutectic or near eutectic composition of Sn/Pb is used as an etch resist it may be necessary to selectively clear the solder resist from large areas of the pattern, such as earth/power plane areas, to minimise surface disruption of the solder resist that may be caused due to Sn/Pb reflow during the pcb assembly process.
By eliminating solder resist from areas adjacent the pads, the density of the circuit layout can be increased. Preferably the density of the pattern is such that at least some features of the pattern are spaced apart by 0.5 mm or less. Preferably the features of the pattern further comprise a via connected to one of the pads and the density of the pattern is such that the via is spaced apart from the pad by 0.5 mm or less.
Preferably the pattern comprises a land surrounding a via, the land adjoining the pad without an intermediate connecting track. The land can adjoin the pad by having the land positioned tangentially with an edge of the pad or by having the land positioned with a diameter across an edge of the pad.
Preferably two or more pads adjoin the via without the use of intermediate connecting tracks between the pads and the land. The pads may be located on opposite sides of the board. This provides further improvements in the density of the circuit layout.
Preferably the coating is formed without the solder resist being formed anywhere on the board. Eliminating solder resist from the board reduces process steps in the manufacture of the board. Preferably the step of forming the pattern comprises the step of applying etch resist and wherein the etch resist is used in the step of forming the coating. This further reduces the number of steps in the manufacturing process.
A further aspect the present invention provides a high density printed circuit board comprising a high density conductive pattern comprising at least one pad suitable for forming a solder connection with a surface mount component, the pattern having a protective coating which has been formed by heating an etch resist coating that was used in etching the conductive pattern, the protective coating having a solderable surface for receiving solder to solder the surface mo
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
Talbot Brian K.
LandOfFree
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