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Design and placement of pads for mechanically assembling multiple printed circuit boards together to form one intergraded printed circuit board.
Description
This invention is a reliable and economical method of connecting modular electronic/electrical circuits or printed circuit boards together to form custom circuits or a custom printed circuit board. All circuit boards using this system will be compatible with each other irrespective of the manufacturer. The design of the interconnecting pads and placement of these pads make the method unique. The other unique feature of this system is the use of blank boards and boards for mounting to achieve the desired shape and mounting holes. This method varies from other patents in that it is economical and practical to produce.
It must be noted that this is a tested and proven method of connecting circuit boards together and not the electrical circuits, these circuits are connected by standard methods for example wiring of circuits together or through connectors etc. Connecting boards together via this method does not require the board to be on the same plane, boards can be connected at right angles to each other. This method of connecting boards together requires no knowledge of other boards to which they may be connected allowing persons without knowledge of other products to produce a product that would integrate with the system. Different boards can be connected together irrespective of size, shape, orientation or alignment as long as connection sides are parallel with each other.
Background of the invention
Producing customised electronic circuit boards is expensive and time consuming whether being used for prototyping, testing a concept or small volume productions. The use of connecting modular circuits together simplifies the task thus saving money and effort.
With the size of electronic components getting smaller and smaller, there are difficulties associated in using these components due to their size. Some of these components are so small they require special skills and equipment to use them. To add to this problem certain components are only manufactured in these small packages. This prohibits the use of these small components to the majority of people.
At present some component manufacturers still produce product in a variety of packages for the older components but the tendency is to change to the small packages as the market dictates. These larger packages can be easily used but these have the following disadvantages:- 1. Costly to manufacture.
2. Expensive to ship and store due to size and weight.
I Circuits produced with these components are large and therefore costly.
Most of the present production of electronic components is in surface mount packages which are difficult to use by students and developers. This system would alleviate this problem by supplying a method by which they can easily be used. There are a limited number of adaptor boards available but have size disadvantages and require additional circuits to be incorporated into, for example, a proto board.
Development boards supplied by component manufacturers are designed to prove the concept of a product and contain additional circuitry that is usually irrelevant to the required design, thus taking up unnecessary space and cost. These boards would be of more benefit if they could be made in functional modules or components and combined as required.
It is apparent that a number of circuit boards that can be connected together to form one circuit board would be advantageous to a number of people especially if they can produce their own designs which do not have any restraints due to existing circuits.
The practical implementations of these ideas present challenges.
These are explained below:-The requirement to connect boards of any size, shape, orientation or alignment together. This has two challenges, the first being a uniform pattern on the edge of the board that will allow alignment of pads irrespective of the arrangement or orientation of the boards. The problem arises with the pad at the corner of the board. This problem is easily illustrated with a photo shown in fig2. This is a photo of standard mats that can be placed side by side to form a larger mat. This works well until you try to connect them in a different arrangement from what was intended, the pattern does not allow for this. To correct this problem and still have the required strength in this connection the corner pads need to be offset to the centre of the board. This is of particular importance on very small boards where only the corner pads are used.
Illustration figure 5a and 5b shows that different boards of different sizes and orientation can be interconnected using the correct placement of pads.
Mechanical interlocking patterns have the disadvantage of being very expensive to implement, especially due to the high accuracy required because of rigidity of the boards.
The advantage of using pads placed along the edge of the board is low cost therefore making the mechanical method uneconomical to use for a low cost system.
The other challenge is in the design of the interconnecting pads. Standard pads do not have the required bonding strength with the circuit board substrate or strength in the copper track.
This is illustrated in photo figure 3 where it can be seen that the pad will easily break away from the board even if there are tracks connected to the pad.
The way to overcome the problem is to place a plated through-hole through the pad as this does provide mechanical strength via the plated hole, but as standard pads where the hole is placed in the centre of the pad a different problem arises where the through-hole plating is pulled through the side of the board making this an impractical solution. This is illustrated ip photo figure 4.
To overcome these problems the design of the pad and its associated plated through-hole is very important. The pad must reach the edge of the board to allow easy connection to the adjacent board via a solder joint or wire link. The exact position for the plated through-hole is critical with regard to its placement from the edge of the board. The pad will easily become disconnected from the board. If the plated through-hole is far from the edge of the board and if the plated through-hole is close to the edge of the board, the plated through-hole and pad will easily become disconnected from the boards.
It can be seen that the design of the interconnecting pads with the associated plated through-hole is an important part of the design feature and critical to the success of this type of assembly.
To understand the importance of the design of the interconnecting pads we need to consider the forces that can be applied to the interconnection. For example if we joined two 100mm wide boards together, fixed the two side and applied a force at right angles to the connection between the boards, the force exerted on the interconnection would be the width of the board divided by the thickness multiplied by 2. A standard board is approximately 1.6mm thick using the formula below.
F = length/thickness x 2 x applied force Using a force of 1Kg we get Force = 100 x 2 xl/1.6 Force 125Kg With the need to be RoHS compliant we need to use lead free solders, these solder joints are stronger than the copper bonding of printed circuit boards thus requiring a special design for the interconnecting pads.
In practice the size of the plated through-hole would determine the distance from the edge of the board. The size of the hole to provide the strength required should be greater than 0.6mm, illustrated in drawing figure 1 Dimension A. The distance from the edge of the board to the edge of the hole shall be greater than 1.0mm but not greater than 2.5mm, illustrated in drawing figure 1 Dimension B. The pad size would be greater than these dimensions, illustrated in drawing figure 1 Dimension C and D. For example plated through-hole size of 0.7mm and the distance from the edge of the hole to the edge of the board, a distance of 1mm, the associated pad size for this combination would be greater than this typically 1mm x 1.5mm size or larger.
An example of a board connected at right angles to each other is illustrated in photo figure 5.
An example of a number of boards connected together using this interconnecting system is illustrated in photo Figure 6