Note: Descriptions are shown in the official language in which they were submitted.
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A ROUND SOLDER PAD FOR SURFACE MOURTING ELECTRONIC DEVICES AND A
SURFACE MOUNTING POS~TION INCORPORATING SUCH SHAPED PADS
This invention relates generally to solder pads, for
surface mounted devices, and more particularly relates to a specific
shaping and slzing of such pads. A feature of the invent-ion is the
ability to hold devices in place, on or under the surface of a circuit
board, duriny soldering of further devices on the obverse of the
circuit board.
~ackground of the Invention
A very large proportion of electronic devices are
surface mounted on circuit boards. In such a process, a circuit
pattern is defined on a circuit board, usually in copper, the copper
pattern being afterwards tinned, to assist soldering amongst other
reasons~ Such circuit boards are generally referred to as printed
circuit boards (PCB). In the circuit pattern, lands (or pads) are
defined, to which devices are to be soldered. The land sizes, and
spacing, depend generally on the size of device to be mounted. Two
lands may be provided for a device which is to be connected at each
end. A patkern of lands may be provided ~For connection of leads, or
contact pads, of encapsulated devices.
For good electrical connectiorl it is desired that a
device be centered upon the lands. Centering is also desirable to
ensure satisfactory spacing of the devices, one to another. With
conventional forms of circuit pattern, the contact pads for devices
are usually rectangular. Although devices may be positioned so as to
be aligned correctly~ and are central on the contact pads, they can
move prior to soldering. Posltioning of the devices may also not be
accurate and m~salignment and o-ff-center posi-tioning occur,
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Soldering itself may also cause devices to move from the desired
position.
Prior patents which relate generally to soldering
systems and to which attention is directed are as follows: U.S.
patent 4,127,692 dated November 28, 1978 by K.G. Boynton; U.S. patent
4,39~,660 dated August 16, 1983 by T.R. Pampalone et al; and U.S.
patent 4,~45,114 dated February 24, 1987 by Jacques Van Den Brekel,
Carlyle W. Crothers, and Dale B. Squires.
Summarv of the Invention
The present invention is concerned with the specific
shaping of contact pads. By suitable shaping of a contact pad, a
device can be held in a desired position or moved to the desired
position is mispositioned, on the melting of the solder at the contact
pads, during soldering. Substantial improvement in alignment and
positioning is thereby obtained. Broadly, in accordance with the
present invention, a contact pad comprises a body having a deflned
shape and size; the shape being square with rounded corners and at the
limit being round and having a diameter of between 0.058 inches and
0.064 inches.
A further feature to which the present invention can
also be applied, is the holding in positio~ of devices on an
undersurface of a circuit board, during reflow soldering of further
devices on the obverse of the circuit board. By providing a
predetermined amsunt of solder, related to the weight of the device,
the device will stay in position on the undersurface even though the
soldering attaching it is molten.
Stated in other terms, the present invention is a
solder pad on a printed circuit board for the mounting by soldering of
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surface mounted devices to a printed circuit board to thereby
establish both electrical contact to, and mechanical connection to,
the printed circuit board, the solder pad being characterized by being
approximately circular in shape.
Stated in yet other terms, the present invention is a
solder mounting position both for mechanically attaching a surface
mounted electronic component to a printed circuit board, and for
establishing electrical conta~t to conductors on said printed circuit
board, each said position characterized by: a plurality of solder
pads spaced in a predetermined pattern to facilitate alignment of
surface mounted components, each said solder pad being defined by an
approximately circular shape; wherein said plurality of solder pads
are three ;n number and positioned so as to form approx;mately an
equilateral triangle.
Brief Description of the Drawinqs
The invention w;ll be readily understood by the
following description of certain embodiments, by way of example, in
conjunction with the accompanying diagrammatic drawings, in wh;ch:
Figure 1 is a plan view of a surface mounted device;
Figure 2 is an elevational view of the device of Figure
1, taken along the section lines 2-2 of Figure l;
Figure 3 is an end view of the device of Figure 1,
taken along the section lines 3-3 of Figure l; and
Figure 4 is a plan view of the device of Figure 1,
additionally depicting solder pads for the mounting of the device.
Detailed DescriPtion
Figures 1, 2 and 3 illustrate a typical electronic
device or component 10, which is rectangular in plan form, usually
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elongate, and is usually rectangular in cross-section viewed from an
end. Component 10 has three Formed leads 11 protruding from the
longer sides, such that there are two leads 11 on one side and one
lead 11 on the other, midway between the two leads 11 on the opposite
side. One example of component 10 is a small outline transistor (SOT)
3a
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such as a model MM~T390~ rnanufactured by Motorola.
Figure 4 illustrates a typical mounting position having
three contact pads 12 spaced apart. Each pad 12 has a circular shape
with a diameter of approximately 0.062 inches. The tinned contact
5 portions of leads 11 of the device 10 are positioned on the round pads
12, as shown in Figure 4.
The action of the round contact pad 12 is to centralize
the leads 11 of the device or component 10 during soldering. The
process for the mounting of components is generally as follows,
although variations in the specific process can occur:
1) The circuit board has a circuit pattern defined on
one or both surfaces - this is done by conventional means, such as by
photolithographic etching;
2) The circuit pattern - generally of copper - is tin
plated, this can comprise several steps including a thin copper
plating before the tin plating, and is conventional;
3) Solder paste (flux and solder particles) is
applied, as by screen printing or dot transfer deposition,
4) After step 3, the circuit board has a pattern of
solder pads arranged in the form of an equilateral triangle, as in
Figure 4. A device 10 is then positioned on the circuit board at the
desired positions. The devices 10 are kept in place by the flux in
the deposited solder paste, the flux acting as an adhesive;
5) The solder paste is melted by heating the circuit
board by lnfrared radlation to fusion temperature - generally referred
to as reflow soldering - a conventional step or process.
Once the solder has melted, then the shape of a pad 12
acts to pull the leads 11, or contact point, o-f the device 10 into a
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central position. The solder on the pad, when molten, because of
surface tension, tends to move the related leads 11 of the device 10
into alignment w~th the center or focus of -the round pad 12. The
center, or focus, is generally on or close to the geometric center oF
the pad 12. The equilateral triangular positioning of the round pads
12 tends to centralize the device. Again, this occurs because of the
surface tension in the molten solder at the pads 12. The surface
tension will be equalized when each of the three leads 11 and hence
the device 10 itself is symmetrical relative to the focus of each
round pad 12.
The distance between the centres of round pads 12 -
(D and E in Figure 4) - is generally equal to the relative centre
positions of the leads 11~
A further feature of the invention is the ability to
hold devices in position while the board is reversed, that is with
surface mounted devices 10 on the undersurface, while reflow
soldering further devices on the top surface. The devices 10 on the
bottom surface are held in place, even though the solder is molten.
The disclosed pad arrangement is applicable to packages
of the form SOT-23 and T0236.
The size of the pads 12 is of course a factor of the
size of component 10 and its leads 11. The amount of solder paste
applied to the pads 12 is important; it has been found that the
preferred ratio of solder paste to pad area is 0.81 milligrams to
~5 1 square millimeter. The preferred ratio of the solder paste is
99~1 parts solder to 0.1 parts flux by weight. One example of a
commercial solder paste suitable for this application is part No.
255-EM manufactured by Qualitek.
