Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Circuit Board with an Electronic Component and a Method for Producing a
Connection between the Circuit Board and the Component
FIELD OF THE INVENTION
This invention relates to a circuit board with at least one electronic
component
and a method for producing a connection between the circuit board and each
component. The connection provides the mechanical fastening of the component
onto
to the circuit board as well as the electrical connection of the component to
the circuit
board.
BACKGROUND OF THE INVENTION
Modern electrical appliances, especially measuring instruments, normally have
at least one circuit board containing electronic components. These components
must
be mounted onto the circuit board mechanically and must be electrically
connected to
wires running to, in or on the circuit board. In order to reduce production
costs,
2o surface mountable components, so-called "Surface Mounted Devices," in short
"SMD" components, are preferably used. SMD components require no circuit board
holes for their assembly, rather their contacts are soldered directly onto
pads provided
on the circuit board. SMD components can be placed on the circuit board
mechanically using fitting machines.
Typically in a first step of the manufacturing process for connecting
components to a circuit board, soldering paste is applied to the circuit board
in all
places where components are later to be placed using a screen printing
process. Any
place where solder has been applied or where a soldering connection exists
after the
completion of this initial step is called a soldering point.
3o In the next step, the components are mechanically placed on the circuit
board.
The circuit board with the components is placed in a furnace. The soldering
process is
carried out in the furnace, for instance in a controlled protective inert-gas
atmosphere,
which cycles through a solder-specific temperature cycle.
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Alternatively the application of solder to the circuit board and the soldering
process can also be carried out using a soldering bath. Since the soldering
points are
located in between the circuit board and the components, the circuit board
must be
placed in the soldering bath in such a way that the components project into
the bath.
In order for the components not to fall into the bath, they are fixed to the
circuit board
with adhesive before they are placed in the soldering bath. The type of
adhesive used,
usually a liquid tin alloy, must be able to withstand the temperature of the
soldering
bath. Such adhesives are commercially available. In order for the adhesive to
adhere,
it first has to harden after being placed on the circuit board. For this
purpose, the
1o adhesives usually have to be heated. Thus, an additional step is required
before the
soldering process can be carried out, specifically the gluing on of the
components to
the circuit board. In this case, the adhesive serves exclusively for the
mechanical
fastening of the components.
Many appliances, such as measuring instruments for measuring pressure,
temperature, fluid levels, pressure or flow, are used under conditions in
which very
high temperatures can occur. Such applications include, for example, the
chemistry
industry and the food industry. Temperatures of more than 100 °C can be
used in the
food industry during the cleaning and sterilization processes.
One example of a measuring instrument is a fluid level limit switch. Fluid
level limit switches are commercially available, and are used to detect when a
fluid
limit has been reached, as protection against overflow, or as protection
against
running a pump dry. These limit switches may include a mechanical vibrating
structure extending into a receptacle. The mechanical structure is caused to
vibrate by
means of a piezoelectric element. The resonance frequency and/or the amplitude
of
the stimulated vibration is measured. Using this information it is determined
whether
the vibrating structure is vibrating freely or whether it is covered by a
fluid material.
A fluid level limit switch is described in CA Patent Appl. No. 2,281,455 filed
on Sep.
09, 1999, in which the vibrating structure extending into the receptacle has
two
vibrating rods which are connected to a membrane. A piezoelectric element is
3o mounted on the side of the membrane facing away from the vibrating rods.
The
piezoelectric element may, for example, be glued on the membrane. The
vibrating
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rods are excited into vibration through the membrane by the piezoelectric
element..
On the side of the membrane facing away from the vibrating rods, the
piezoelectric element is electrically and mechanically connected to a flexible
circuit
board. This circuit board contains electronic components, preferably SMD
components, in addition to the piezoelectric element. These SMD components are
for
instance elements restricting electricity and voltage, or parts of a signal
receiver
and/or signal processing circuit. Until now the circuit board was first
equipped with
SMD components in the manufacturing process, the SMD components were soldered,
and then the piezoelectric element was connected.
to In addition to the aforementioned example, there are a great number of
further
applications in which other electronic components besides SMD components must
be
placed on the same circuit board. In these applications it is desirable to
mechanically
connect all components, preferably in one process, and to connect these
components
electrically and mechanically with the circuit board in a mass production
process.
Soldered connections are only stable at low temperatures, for instance
temperatures that lie far below 150° C. At temperatures above a solder-
specific
maximum level, the solder becomes soft and the mechanical and electrical
connection
is no longer guaranteed. When using piezoelectric elements, this problem
becomes
evident at lower temperatures as compared to conventional SMD components. This
is
2o due to the mechanical vibrations of the piezoelectric element, since the
connection is
exposed to mechanical stress.
There are adhesives available on the market which make possible connections
that still guarantee reliable mechanical connections at higher temperatures,
for
instance at temperatures of about 150 °C. Some of these commercially
available
adhesives are electrically conductive, so that there is also an electric
connection when
they are used.
These adhesives are normally very watery and must set at the desired gluing
point before they initially become sticky, and then finally completely harden,
and
adhere. These adhesives are therefore not suitable for fast mass production.
3o Transportation and/or the processing of an assembly unit consisting of a
circuit board
and at least one electric component is not possible until the adhesive has
hardened
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enough for it to at least become sticky.
One objective of the present invention is to provide a circuit board with at
least one electric component, and to provide a method for the manufacture of a
connection between the circuit board and the component so that fast mechanical
mass
production is possible and the circuit board can still be used at higher
temperatures.
SUMMARY OF THE INVENTION
to The invention includes a circuit board containing at least one electronic
component. The component has at least two electrical contacts, of which the
first is
initially glued to the circuit board using a conductive adhesive and the
second of
which is connected to the circuit board by soldering using a solder.
In accordance with one embodiment of the present invention, at least one
15 component is a piezoelectric element.
In accordance with another embodiment of the present invention, the solder is
free from lead and contains, in part, silver.
In accordance with another embodiment, the solder is a tin-silver solder.
In accordance with another embodiment, the circuit board is a flexible circuit
2o board, especially one made from polyimide.
In accordance with another embodiment, the two contacts are connected to
copper plates on the circuit board.
In accordance with another embodiment, the copper plates contain a
nickel/gold connecting metal plating or a layer of tin.
Furthermore, the invention comprises a method for the manufacture of a
connection between a circuit board and at least one electronic component, in
which:
- solder is applied to soldering points and a conductive adhesive is applied
to adhesive
points, spatially separated from each other, on each surface of the circuit
board where
a component is to be placed,
- the components are placed on the appropriate surfaces,
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-the circuit board with the components is placed in a
furnace and
-the furnace is cycled through a temperature cycle, which
results in a solder connection between the component part
5 and the circuit board, and through which the adhesive
develops its adhesive effect.
According to a further feature of the method, the
components are SMD components or components on which all
contacts to be connected to a circuit board are in one plane
on one side of the component and which form flat surfaces.
The components are placed on the circuit board mechanically.
According to one embodiment of the method, the
solder is applied mechanically using a screen printing
process and the adhesive is applied using a dispenser.
A broad aspect of the invention provides a method
for the manufacture of a connection between a circuit board
and at least one electronic component, in which: solder is
applied to soldering points and a conductive adhesive is
applied to adhesive points, spatially separated from each
other, on each surface of the circuit board where a
component is to be placed, the components are placed on the
appropriate surfaces, the circuit board with the components
is placed in a furnace and the furnace is cycled through a
temperature cycle, which results in a solder connection
between the component part and the circuit board, and
through which the adhesive develops its adhesive effect.
BRIEF DESCRTPTION OF THE DRAWINGS
The invention and further advantages are now
explained in more detail with reference to the figures in
which two embodiments of the present invention are shown.
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5a
In these figures, the same elements have the same reference
numbers.
Fig. 1 shows a diagram of a circuit board with an
SMD component and
Fig. 2 shows a diagram of a circuit board with a
piezoelectric element.
While the invention is susceptible to various
modifications and alternative forms, exemplary embodiments
thereof have been shown by way of example in the drawings
and will herein be described in detail. It should be
understood, however, that there is no intent to limit the
invention to the particular forms disclosed, but on the
contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the spirit and
scope of the invention as defined by the appended claims.
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DETAILED DESCRIPTION OF THE INVENTION
Figures 1 and 2 each show a circuit board 1 with an electronic component. In
the example shown in Fig. 1, the component 3 is a conventional SMD component
and
in the example shown in Fig. 2 the component part is a piezoelectric element
5.
The circuit board 1 can be a commercially available stiff circuit board, for
instance one made from insulation with an epoxide resin as a base, or a
flexible circuit
board, for instance made from polyimide. Strip conductors on the circuit board
usually consist of copper and have copper plates 7 where a connection is to be
made.
There can be a connecting metal plating 9 on the copper plate 7 as shown in
Fig. 1. The connecting metal plating 9 could be, for example, a nickel layer
9a and a
gold layer 9b. The nickel is applied using the known galvanic method and the
gold is
applied chemically. In this embodiment, the nickel layer 9a is disposed
between the
copper plate 7 and the gold layer 9b. The nickel layer in this embodiment
serves as a
diffusion barrier and prevents the migration of the copper into the gold layer
9b. This
type of surface improvement is frequently used in soldering connections. Since
the
gold layer 9b has a very smooth and even surface, it is also very well suited
for
gluing.
As an alternative, the soldering can be improved through hot air tin plating,
as
2o shown in Fig. 2. In this case, the tin is applied to the surface of the
copper plate 7 in
an immersion bath and is blown off by means of hot air. The liquid tin forms a
tin
layer 11 having a very flat surface which is advantageous for the soldering
process
that follows. The process is also known as Hot Air Leveling (HAL). In this
case as
well, the flat surface is advantageous for gluing.
Both types of surface preparation, i.e., the connecting metal plating and the
hot
air tin plating, are therefore well suited for soldering and gluing. For this
reason the
soldering and adhesive points can be simultaneously processed. A
differentiation
between the two, which would mean additional expenses in production
engineering, is
not necessary.
3o In accordance with the present invention, each component has at least two
electric contacts. The SMD component 3 displayed in Fig. 1 has a first and a
second
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contact 13, 15. The contacts of commercially available SMD components consist
of a
tin-lead alloy.
The piezoelectric element 5 shown in Fig. 2 also has a first and second
contact
17, 19. These contacts are electrodes applied to the element. Two separate
electrodes
are shown here. However a single electrode can have contacts at several
positions,
which means one electrode can correspond to several contacts. The electrode
preferably consists of silver.
The circuit board 1 shown in Figs. 1 and 2 also each have a first and second
connection 21, 23, and 25, 27, respectively. The implementation of the
embodiment
to illustrated in Fig. 1 shows the first and the second connections 21, 23,
each of which
consists of a copper plate 7 and an applied connecting metal plating 9. The
implementation of the embodiment illustrated in Fig. 2 shows the first and the
second
connections 25, 27, each of which consists of a copper plate 7 and the tin
layer 11.
In accordance with the present invention, in the embodiment shown in Figs 1
and 2, the first contact 13, 17 are connected to the corresponding first
connections 21,
on the circuit board 1 by gluing 29 then with a conductive adhesive 29 and the
second contacts 15, 19 and connected to the corresponding second connections
23, 27
on the circuit board 1 with solder 31.
In the case of the SMD component in Fig. 1, the solder and adhesive touch the
2o contacts 13, 15 which consist of tin-lead, and in the case of the
piezoelectric element 5
in Fig. 2, the solder and adhesive touch the contacts 17, 19, which consist of
silver.
On a circuit board containing several components with contacts made up of
various materials, it is preferable that one single solder and one single
adhesive be
used, which work well together with the different materials.
On a circuit board which, for example, consists of one or more SMD
components and one or more piezoelectric elements, which are set up side by
side as
shown in Figures 1 and 2, a solder and an adhesive are preferably used, each
of which
work well with silver as well as with a tin-lead alloy. A particularly
suitable solder is
one that contains, in part, silver. Very good results are achieved using a tin-
silver
solder. The conductive adhesive also should preferably contain, in part,
silver. For
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instance, the adhesive Amicon CE 3511 produced by Grace Emerson Cuming can be
used. The silver part in the solder and adhesives prevents the silver in the
electrodes of
the piezoelectric element from leaking into the solder and the adhesive. The
silver part
therefore protects the connection between the silver electrodes and the
piezoelectric
elements.
The solder should preferably be free from lead. Lead has a low melting point
and as a result, stable connections can only be made using ferrous solder at
low
temperatures. A tin-lead solder has a melting point of approximately
180° C. And a
tin-silver solder has a melting point of approximately 220" C.
to The connection between the circuit board and at least one electronic
component shown in the two embodiments in Figs. 1 and 2 is produced in
accordance
with the present invention, in which solder is applied to the soldering points
and
adhesive is applied to adhesive points, spatially separate from each other, on
each
surface of the circuit board where a component is to be applied. The solder is
preferably applied mechanically using a screen printing process and the
adhesive is
applied using a dispenser.
In the next step, the components are positioned onto the corresponding
surfaces. The components used are preferably SMD components or components
which have similar characteristics for connection to a circuit board. If all
the
2o components have similar connection characteristics, they the can be
positioned onto
the circuit board mechanically in the same process. A machine for positioning
components on a circuit board should be used for this procedure.
A component can be handled as an SMD component if all contacts that are to
be connected to a circuit board are arranged in one plane on one side of a
component
and form flat surfaces.
It is possible to treat a piezoelectric element like an SMD component in a
production process. Thus, the mechanical application and the connection of the
circuit
board with SMD components and piezoelectric elements can take place in mass
production.
Because of the wet adhesion properties of the solder, all components are fixed
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onto the circuit board and, thereafter, the circuit board can immediately be
transported. This makes it possible to transport the circuit boards during the
entire
manufacturing process over conveyer belts. Vibrations, as they inevitably
occur
during this mode of transport, do not lead to a shifting or even a loss of
components.
Further processing steps can be carried out immediately, if necessary.
After this step, the circuit board with the components is placed in a furnace.
Because the wet adhesion properties of the solder, this can take place
directly
following the positioning and fixing of the components onto the circuit board
via a
conveyer belt. The furnace is cycled through a temperature cycle, which causes
an
to electrical and mechanical solder and adhesive connection between each
component
and the circuit board. Because of the heating, the solder melts and the
adhesive
hardens. Accordingly, the adhesive and soldering connections are carried out
simultaneously in a single operation.
The temperatures occurring in this process are not a disadvantage for
piezoelectric elements, but in fact are advantageous. Provided the
piezoelectric
elements are only connected to the circuit board in the described manner and
are not
mechanically tightly fastened, no noticeable depolarization of piezoelectric
elements
occurs at the temperatures necessary for the gluing and soldering process.
However,
there are sediment effects, which cause an artificial aging of the
piezoelectric
2o elements. Signs of aging or slackening can therefore be found immediately
following
the manufacturing process. However, these effects can be taken into
consideration
through an adjustment of the circuit in which such a piezoelectric element is
contained
or with the initial calibration of an appliance in which the circuit board is
placed.
The remaining free surfaces of the piezoelectric elements are cleansed using
heat. This can be an advantage, for example, when such a piezoelectric element
is to
be adhered to the circuit board on site. Adhesives can adhere better on clean
surfaces.
Tests have shown that the described connection produces outstanding results
in terms of its electric and mechanical characteristics. Circuit boards with
3o interconnected components were exposed to cycles of extreme temperature
changes.
The transitional resistance of the connections was recorded to test the
electrical
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characteristics, and tearing and peeling tests were carried out to test the
mechanical
characteristics.
The tearing strength of the adhesive connections is clearly greater than that
of
the soldered connections. On the other hand, the soldered connections have a
higher
5 solidity than the adhesive connections when they are stressed for peeling,
i.e., when
normal stress is added. The connection between the components and circuit
boards is
therefore resistant to both types of mechanical stress. However, at higher
temperatures, the strength of the adhesive connections in terms of mechanical
stress
capacity increases with respect to the soldered connections.
1o Since the soldering connection protects the adhesive connection depending
on
the different mechanical stresses and vice versa, this connection clearly
holds longer
than the conventional connections described in the prior art.
The measurement of resistance showed that there was no discernible stress
related damage to the soldered and adhesive connections. This is at least in
part
attributable to the high mechanical stress capacity of the connection.
While the invention has been illustrated and described in detail in the
drawing
and foregoing description, such illustration and description is to be
considered as
exemplary and not restrictive in character, it being understood that only
exemplary
2o embodiments have been shown and described and that all changes and
modifications
that come within the spirit of the invetnion are desired to be protected.
