PRINTED CIRCUIT BOARD ASSEMBLYThe invention relates to a printed circuit board assembly for the mounting of a transducer having an unwanted sensitivity to mechanical strain and in particular, but not exclusively, for the mounting of a pyroelectric transducerPrinted circuit board assemblies are known for accommodating pyroelectric transducers within thermal imaging equipment. However, known arrangements have the disadvantage that they are bulky, due to the physical measures taken to minimise unwanted strain on the transducer.
A typical prior-art printed circuit board (PCB) assembly is shown in Figure 1. The PCB assembly 10 inFigure 1 is designed as part of a thermal imaging system (not illustrated) and incorporates a pyroelectric transducer 20. The transducer 20 has a sensing area 22 upon which thermal radiation impinges and the transducer delivers, at terminals 24, electrical signals the amplitude of which is dependent on the amount of thermal radiation sensed by the transducer at particular points on its sensing area 22. A ribbon cable 40 at each end of the transducer 20 is connected to the PCB and carries the signals to the rest of the thermal imaging system by means of tracks (not shown) on either or both of the top and bottom surfaces 32 of the PCB.
The PCB itself is of an epoxy glass construction, which may or may not be multi-layer.
The type of pyroelectric transducer used, while producing an electrical output at the terminals 24 in response to heat radiation, also, unfortunately, produces an electrical output when the transducer is subject to mechanical strain. In this respect the transducer can be said to be a piezoelectric device. Because of this, measures have to be taken to ensure that the PCB 30 is kept as stiff as possible, especially in the region of the transducer 20. This prior art assembly achieves this by means of a solid alumina block 35, approximately 12 mm high, mounted between the transducer 20 and the PCB 30 and bonded to both surfaces.Now if the PCB flexes, either due to temperature expansion effects, or due to distortions in a mounting frame to which the PCB assembly is attached being communicated to the PCB via the mounting points, the area of the PCB directly underneath the transducer maintains a reasonable degree of stiffness, thereby reducing the risk that wanted signals relating to a viewed image will be significantly impaired by the presence of unwanted strain-produced signals.
while the known PCB assembly provides a reasonable degree of compensation for such strain-produced signals, it nevertheless has the disadvantage that it is bulky due to the large, stiffening alumina block, and provides insufficient immunity from bowing of the PCB due to vibrations transmitted from the mounting frame to which thePCB is attached. This problem is all the more acute when state-of-the-art pyroelectric transducers are used, which can output significant unwanted electrical signals when they are subject to very small levels of mechanical strain.
Indeed, in modern devices for use with very low levels of incident thermal radiation, a strain of only 1 part in 109 on the transducer will appear as an electrical signal at the transducer output of the order of only 20 dB below a wanted, thermally produced signal.
It is an aim of the present invention to provide a printed circuit board arrangement for the mounting of a transducer having unwanted strain sensitivity, which overcomes the above-mentioned disadvantage.
According to a first aspect of the invention, there is provided a printed circuit board assembly comprising a printed circuit board and a transducer, a surface of which is mounted substantially flush to a surface of the printed circuit board and which has an unwanted sensitivity to mechanical strain, wherein the printed circuit board consists of a plurality of layers, the material of at least one inner layer having a density less than that of layers on either side of it.
The layers on either side of the at least one inner layer may be directly adjacent to that inner layer.
The advantage of this arrangement is that by means of a low-density inner layer the resonant frequency of vibration of the PCB may be raised.
The inner layer may be composed of a foam, in particular a rigid foam, and the layers either side of the inner layer are preferably composed of epoxy carbon fibre.
In accordance with a second aspect of the invention there is provided a printed circuit board assembly comprising a printed circuit board and a transducer, a surface of which is mounted substantially flush to a surface of the printed circuit board and which has an unwanted sensitivity to mechanical strain, wherein the printed circuit board consists of a plurality of layers, the layers being so arranged that changes in temperature produce substantially zero bending moment in the printed circuit board in a plane perpendicular to the major surfaces of the printed circuit board.
The preferred way of achieving this is to arrange the layers of the PCB symmetrically about a plane parallel to the major surfaces of the board and substantially midway through its thickness, so that each layer on one side of that plane consists of a material having substantially the same coefficient of expansion as the material of the corresponding layer on the other side of that plane and is of substantially the same thickness as the corresponding layer.
The advantage of this second aspect of the invention is that any temperature-induced bending moment introduced into the PCB due to unequal coefficients of expansion of the layers either side of the central plane of the PCB is minimised and therefore the production by the transducer of a strain-induced output dependent on temperature is significantly reduced. The lamination structure is said to be balanced about the central plane, and where there is an odd number of layers the lamination structure is balanced about the inner layer.
Preferably there are five layers: two outer layers, two stiffening layers bonded to these outer layers and an inner layer bonded to, and of lower density than, the two stiffening layers. While the outer layers may be constructed from any PCB substrate material (i.e. a polyimide), they are preferably made from epoxy glass.
Preferably, the stiffening layers are composed of epoxy carbon fibre and the inner layer is composed of a foam.
The PCB assembly may incorporate both the above two features of low-density inner layer and thermally balanced lamination structure, the advantages of both features then being obtained in combination.
Preferably, at least some of the layers are bonded together by means of an epoxy glass prepreg layer. Also present on the PCB will be conductive tracks disposed on either or both surfaces of one of the outermost layers, and the other outermost layer may also accommodate a conductive ground plane.
The transducer may either be surface-mounted to thePCB or it may be mounted by way of a thermal and electrical insulator interposed between the transducer and the PCB.
The insulator is preferably an epoxy glass layer and is kept thin to maintain as low a profile of the transducer above the PCB as possible, i.e. to keep the transducer substantially flush to the board.
In the main application envisaged by the invention the transducer is a pyroelectric detector array.
In accordance with a third aspect of the invention there is provided a mounting arrangement, comprising a printed circuit board assembly as described above and a mounting frame for mounting the printed circuit board assembly, the low density of the at least one inner layer of the printed circuit board of the printed circuit board assembly causing the natural mode of vibration of the printed circuit board to be higher in frequency than that of the mounting frame, the printed circuit board assembly being secured to the mounting frame at a plurality of points by compliant mounting means, such that the printed circuit board assembly is substantially isolated from distortions produced by the vibrations of the mounting frame.
A mounting arrangement in accordance with the third aspect of the invention has the advantage that vibrations generated in the mounting frame are largely decoupled from the PCB by the compliant mounting means; further to this, those vibrations that do reach the PCB do not have the effect of setting the PCB into resonance, since the invention ensures that the resonant frequency of vibration of the PCB is raised, by virtue of the low-density inner layer, above that of the mounting frame.
It is preferable that the printed circuit board assembly be compliantly secured to the mounting frame by means of elastomeric bushes, which bushes act as filters decoupling the distortions produced by the mounting frame vibrations from the printed circuit board assembly.
In the interest of minimising a DC offset strain in the PCB, it is advantageous for there to be three mounting points, such that the PCB may be secured to an uneven surface without experiencing strain.
A motor shaft may be arranged to protrude through the printed circuit board on the side of the board on which the transducer is mounted, the end of the shaft supporting a chopper vane for periodically covering and uncovering the outer face of the transducer. The compliant securing means is preferably arranged to prevent the chopper vane from coming into contact with the transducer. This measure has the advantage of protecting the expensive and fragile transducer from damage in the event that the chopper vane comes too close to the transducer, which may be caused by movement of the thermal imaging device inducing gyroscopic effects within the rotating chopper vane.
A mounting arrangement in accordance with the invention can advantageously be employed in a thermal imaging system.
One embodiment of the invention will now be described, by way of example only, with reference to the drawings, of which:Figure 1 is a pictorial view of a prior-art printed circuit board assembly incorporating a transducer having unwanted strain sensitivity;Figure 2 is a schematic side elevation of a printed circuit board used in a printed circuit board assembly according to a first aspect of the invention;Figure 3 is a side elevation of a theoretical printed circuit board, given by way of explanation only, to illustrate the concept of the first aspect of the invention.
Figure 4 is a side elevation of a preferred embodiment of the printed circuit board assembly according to first and second aspects of the invention;Figures 5A and 5B are diagrams showing the generation of thermally induced strain in a laminate due to lamination imbalance;Figure 6A is an underside view of a printed circuit board assembly in a mounting arrangement according to a third aspect of the invention;Figure 6B is a front elevation along a line X-X' ofFigure 6A;Figure 7 is an enlarged view of a compliant securing means illustrated at Y of Figure 6B, andFigure 8 is a schematic diagram of a thermal imager including a mounting arrangement according to the invention.
Referring to Figure 2, a PCB 60 consists of an epoxy carbon fibre layer 67 sandwiched between two epoxy glass layers 62, 74, the carbon fibre layer being arranged to be substantially thicker than the glass layers, such that thePCB is stiffened and is resistant to deformations.
In Figure 3 a conceptual modification to the PCB ofFigure 2 is shown, wherein, in order to raise the frequency of the resonant modes of vibration of the PCB, the density, and hence also the mass, of the PCB has been reduced by replacing the central portion of the carbon fibre layer 67 by a less dense core of rigid foam material. This has a negligible effect in practice on the strength of the board 60, since it is the outer layers 69 of the carbon fibre layer 67 that define the properties of that layer. The value of raising the frequency of the natural modes of vibration of the PCB is explained below.
In the embodiment illustrated in Figure 4, a practical implementation of the PCB assembly of Figure 3 is illustrated which has the same basic structure of the PCB, but with the main difference that the carbon fibre layer, instead of being "hollowed out" to accommodate the foam core, is divided into two parts, 66 and 70, which are in turn bonded to a foam layer 68. This arrangement is far easier to realise and achieves exactly the same effect of maintaining rigidity. Approximate thickness dimensions are given for the various layers and from these it can be seen that the PCB consists mainly of the foam 68. Despite this, the stiffness of the board is effectively the same as if the foam core were composed of carbon fibre, like the layers 66 and 70.Thus by replacing most of the carbon fibre layer by foam, the mass of the board has been considerably reduced, but without sacrificing strength. Also present in Figure 4 are two epoxy glass prepreg layers 64, 72. These act as a binding resin which bonds, with the application of heat and pressure, the epoxy glass layers 62, 74 to the epoxy carbon fibre layers 66, 70. In practice, however, any suitable form of bonding agent may be used.
The PCB assembly of Figure 4 also comprises copper tracking 76 for the mounting and connecting of components, most notably the pyroelectric transducer 20. This is mounted on the board via a thin insulating layer 80 of epoxy glass 0.1 mm thick. On the other face of the PCB is bonded a l-ounce copper ground plane 78.
In Figure 4 typical thicknesses for the various layers are shown. The outer epoxy glass layers 62, 74 are approximately 0.1 mm thick, as are the two epoxy glass prepreg layers 64, 72, while the carbon fibre layers 66, 70 are 0.3 mm thick and the foam core is approximately 2 mm thick. This makes the lamination structure symmetrical about the central foam core 68. This ensures that the PCB is not subject to bending moments due to unbalanced thermal expansion behaviour throughout the thickness of the PCB.
The principle behind the above thermal balancing is illustrated in Figure 5A, where a two-layer laminate 90 is shown clamped at one end 96 and consists of a layer 92 of a material A and a layer 94 of a material B. When, due to a temperature rise, expansion of the laminate takes place, if the coefficient of thermal expansion of the two materials is different, such that that of material A is higher than that of material B, the end 93 of the laminate 90 experiences a downward moment M, which bends the laminate in the direction shown. In the case where both materials are a metal, this is known as the bi-metallic strip effect.
To remove the effects of the bending moment M it is necessary to balance the structure and this is done by adding a third layer 98, also of material A, as shown inFigure 5B. This then gives rise to a second bending moment equal and opposite to that created by the unequal coefficients of expansion of the layers 92, 94. The net displacement of the laminate 90 at the end 93 due to a change in temperature is therefore zero.
In practice, the balancing of the expansion properties of the laminate can be achieved either by employing correspondingly similar materials either side of the central layer (e.g. material A in Figure 5) together with correspondingly equal thicknesses of those materials, or by employing dissimilar materials together with unequal thicknesses, the choice of material and thickness being such as to ensure that the expansion properties of, say, layer 92 in Figure 5B are the same as those of layer 98 in the sameFigure.
Figure 6A shows an embodiment of the PCB assembly, as previously described, in a mounting arrangement. A pyroelectric transducer 20 is mounted near the periphery of an essentially circular board 60, the edge 140 of the board nearest the transducer being extended to accommodate the transducer. The PCB assembly is mounted to a frame 150 (seeFigure 6B) via fixing holes 100. This frame forms part of a thermal imaging device (not shown). Also shown in Figure 6A is an area 130 of the board reserved for terminal connections to the rest of the thermal imaging device and two holes 110 and 120.The hole 110 is included to allow the shaft 162 of a chopper motor 160 (see Figure 6B) to protrude through the PCB above the height of the transducer 20 above the board, the shaft being attached to a chopper vane 164 which rotates with the shaft and has the effect of progressively covering and then uncovering the face 22 of the transducer as it rotates, such that it periodically interrupts the radiation incident on the transducer. The hole 120 is provided to allow the chopper motor 160 to be mounted in a different position, if a larger chopper vane or a chopper vane of a different shape is used.
The PCB assembly 50 is mounted to the frame 150, as shown in Figure 6B, via fixing members 102 passing through the fixing holes 100. The chopper motor 160 is also mounted to the mounting frame 150. The PCB assembly is not tightly mounted to the frame 150 but is compliantly mounted by elastomeric bushes 104, which in this embodiment are composed of rubber. The rubber bushes 104 substantially isolate the PCB assembly from distortions created in the mounting frame 150 by vibrations, especially vibrations occurring at the natural resonance frequency of the frame 150. Such distortions, where they are allowed to reach the board, have the effect of creating unwanted strains in the board and hence also in the transducer 20. Thus the rubber bushes act as filters, attenuating the mounting frame vibrations before they reach the PCB assembly.The filtering effect of the rubber bushes 104 is, however, limited and some of the distortions will in practice reach the PCB. Due to the raised frequency of the PCB resonances referred to above, those distortions that do reach the PCB are not able to set the PCB into resonance.
From Figure 7 it is seen that the fixing member 102 is a shouldered fixing which passes through rubber bush 104 and is secured to the mounting frame 150. Thus the PCB is secured by the rubber bush to the mounting frame 150 and the shoulder 107 on the fixing ensures that when the latter is tightened the rubber bush is not unduly compressed and therefore stiffened..
The rubber bush 104 comprises two pieces, 105, 106, which are pressed lightly together when the fixing 102 is tightened, thus ensuring a firm but compliant seating for the PCB 60. Alternatively, the bush 104 may be in one piece, and this must then be squeezed through the hole 100 in the PCB 60.
Referring again now to Figure 6A, the PCB assembly is fixed by means of three fixing holes 100, each comprising an arrangement as illustrated in Figure 7. This ensures that no constant strain is introduced into the PCB by the forcing down of the PCB onto its fixing positions, as could be the case if more than three fixing points were used. The use of four or more points nearly always results in the board's having to be bent down at one point when it is being mounted to the mounting frame, since either the mounting frame will not be perfectly flat, or the PCB may be slightly warped, or both. Thus strain, which appears as a DC offset signal at the transducer outputs, is avoided.
The fixing members 102 of Figure 6B stand proud of the transducer 20 such that if the vane is deflected, it comes into contact with the fixing member and not the transducer.
Use of the mounting arrangement within a thermal imaging system is illustrated in Figure 8. Figure 8 shows a thermal imager 200 including the mounting arrangement described above, comprising a PCB 60 on which a detector 20 is mounted and a mounting frame 150 to which the PCB is secured via elastomeric bushes 104. The mounting frame 150 is, in turn, secured to the casing 220 of the imager.
Thermal radiation enters the imager 200 via the lens 210 and is directed to the transducer 20, which is in the form of a pyroelectric array, where it forms a thermal image. The thermal radiation to the transducer 20 is interrupted by the chopper blade 164 driven by the motor 160 attached to the mounting frame 150. signals from the transducer array 20 are processed either on or off the PCB 60 and are output by the imager along a cable 230 to an external image-forming means, e.g. a TV monitor.