GB2239739A

GB2239739A - Electronic circuit assemblies

Info

Publication number: GB2239739A
Application number: GB8925958A
Authority: GB
Inventors: Graham Leonard Adams; Mak Nagi
Original assignee: Motorola GmbH
Current assignee: Motorola Solutions Germany GmbH
Priority date: 1989-11-16
Filing date: 1989-11-16
Publication date: 1991-07-10
Also published as: GB8925958D0

Abstract

In a circuit assembly 10 a relatively rigid carrier member 11 formed of aluminium supports relatively massive circuit components such as transformer 12, power transistor 14, connector 15 and pressure sensor 16. Relatively less massive components such as surface mount integrated circuit 18, surface mount resister 19 and leaded capacitor 100 are supported by a printed circuit board 17 to provide interconnection between the components. The circuit board 17 is itself secured to the carrier member through the agency of fasteners 101. <IMAGE>

Description

Improvements in or relating to Circuit Assemblies The present invention relates to circuit assemblies, and in particular to circuit assemblies including a printed circuit board component.

In order to provide a useful electronic circuit, a plurality of components must be assembled together. A typical circuit might comprise some active components (for example integrated circuits, transistors) some passive components (for example resistors, capacitors) some power components (for example large transistors, thyristors with their associated heatsinks) and some wound components (for example inductors, transformers) together with some interface components such as connectors, transducers and relays. Typically components are assembled together with a component providing the interconnections between other components, such as a printed circuit board. In many arrangements, the printed circuit board provides not only the interconnections but also support for other components acting as a carrier which is itself assembled into a supporting frame or enclosing case.

Presently there is a wide range of component types which may be required in a circuit, from relatively large transformers, connectors and heating generating power devices to relatively small passive components, small signal active devices and integrated circuits, particularly if the latter are of the surface mount type.

A noticeable trend in circuit assemblies is the significant "mix of many present circuits, that is the wide range of components of different types making up a single circuit assembly.

In assembly methodology, there is automation and parallelism. Typically starting with the printed circuit board component, other components are temporarily attached thereto either by adhesive (for surface mount devices) or by mechanical engagement before being permanently fixed and connected by soldering, which might be a flow soldering (passing theboard through a wave of molten solder) or a reflow (melting prepositioned solder paste) operation. The initial placement of components may be automated (pick-and-place machinery), semi-automated (component supply and position indicating equipment) or manual.

Clearly, the assembly task is likely to become more difficult as components mix increases. There is also a trend to integration, that is providing circuit assemblies that may themselves be considered as a functional components of a larger assembly, leading to increased circuit complexity and mix.

As for the printed circuit board component, strength and rigidity is often dictated by a relatively small number of large or bulky components, being a in excess of that required to interconnect and carry the relatively much larger number of small and light components. Even so, boards carrying massive components tend to buckle and warp during the heat intensive soldering operation in a random and unpredictable way which can vary greatly from one nominally identical assembly to another.

Hence, residual stiesses are present within circuit assemblies which can compromise reliability. This is particularly true when the circuit assembly must itself be mated with other components, notably a case for example.

Misalignment of connectors or transducers that must engage with the case due to warpage or shrinkage of the printed circuit board component can cause stresses on the soldered component joints when such components are forced into register. Power devices engaged with external heat sinkage are equally vulnerable In accordance with the present invention in a first aspect thereof, a circuit assembly includes a relatively rigid carrier member arranged to support relatively massive circuit components and adapted to be assembled with a circuit board component providing support for relatively less massive circuit components.

In accordance with the present invention in a second aspect thereof,a method of fabricating a circuit assembly including the steps of mounting relatively less massive circuit components on a circuit board, mounting relatively massive circuit components on a carrier member, engaging the carrier and the components mounted thereon with the circuit board components, and performing a soldering operation on said circuit to thereby effect its assembly Preferably the carrier member includes a heats ink portion and the relatively massive components are secured to the carrier member by means of a spring clip.

Advantageously, the circuit assembly is supported in a case structure by means of the carrier member.

In order that features and advantages of the present invention may be further appreciated, an example and embodiment, by way of example only will now be described, with reference to the accompanying diagrammatic drawings of which: Fig 1 represents a circuit assembly, Fig 2 represents an alternative circuit assembly, Fig 3 represents a detail of the arrangement of Fig 2, and Fig 4 represents an alternative embodiment of the present invention.

In a circuit assembly 10 (Figure 1) a relatively rigid carrier member 11 formed in aluminium supports relatively massive circuit components such as transformer 12 power transistor 14 connector 15 andpressure sensor 16.

Relatively less massive components such as surface mount integrated circuit Is, surface mount resistor 19 and leaded capacitor 100 are supported by a circuit board component 17 in the form of a printed circuit to provide interconnection between the components. The circuit board component l7 is itself secured to the carrier member through the agency of fasteners 101.

The circuit assembly is of the type that might be found for example in automotive engine control applications, having as it does a high component mix ranging from the large connector 15 and the remote sensing pressure transducer 16 to small leaded components such as resistor 100 and surface mount devices such as component 18.

The assembly of the components into an electrical circuit will now be considered in more detail.

Firstly, surface mount components such as resistor 22 and integrated circuit 27 (figure 2) are mounted on the upper surface of the printed circuit board 17. This may be an automated operation performed by a pick and place machine and in accordance with surface mount technology the components are initially held in place on printed circuit board component 17 by adhesive. Once components on the upper surface have been placed they are reflow soldered by passing the printed board component 17 through a soldering oven. The next operation is to mount components destined for the underside printed circuit board 17 such as surface mount component 21. These are mounted in place on the under side of the board and held by adhesive for example.

Leaded components such as resistor 28 may now be placed on the board, to be held by mechanical resistance. In parallel with the above operations the relatively massive components are mounted onto the carrier member 11. These components include remote sensing device 16 connectors 15 and power devices 14. Such devices are held in place on the carrier for example by the location of lugs such as lug 29 with cooperating holes in the relevant component, or by bolting the power devices such as transistor 14 to the support portion 103. The printed circuit board component 17 with the relatively less massive components already mounted thereon may now be introduced to the underside of the carrier member 11. In this operation the electrical connecting pins of the massive components are mated with their connection pads on the printed circuit board 17.Now the entire sub assembly may be flow soldered. During flow soldering the assembly may be supported by the carrier member itself.

Finally case components 23, 24 and 25 may be assembled around and to support the circuit assembly by means of the carrier member 11. The case may then be held together by means of fasteners 26.

As an alternative to mounting the power devices such as power device 14 to there supporting portion 103 by means of bolts, a spring clip 20, as may be more clearly seen in figure 3, may be employed. Mounting portion 103 has a position 30 adapted to locate the spring. Spring clip 20 also acts to damp the board 17 to the carrier by means of upwardly urging the board 17 in a direction 30.

The intimate contact between the power device 14 and the mounting position 103 enables this portion to act as a device heats ink. Heatsinks is thus formed integrally with the carrier member 11 and it will be appreciated that heats inks would otherwise constitute massive components to be supported by the board in prior out assemblies. Also from figure 3, details of an arrangement of case components can be seen, co-operating to give an environmentally sealed assembly.

From the foregoing descriptions a number of features of the present invention will be apparent. The relatively massive components are supported by the carrier member 11 and not by the board component 17, particularly during flow soldering, hence, warpage of the board due the weight of these components cannot occur. The components are soldered into position after having been fixed to a structural component and hence no subsequent assembly is required; this is particularly important since in prior art constructions warpage or shrinkage of the board 17 occurs and such components may need to be forced into register with their fixtures, placing a strain upon the soldered joints. The presents invention avoids this problem. The case construction is such that (as may be seen from figure 3) the carrier and the board are secured o the case as a single sub assembly.This provides a rigid and reliable construction, with the weight of the relatively massive components transmitted to the case via the carrier member and not by the printed circuit board. Thus one of a lighter gauge or flexible board may be permitted. Assembly of the printed circuit board 17 to the underside of the carrier member 11. Means that the carrier member need not enter a flow solder wave and may hence be made of a material such as aluminium which would otherwise contaminate the solder bath. The only metallic components to come into contact with the solder wave are the tracks of the printed circuit board, component leads, and if it is used, the spring clip 20 which secures power devices to their support portion 103 and which will be formed in non contaminating steel. Any surface mount components will be soldered in place during soldering by reflow of their solder paste applications. Thus soldering in a single pass for a single sided board is permitted. It will further be appreciated that the invention permits a substantially fastener less sealed construction suitable for use in a hostile environment, such as an automotive application.

In an alternative embodiment of the present invention (Fig 4), a carrier member 11 forms part of a case structure inconjuction with covers 40 and 41.

Claims

1. A circuit assembly including a relatively rigid carrier member arranged to support relatively massive circuit components and adapted to be assembled with a circuit board component providing support for relatively less massive circuit components.

2. a method of fabricating a circuit assembly including the steps of mounting relatively less massive circuit components on a circuit board, mounting relatively massive circuit components on a carrier member as claimed in claim 1, engaging the carrier and the components mounted thereon with the circuit board components, and performing a soldering operation on said circuit to thereby effect its assembly

3. A circuit assembly as claimed in claim 1 and wherein the carrier member includes a heatsink portion.

4. A circuit assembly as claimed in claim 1 or claim 3 and wherein the relatively massive components are secured to the carrier member by means of a spring clip.

5. a circuit assembly as claimed in claim 4 and wherein the spring further urges the board to retain it to the carrier member.

6. A circuit board assembly as claimed in any preceding claim and wherein the carrier member forms part of a case structure.

7. A circuit assembly as claimed in any preceding claim supported in a case structure by means of the carrier member.

8. A circuit assembly substantially as herein described with reference to the drawings.