The complexity of electronic systems has seen a continuing increase in the past years.
This tendency will continue in the future, and it will only be possible to master such developments by applying highly integrated circuits. The call for small dimensions for all system components places further exacting demands on the hardware development process.
Efficient and cost-effective hardware development is now only possible by using standard modules that are produced in large numbers.
Project starting point
A schematic diagram sets out all the func-tions that are to be performed by the ECU being developed. From that point, the fol-lowing aspects can be clarified:
definition of hardware required,
cost estimate for the hardware,
extent of development work required, and
cost of tools.
Hardware design prototypes
Once the project is underway, hardware de-sign prototypes are produced and subjected to quality tests. These design prototypes may be one of four categories representing suc-cessive stages along the road to the final pro-duction ECU. Each prototype category is based on its predecessor and is designed for a particular purpose in each case.
‘A’ prototype
The ‘A’ prototype is derived from an existing or modified ECU or a development circuit board. Its range of functions is limited. Its technical function is largely in place but the
‘A’ prototype is not suitable for continuous testing. It is a function prototype that is used for initial trials and to confirm the basic via-bility of the design.
‘B’ prototype
The ‘B’ prototype includes all circuit compo-nents. It is a trial prototype that is used to
test out the full range of functions and the technical requirements. At this stage it is ready for continuous testing in prototype vehicles.
The connection sizes and ECU dimen-sions are as required for final production. It may be, however, that not all of the vehicle manufacturer’s specifications are satisfied at this stage, perhaps because different materi-als have been used, for instance.
‘C’ prototype
The ‘C’ prototype is the approval prototype on which the vehicle manufacturer’s tests for “technical approval” are performed. This version of the ECU must reliably conform to all specifications. On successful approval of the product, the development process is complete. As far as possible, ‘C’ prototypes are produced using full-production tools and production methods as close as possible to full series production.
‘D’ prototype
The ‘D’ prototype is the pilot-series prototype which also carries the full-production iden-tification plate showing the version number.
‘D’ prototypes are fitted in pilot-series vehi-cles for large-scale vehicle trials. This version of the ECU is produced using volume-pro-duction methods and is fitted and tested un-der volume-production conditions. It is the version with which the reliability of produc-tion is verified.
Preparations
The preparations for the ‘B’ prototype start from the beginning of the project and in-volve
definition of connector pin assignment,
definition of the casing design,
ordering and developing new circuit modules (function groups),
producing the circuit diagram,
defining the components (only approved components may be included, or inclu-sion must be subject to approval).
When selecting the circuit modules (e.g.
knock-sensor analyser circuits integrated in an IC), developers check whether existing circuits – with modifications if necessary – can be used. If not, new modules have to be developed.
Circuit diagram and components list A CAD (Computer-Aided Design) system is used to create the circuit diagram (Fig. 1 a) and the list of all components used. The components list also details the following information for each component:
its size,
its pin assignment,
its casing design, and
the supplier and terms of supply.
Layout
A circuit-board layout (Fig. 1 b) is required for production of the circuit board. It shows the positions of the conductor tracks and the connector pins.
The layout is produced on a CAD system.
The process starts by taking the circuit-dia-gram data and converting it. The compo-nent connection schedule (list of connec-tions between components) then produced shows how the various components are con-nected to each other. From this component connection schedule and the CAD data for the components (size and connector pin as-signment) the layout can then be produced.
Fig. 1
a Circuit diagram b Layout c Circuit-board
production d Prototype
construction e Testing
GND GND
GND
IC7
VCC
+ VCC VCC
1 IC7A 2 3
IC7C 8 9 10 4 IC7B
5 6
IC7D 11 12 13
GND D1
C10 R2 S2
C7
GND
a
b
c
d
e
Hardware development process
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NAE0912YThere are specific criteria that have to be taken into account in the creation of the layout. In connection with the positioning of components these can include the following considerations:
the power loss from specific components (possibilities for heat dissipation),
EMC (electromagnetic compatibility) effects,
convenient positioning of components in relation to connectors,
observance of restriction zones (component size),
ease of fitting of components by auto-mated production machinery,
accessibility of testing points, and
space requirements for testing adaptors.
Circuit board
The layout data can be used to produce films for manufacture of the printed-circuit board (Fig. 1 c). The films are used to photographi-cally expose the blanks (which are covered with a light-sensitive coating) and then de-velop and etch them. The individual layers of multilayer printed-circuit boards are placed on top of one another and hardened.
Finally, the component print, the solder resist and a carbon lacquer are applied to the circuit board.
Prototype construction
The finished circuit board has to have the components mounted on it (Fig. 1 d). In the case of prototype products, this is part of the process of prototype manufacture. Because of the miniaturisation of components and the high degree of integration on the printed circuit, even prototypes have to have their components fitted by machine. The machine is controlled by the CAD layout data.
Following fitting of the components, their connections are soldered. There are two alternative soldering methods:
wave soldering and
reflow soldering.
Testing the finished circuit board Electrical testing
Once all components have been fitted and all connections soldered, the circuit board must be tested. To this end, electrical testing sequences are defined which run on a com-puter. These automatic tests check that all components are fitted and that the circuit functions properly.
Thermographics
Thermographic images of the printed circuit show the heat generated by the components during operation (Fig. 2). Different temper-ature ranges appear on the film as different colours. In this way, components that are too hot can be identified. The information obtained is incorporated in the list of modi-fications between the ‘B’ and ‘C’ prototypes.
Changes to the circuit layout (e.g. heat-dissi-pation through-contacting) can reduce the amount of heat generated.
Fig. 2
Engine control unit Power supply voltage U = 14 V
Idling speed n = 1000 rpm
62°C 58°C 63°C 71°C
Thermographic image of a printed-circuit board
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SAE0910YElectromagnetic testing
The electromagnetic fields created on the circuit board can be scanned with a mag-netic field detector (Fig. 1 e). The readings are analysed on a PC. Different field strengths show up as different colours. If necessary, modifications to the circuit layout have to be made and additional components fitted that reduce the magnetic field or make the ECU immune to interference.
These tests are performed on the ‘B’ proto-type so that the necessary modifications can be incorporated in the ‘C’ prototype.
EMC tests
Readings taken in the EMC testing cell or testing chamber (Fig. 3) test the ECU re-sponse to external and internal sources of electromagnetic interference. Measurements are taken both with the ECU fitted in the vehicle (in-situ tests) and in the laboratory (e. g. stripline method).
The difficulty with the in-situ tests is that they cannot be carried out until develop-ment of the vehicle and all its electronic sys-tems is at a very advanced stage. If the EMC characteristics are found to be unsatisfactory at this stage, the scope for modifications is then severely limited. For this reason, labora-tory testing is extremely important because it permits potential problems to be identified on the hardware prototypes at an early stage in the process.
The EMC readings are taken at a variety of frequencies and with a range of electric field strengths. The immunity of the output nals (e.g. ignition signals, fuel injection sig-nals) to external interference is examined as well as the level of interference generated by the unit itself.
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3 1
Vehicle in the EMC testing chamber
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SAE0911YThe manufacture of microelectronic com-ponents and circuits demands ultra-high precision as well as extremely high stan-dards of cleanliness in the various stages of production. Even the minutest speck of dirt can irreparably damage a circuit.