functional laser trimmingofautomotive electronics

Electrocomponent Science and Technology, 1981, Vol. 8, pp. 45-460305-3091/81/0802-0045 $06.50/0

(C) 1981 Gordon and Breach Science Publishers, Inc.Printed in Great Britain

FUNCTIONAL LASER TRIMMING OF AUTOMOTIVEELECTRONICS

JUNJI MITOBE and CHRIS DAVISON

Teradyne K. K., Meguro-ku, Tokyo 153

This paper discusses the use of laser trim systems for the trimming of automotive hybrids. Being high volume partsthese devices are best trimmed in two stages; first, a passive trim of the resistor substrate and secondly, a functionaltrim. The requirements for such a functional trim are such as to demand a fully integrated system.

1. INTRODUCTION

The use of lasers to adjust the performance of hybridcircuits has added a new dimension to the electronicsindustry and created a new production technology. Nolonger is hybrid technology reserved to the laboratoryor small run production. Hybrid construction can bechosen on its own merits for such high volume productsas automotive ignition modules and telephone activefilters.With the use of laser trimming systems for these high

volume parts it is necessary to consider issues such asproductivity and reliability in addition to the perfor-mance of the test system instrumentation and the laseritself. As an example of functional laser trimming andits use in high volume production we have chosen totalk about automotive modules but we could equallywell have chosen active filters or d/a converters asother examples of extremely high volume parts.

In recent years, the hybrid circuit technique has beenused in manufacturing electronic modules for theautomobile that cover the range from voltage regu-lators and electronic ignitions to various sensors.Given the __+20% tolerances usually associated withthick-film resistors, some sort of laser trimming isessential in order to meet performance goals.

2. SEPARATE PASSIVE AND FUNCTIONALTRIMMING

The most economical way to trim automotive electronicmodules is to first passively trim the resistors on a gen-eral purpose resistor-trim system and then actively trimthem on a system specifically designed to recreate the

45

actual automotive environment. Using two separatesystems retains the high throughput advantages ofpassive trimming and permits more reliable activetrimming.The Teradyne W411 high-volume passive trim sys-

tem and the A300 solid-state ignition adjust systemoffer the necessary capabilities for passive and activeautomotive trimming needs. The W411 utilizes a high-speed galvanometer beam positioner, a high-powerYAG laser, a 7 7 in. step-and-repeat carrier, andspecialized software to provide the throughput andreliability required for passive trimming.The A300 configured for active trimming of ignition

modules, simulates actual operation in an automotiveenvironment so that the trim is relevant. For example,specialized high-current and high-voltage sources sup-ply the required currents and voltages, and detectorsmeasure peak voltages and currents. A waveformgenerator allows users to program any waveform intothe device under test, and a universal interface permitsuser to interchange loading coils, without systemmodification.A system such as the W411 first passively trims most

of the resistors on a substrate to bring them withinacceptable tolerances. Following this the remainder ofthe components such as capacitors, discrete diodes, andany silicon integrated circuits, are added to the device.The silicon integrated circuits are usually mounted in aflip-chip manner, again allowing freedom from vibra-tion effects.At this point, a system such as the A300 makes a

final active trim to bring the unit into final specification.In the case of the voltage regulator, for example, anactive trim is made for the final regulated voltage. Dur-ing the past four or five years electronic voltage regu-

46 J. MITOBE AND C. DAVISON

lators have appeared in virtually all cars in the UnitedStates, Japan, and Europe.

In the case of sensors such as manifold pressure, air-flow, or temperature, the final active trim is made on thedevice in order to calibrate the output of the devicewith respect to the parameter being sensed. The use ofsuch sensors in automobiles is just beginning to comeinto widespread use spurred on by the need for central-ized engine processors to handle such functions as fuelmetering, pollution control, and the like.

3. LASER TRIMMING OF IGNITION CIRCUITS

Trimming ignition modules is a bit more complex. Inthe current version of electronic ignitions, the contactpoints are replaced by a Darlington transistor that cantake the on-current of the coil (about 7 A) and can alsowithstand the inductive kick when the current is turnedoff at spark time. The combination of 7 A and up to400 V, in addition to the very high levels of reliabilityrequired, added to the heat and vibration present insidean automotive engine, tend to impose severe require-ments on these output transistors. In order to limit theenergy that the device must switch, a means of limitingthe current in the output device must be found.

This is accomplished by having the electronic moduleregulate the maximum current at some preset valueusually in the range of 5 to 7 A. An active trim is made

on the ignition device in order to ensure the correctcurrent. Another requirement in addition to limitingthe maximum energy that the device must switch islimiting the peak voltage to which the device is subjectwhen the ignition coil is shut off, as this voltage caneasily reach 500 or 600 V. While it is possible to limitthis voltage through clever design of the ignition coil, amore common technique is to employ a feedback clampto the base of the output transistor. A second activetrim is thus required to ensure that this peak voltagedoes not rise above allowable limits, usually in theneighbourhood of about 400 V.A further problem with ignition devices occurs

because most inductive or Hall-effect type pickups .pro-duce an ON signal that is a constant "dwell" or con-stant percentage of an engine revolution. If this signal isused directly to switch the output transistor, at lowspeeds the device would spend most of its time in cur-rent limit, thus dangerously raising its power dissi-

pation. A variety of techniques are used to changethe constant dwell of a pickup device into an ONsignal which is of constant-time duration. For some ofthe latest ignition devices, an additional trim is re-

quired to ensure that the ON time specifications aremet.The laser has proved indispensable in manufacturing

low cost devices with the desired high precision. In thelast ten years, laser trimming has moved from labora-tory curiosity to a reliable flexible production tool.

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