IMPROmflS MADE IN ELECTRONIC PARTS DURING THE PAST TEN YEARS H. V. Noble

Wright Air Development Center Wright-Patterson Air Force Base, Ohio

Since other presentations of this confer- ence will cover the different electronic parts areas9 this paper is limited to the improve- ments made in the general areas of resistors, transmission lines, mechtronics and relays.

Changing environments along with the de- mand for improved performance, smaller size and longer life have been the motivating factors in bringing about electrcmnic part improvements. Briefly these environmental changes are listed in Figure 1.

It should be noted that there is a consid- erable spread in some of these conditions - as for example - temperature. This is because some electronic equiments are subjected to different environments than others and parts fulfilling these requirements may be so vastly different in size, cost and quantity requirements that it would be impracticable, for example, to use a 500°C component for all lower temperature appli- cations.

Resistors --

Fixed Resistors

Resistors are one of the most important components of electronic circuitry, since they make up approximately 70 per cent of the total components for most circuits.

The MIL (JAN) specifications dated prior to 1950 are a reasonabsly true reflection of the state of the art at that time. The character- istics of these units are shown in the "Early" columns of Figure 2.

Since that time much effort has been spent in searching for improved resistor mater- ials and fabrication techniques. The develop ment and applications of new materials, process- es, designs and techniques has resulted in a larger variety of resistors smaller in size, with increased reliability, greater stability, greater precision, little change under humidity, and capable of operating at higher temperatures for long periods. The increase in variety of resistors is reflected in "on the shelf" items such as general purpose film resistors, low temperature coefficient precision film resistors, film type precision potentiometers and trimmers. Many resistors have been developed suitable for new solid state circuitry as well as present day analog and digital computers, accelero- meters, etcetera.

The characteristics of these improved units are also shown under Yresent~~ in Figure 2 and represent types available now as standard items. For the case of more than one style

under the "Present" item column, the smallest item was selected since the :military is interest- ed in small size.

It should be noted that it is possible to operate resistors in higher ambient temperatures than indicated although it may be necessary to shorten the operating time or allow a larger change in resistance. For example, it was possi- ble to operate the early film type resisto:rs ful:L load for a long time period if a maz&num resist- ance change of 3 per cent could be tolerat'ed. Another example is the present molded accurate wirewound type. These resistors may be operated for an extended period of time of 1500 hours, if the circuit could tolerate a resistance change 0:' 1 per cent.

The fixed composition resistor has been the work horse of most electronic circuits probably because of the wide choice in size and resistance range (Figure 3). Recently several new types of fixed composition resistors have been made avail- able for military applications giving better performance under environment of humidity *and temperature. These resistors are constructed by sealing the molded composition resistor in a ceramic tube,

With the develoment of new encapsulating compounds, many new types of precision wirlswvund resistors have come into existence. Although there is not much difference in physical size between these types and the earlier varnished coated types, the encapsulated type possesses greater stability and long life under adve:rse conditions of high humidity and high ambient temperatures. Since the bobbin and encapsulation compound are generally of the same wterial or closely matched materials, there are less :internril strains, which yield greater stability ove:r a wide temperature range.

Considerable progress also has been made in the developent of various types of film re- sistors. These resistors are formed by de:posit- ing on the surface of a ceramic core a thin film of carbon by the pyrolytic deposition from a hydrocarbon gas such as methane. Sealed types are made by enclosing the resistive element in a metal, ceramic, or glass enclosure.

The noted features of the film resistors are smaller size and wider resistance rangls as compared to the wirewound types. They have ex- cellent resistance temperature characteristics and smaller percent resistance changes due to various operational and environmental tests as compared to the composition type. All charact- eristics of the film types have been developed or improved wer the past ten years.

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Figure 4 shows some of the present fixed film resistors. These resistors are either en- closed in ceramic or molding compound.

During the past several years, there have been great strides in metal film resistor devel- opnent. High stability, precision metal film resistors are being manufactured which in many applications can replace precision wirewound resistors. These units can be easily fabricated to temperature coefficients of resistance of less than 50 ppm/OC (as compared to 250 p~pn"C) for the earlier film types.

In the past great advancements have been made towards miniaturization. Figure 5 shows some of these types which are particularly adapt- able to transistor circuits.

In the area of fixed power wirewound resis- tors there have been great advances, also. Fixed power resistors are now being produced without the usual heavy ceramic core0 The resistance wire is wound on a low temperature form that is coated 1tit.h a special ceramic compound. After the winding is terminated and recoated with this compound, the resistor is then fired to cure the insulating material and to burn off the form. This makes for a lighter weight unit that is capable of operation in higher ambient tempera- tures.

Another class of fixed power wirewound resistors that are being frequently used by the military is the chassis mounted types. Figure 6 shows a few of these types. These resistors depend primarily on conduction for heat transfer rather than convection. The connnercial type shown has been widely used by the TV industries.

Variable Resistors

There have been similar improvements in variable resistors during the past decade,

General purpose non-kdrewound controls are characterized by Specifications JAN-R-94 and ML-R-94(superseding JAN-R-94), which were rated at 50°C in 1949. Most of the types presently available were in existence then. Improvement in this area has been evolutionary in nature, rather than revolutionary. Sufficient improve- ment in materials and construction had been made by 1954 to allow these controls to be rated 70°C (Figure 7).

There have been operational capability improvements effected in all areas since 1949, most notable of which is rotational life, which is now 25,000 cycles, five times what it was in 1949.

General purpose wirewound controls avail- able in production quantities throughout the period 1949 to 1958, covered by Specification NIL-R-19 and its predecessor JAN-R-19, were all rated at 40°C, and derated to zero wattage at 105oc. In 1954 the Signal Corps sponsored the

development of a general purpose wirewound con- trol rated at 85"C, and more recently an Air Force development has extended this to 140°C as shown in Figure 8.

Militaxy items now use almost wholly non- hygroscopic plastics or metals. Noble metals are being used in many of the wirewound types. With the use of these new materials, such as platinum- tungsten and platinum-ruthenium-rhodium, much better performance is possible under moisture conditions and use of finer wire sizes becomes practicable. F'resent capability of production without selection approaches 0.01 per cent lin- earity tolerance along with the capability of operating at higher temperatures (Figure 9).

Since 1949 plastic element potentiometers, metal film potentiometers, and carbon film potentiometers have become available and will provide capabilities not previously available in the wirewound types (Figure 10).

Transmission Lines

Under the present concept of transmitting direct and alternating current power by tire and cable and radio frequency energy by coaxial cable and waveguide, revolutionary improvements in transmission line components have been limited by deficiencies in conducting, dielectric, and insul- ating materials and basic physical laws. In the case of coaxial cables, for example, it is pass- ible to fabricate a semi-rigid or rigid, low loss cable by using a solid sheath outer conductor and a semi-air dielectric core, or a flexible, standard-loss cable by using a braided outer con- ductor and a solid dielectric core, but it is not possible to fabricate a cable with both low loss and flexibility. Flexibility and low loss are not presently compatible.

In spite of these "road blocks", consider- able improvement in transmission line components has been made. By using stainless steel and magnesium, the weights of waveguides have been reduced by the factor of 5 to 1 while maintaining the electrical characteristics of standard brass guide.

By changing from air to a solid dielectric of Teflon, a dielectric filled waveguide was developed-having a size approximately l/3 that of the standard equivalent brass guide. Due to bonding problems, a loss in high temperature characteristic was an undesirable result. The filled guide will withstand a temperature range of only -65% to +71"C; and because of the small size, there is a possibility of its use where temperatures are not beyond these limits.

Wave&de twists, both fixed and rotatable, have been developed having a reflection loss and a length of approximately l/3 of the old stand- ard brass twist as shown by Figure 11. More recently circular waveguide bends have been dev- eloped for the l'Xtt band frequency having radii of approximately one foot with the same loss as

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earlier bends with a :radii of several hundred feet,

Radio frequency coaxial cables have been improved as far as basic physical laws, concepts, fabrication techniqueis, and materials will allow, By using Teflon as a :jolid dielectric core and silicone impregnated fiber glass as a jacket, the operating temperature range of the cables has been increased from -65°C to +250°C as compared to the old polyethylene cables with a temperature range of -40°C to +85'C.

By using a solid copper or aluminum outer conductor and semi-air dielectric core9 rigid and semi-rigid, low loss cables with an attenua- tion of approximately one-half that of standard solid core cables have been developed.

By fabricating -the dielectric core from wrapped Teflon tape instead of solid Teflon, cables have been developed capable of withstand? ing thermal shock from -65°C to +25O"C without dimensional deformation.

Vast improvements have been made in RF coaxial connectors. The upper temperature limit has been raised from +lOO°C to approximately +25ooc; the msxlmum operating frequency has been raised from 3000 me/s to 10,000 me/s; clamping mechanisms have been improved both mechanically and electrically; satisfactory hermetic seals have been developed; and contacts have been cap- tivated to eliminate connector and cable separa- tion during thermal shock.

Pulse connectors have been made smaller, capable of withstanding a temperature range of -65% to +150°C instead of -40°C to +71°C, and with corona levels up to 7000 volts peak at 50,000 feet altitude, Figure 12 shows a size comparison of such a recently developed miniature pulse connector with one of the old standard connectors on the right. This miniature connec- tor has the same electrical performance and has an O.D. of 0.75'19 a length of 2.90", while the standard connector has an O.D. of l.Of', a length of 3.25".

Multi-contact (type AN) connectors have been similarly reduced in size and weight while retaining the same electrical characteristics and capable of operating at much higher temperatures. For example, a recently developed connector with 26 nonshielded contacts occupies a panel area of 1.0 square inch, weighs 1,76 ounces, is moisture proof end capable of operating in a temperature range of -65"~ to +260°C. It replaces an old AN connector rifich occupied a panel area of 4.0 square inches, weighed 8.6 ounces, was not mois- ture proof, and capable of operating in a temper- ature range of only -55°C to +G5'C.

Improvements in RF coaxial switches have been tenfold with respect to reliability, switch- ing time, size, and weight and all of this with the same power handling capacity. Figure 13 is a size comparison between a recently developed

solenoid driven miniature switch on the left and the old motor driven type on the right.

By the use of new insulation materials such as Teflon, Nylon, and new vinyls, hook-up wire and aircraft harness wire have been reduced.in wall thickness by as much as one half the limi-;- ing temperature raised from +90°C up to as high as +200°C (continuous) and +250°C (short time), By the use of polyester films end silicones, the limiting temperature of magnet wire has been raised from +85% to approximately +200°C, and the space factor reduced as much as twenty-five per cent.

Mechtronics

Mechtronics, as the word is used here, denotes the mechanical aspects of electronics. All electronics, whether large ar small, are always supported by some type of structure (tuse elements, resistors, relays, chassis, etc.) which oftentimes present stumbling blocks in efforts to produce the electronic functional objective. For instance, the active portion of a transistor is indeed small, but the supporting matrix contains by far the greatest portion of the volume, the leads for attachment, and captivating shells for structural strength introduce space compromises. Construction techniques of normal electronics and induced dynsmic stresses are frequent problem areas. Progress in this area has not been rapid, when compared with electronics in general; per- haps because it is considered a hardware area of uninteresting factors.

Design

Many programs have been carried out to define the amount of ruggedization needed for our modern military electronics. The results have indicated a gross variance in degree. Therefore, in order to cover all requirements, the standard policy is to design to the greatest amount of ruggedization. This is especially true in com- ponent parts due to the fact they x go into a drastic environment. Efforts to provide guidance to design engineers have produced a report entitled, 'Electronic Designers Shock snd Vibra- tion Guide for Airborne Electronics." This guide contains information on the environment, compon- ent parts, chassis construction, equipment construction, methods of isolating and testing techniques. It is the first guide, of this scope, on electronics to be disseminated.

Vibration Protection Devices

Advances in the area of vibration :protecl;- ive devices have been gratifying. In this past:, these items (mainly vibration isolators) were unreliable due to the use of organic materials with very little resonance control plus <a short life brought about by drift characteristics.

The primary advance has been in thee use of metal springs for load supporting functions; this in conjunction with friction devices for damping

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has produced a reliable device that till operate at elevated temperatures ard for long periods of time. Always, in the past, when introducing a large amount of damping, a considerable comprom- ise in protective value was involved. Recently as a result of research and develoment effort, a dual stage damping device was developed as shown in Figure I& The incorporation of the techniques of locking-in the large damping forces when small displacements are involved is the trick of decreasing the compromise.

Efforts are presently being conducted to develop a blanket or sandwich type material to replace the unit type vibration isolator. It is contemplated that with new type materials be- coming available an exploitation of their potential can result in a considerable space savings and increased performance. Some VISCO elastic materials, such as the silicone, fluor- inated and butyl rubbers, with internal damping properties are proving to be very promising. Figure 15 indicates one of the samples that has been originated and evaluated. This sample con- tains a silicone rubber-like matrix with imbedded layers of teflon rods. The teflon rods provide additional damping.

Dynamics

As previously stated, the environmental motions which electronics receive from structural attachments points on new type vehicles are quite varied. Progress in the last several years in defining the environment is quite good. It was not too long ago that one considered the vibra- tion environment as being steady sinusoidal motion with a varying amplitude; however, recent investigations of missile environment with its rocket type propulsion systems indicate that the environment is so unlike the steady-state motion that an entirely different approach may be just- ified. This has led to a statistical definition of the vibration with subsequent approaches to random motion. It is to be noted that some vibration engineers tend to express this pheno- mena as being a type of motion instead of the classical verbiage %ibration."

Sound Enerm Environment

Ten years ago the acoustical environment was unknown. Because of fatigue and malfunction difficulties in the newer airframe types, this phenomena has become an increasing headache. It appears to be an established fact that the pro- pulsion sources with their increased thrust are producing an acoustical energy that is creating the basis of most vibration problems. Elec- tronics, normally more fragile than structures, are being subjected to serious stresses. It is difficult to make real progress in a phenomena such as this , primarily because no controlled enera source is immediately available to conduct research to study the problem.

In our efforts to provide solutions to acoustic problems, as related to electronics, a

facility for studying the effects on electronics is being constructed. It is a unique and flexible acoustical facility and has been designed to study the behavior of flight vehicle electronics under the influence of high level acoustical environments, It consists of a concrete horn mass coupled to a siren complex that is supplied by a modified aircraft compressor unit. The siren will deliver to the near field test section a maximum sound pressure level of 174db (ref. 0.002 dynes/sq. cm.) from 50 to 10,000 cps. It is interesting to note that this corresponds .to approximately 24,000 watts over a cross-sectional area of one square foot. The acoustical output is essentially sinusoidal and is capable of amp- litude modulation over a narrow bandwidth for fatigue-response studies. It is anticipated that several by-products of this study will be forth- coming. These are essentially in the area of test-techniques, protective criteria and corre- lation of mechanical vibration versus acoustic excitation.

Relays

Ten years ago the most widely used type was the so-called clapper relay, a very simple design consisting of a YJ" shaped electro-magnet with a bar or armature that in operation clsmps doi,rn over the pole faces. In many cases these relays were not developed from any exacting design analysis but were developed by cut and try methods. For this type of relay, low price was the main objective with nominal performance gen- erally a secondary goal.

The first wide-spread application for relays that demanded a good product in terms of life and reliability was in the automatic or dial telephone industry. This type of service demand- ed a radically different relay design to handle small load currents and have an extremely long life. The design is on the ultra conservative side having extra large coils and extremely long, short-travel spring contacts. Mechanically the device is capable of many millions of cycles. In an effort to improve the useful life, two sepa- rate contact pairs are used on the end of the same contact spring. Such redundancy of contact points statistically gives a greatly increased, more reliable operating life, and with the high degree of preventive maintenance enjoyed by the telephone relay, it turns out to be a reliable long life device for ground application, where size, weight, and first cost are immaterial.

During the second world war, aircraft operational altitudes were raised to such a point that due to the rarified atmosphere the open relay was no longer usable. Some work had been done with hermetic sealing for high alti- tude use so this technique was quite naturally applied to relays. However, short operating life was the result, caused by organic vapors given off by the insulation used in the construction of the relay. These gases in the presence of elec- tric arcs decomposed into solid carbonaceous compounds, some of which deposited on the contact

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surfaces. These deposits were largely non- conductive and sooner or later actually blocked the contacts open.

The use of inorganic insulations and clean- ing operations during assembly were used to overcome this problem.,

About the same t:ie another problem, that of an increased shock and vibration environment was encountered in high performance aircraft, esneciallv jets. Vibration from piston engines

” I

is a low frequency (under SO), and resonance is not a problem in most lelectronic components be- cause of their small size. With jet engines, the exact opposite is the case. Vibration fre- quencies in the range of 500 to 2000 cps cause small structures to re,sonate. Most all of the early relays were no longer suitable for jet aircraft use, as the contact springs would reson- ate and cause malfunction of switching.

In addition to vibration resistance and low contact contamination, the developmental push was on for miniaturization as well as an increase in the top operating ambient temperature.

Quite a few radical developent programs were started, most of .which were sponsored by the military services. The only one of note, which is still with us today, is the six-pole, double-throw miniature relay developed for air- borne use (Figure 17). There have been many internal improvements over the years, but the same general design approach is used today (Figure 18). All the many products shown here are directly interchangeable and represent collectively the best relsy performance in the industry today. As may be seen, the relays look similar but actually the majority are internally different. There are rotary as well as solenoid structures with many variations in the magnetic circuit. Brieflv their general capabilities are -65" to-+125OC ambient t&perature-operation; 50 g, 11 ms shock, 10-20000 cps at 15. g vibra- tion, 100,000 operation life in a maximum ambient temperature at msxinnun rated contact current.

Figure 19 shows present relays of a size that is applicable to printed circuits. Here miniaturization is gained by several design changes. Switching poles are reduced from six to two and the switching structure has been located on a reduced header area. All current carrying parts and insulating beads are rated the same as before, but all have been reduced in size. A more efficient magnetic circuit is used and made more sensitive by the use of double coil construction. In some designs even greater contact pressures and stability are achieved by the use of a permanent magnet in series with the electro-magnet.

At first life and reliability of these subminiature units naturally suffered from the cutting and trimning operation. Later, as experience was gained:, detail improvements

raised the quality of the device to previous levels. From an over-all viewpoint, the signifi- cant developments of the last ten years have started out as major increases in performance, particularly environmental performance. Relia- bility improvement has come about rather slowly through detail marginal improvements to produc- tion models. No real attempt at improving: reliability was made until the advent of an Air Force program. Figure 20 shows an exploded view of the latest interim developent single-pole model. It is characterized by a very small number of parts. The central piece is the coil bobbin which also serves to locate the other p3l-t.S. The top and bottom pieces serve several purposes. They are contact, magnetic circuit, electrical circuit and terminal. The small square piece to the left of the bobbin is the armature which is the only moving part in the relay. The corresponding part to the right with. a terminal extension attached is the armature base which does not move in operation. These parts are flanked by the outside can and c:eramic header for hermetically sealing the completed unit.

The parts are all assembled in straight line motion from cardinal directions., No screws or springs are used. One pole piece is permanently magnetized which provides the return force for operation. Obviously it is adapted to mechanized assembly.

Evaluation of Improvements

The question is often asked, "Just what improvements have been made in conpnents during the last several years?" It is easy to answer this question by giving some specifics but a generalized answer is often quite difficult. If one considers those parameters which normally might be considered open for improvements,, a lielt might be:

1.

2.

3.

4.

2:

7.

8.

Increased performance, i.e., higher power, greater efficiency, higher frequency, etc. Longer life, i.e., shelf-life, electron tube cathodes, dielectric breakdown, etc. Reduction in size, such as improved form fac- tor, miniaturization. Reduced cost, through labor savings, mater- ials, tooling, etc. Reduction in usage of critical materials. Increased availability (might include a part of or be dependent on i';ems 3, 4, and 5 above). Operation in a more rugged environment, i.e,,, higher temperature, radiation fields, etc. Increased reliability with a greater probab- ility of survival in a given environment under specified operating conditions .- in other words,mean time to failure.

The above items could be indicated by a number value which, with additional weighting or

priority factors, might develop into a method for evaluating the improvement 'of electronic parts.

Most of these items can be given a number

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factor immediately based on improvements made. For example, if a new item had twice the wattage reading as its earlier counterpart, it might be said to be twice as good - but if, on the other hand, to attain these increased parameters its size had increased by a factor of 3, the new item might not be considered better than its prede- cessor if a size reduction was also a desired parameter.

Using the product of the improvements, or lack of improvements, for each of the parameters to determine the over-all improvement, would in this case ., give an over-all improvement factor of: 2 (for increased performance) x l/3 (for reduction in size) = 2/3. This would indicate that we really had not improved the product at all but had actually lost ground.

This brings forth, of course, necessity for the use of weighting factors along with the above numerical values. Again taking the same item, and let us say for this particular case the increased wattage reading was of 5 times greater importance than any change in size, then the over-all improvement would be: 5 x 2 x l/3 = 3 j$, or a real improvement.

While a rigorous mathematical treatment as outlined above might well establish the improve- ment of a single item, it would not of itself be an indication of the over-all improvements made in electronics. *%&her it would be like estab- lishing the strength of a chain by a single link.

In other words, the real criteria of improve- ments that had been made in electronics during the past several years would have to be evaluated by comparing two similar electronic equipments.

A typical example might be found in the improvement of an airborne communication set as shown in Figure 21. Here it readily can be seen that real improvements have been made in power output and number of channels along with a con- siderable decrease in size and weight. By using numbers without weighting factors, an improvement factor could be expressed as: Improvement factor

Yes, indeed, worthwhile strides really have been made during the past decade.

Acknowledgment is made of the assistance by I!+. Eugene C. Miller, Mr. Robert E. Conklin, i$r. George F. Duree, Hr. Leon Stratis, Mr. Carl A. Golueke, and Mr. Robert Anderson, in the prepara- tion of this paper. All are with the Blectronic Components Laboratory, Wright Air Develoment Center.

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400

300

200

100

0 1948 1958

YEAR

INCREASE IN RATED MAX. AMBIENT TEMPERATURE OF COMPONENTS USED IN AIRBORNE EQUIPMENTS

Fig. 1.

FIXED RESISTOR COMPARISON Cl-k&R-i- (ONE HALF WATT UNITS)

Fig. 2.

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Fig. 3.

Fig. 4.

217

XED MINIATURE RESISTORS _- .__.-_ e-.-” -”

ACCURATE MOLDED “JVIREWOUND TYPE l/IO WATT

MIL-R- 9444

.tlMlC ENCLOSED DEPOSITED C FlLM TYPE l/8 WATT

MIL-R- 10509

ED DEPOSITED CARBON FlL l/8 WATT MDL-R-IO509

CERAMIC ENCLOSED COMPOSlTlON +YF Ii’8 WATT COMMERICAL RATING l/l

MIL RATING MIL-R-II

Fig. 5.

FiXED POWER WIREWOUND RESISTORS (CHASSlS MOUNTED TYPES)

MIL-R- 8781 MIL-R-26 . .

,5& t gw 40 WATT

COMMERICAL TYPE 22 WATT 0 ‘ -.

20 WATT 15 WATT TO CHASSIS 75WATT FREE AIR

Fig. 6.

GENERAL PURPOSE NON-WIREWOUND VARIABLE RESISTORS (MIL-R-94 TYPES)

1949 1959 40% 70°C

STYLE RV4 2 WATTS

_- Fig. 7.

218

n !N

219

W E 3

a: w -

220

. zl bi E

f . J 8

222