BOURNS Multiturn Trimpot Trimming Potentiometers User Guide

BOURNS Multiturn Trimpot Trimming Potentiometers

Applications/Processing Guide

HOW TO USE THIS SECTION
This Applications/Processing Guide is intended to provide you with points to consider for designing circuits, selecting trimmers and arranging board layouts, to achieve maximum performance and long life for your circuits and systems. We have also included information on steps your manufacturing engineers can take to preserve circuit reliability.

For example, are you aware that the trimmers and other mechanical components on your boards may face a more extreme environment during boardwashing on your own production line, than they ever will in use? For those trimmers that may need to be reset, are you remembering to select and mount the trimmers to provide easy accessibility?
In this section, you’ll find dozens of pointers, reminders and useful facts that will help you be more knowledgeable and successful in using trimmers.

TRIMMER BASICS
In its most common form, a trimmer is simply a device containing a resistive element, and a wiper, or adjustable tap, contacting the element. The wiper can be mechanically moved to vary the amount of voltage or resistance in the circuit. The resistive element is usually laid out in linear or a circular configuration:

The Resistive Element
Trimmers for commercial applications typically have a resistive element made of carbon or cermet (a combination of CERamic and METal), or of resistance wire wound on an insulated copper mandrel.

The main advantages of wirewound trimmers are their low temperature coefficient, higher power dissipation, lower noise, tighter resistance tolerance, and, when used as a variable resistor, the excellent current- carrying capacity through the wiper due to the lower contact resistance. Also, their long-term resistance stability with time and temperature is slightly better than cermet.

Cermet trimmers provide a wider resistance range (10 ohms to 5 megohms, versus a maximum of 50K ohms for wire-wound). Also, the wiper output can be set closer to the desired value since the resistive element presents a continous contact surface for the wiper, as opposed to the discrete turns (resolu-tion) of the wirewound. Other advantages with cermet are the lower reactance in high-frequency applications, the smaller sizes available, and the generally lower price than wirewound types.

Soldering And Cleaning Processes

This application note is designed to provide step-by-step processing recommendations. It covers the popular SMD soldering processes currently in use and provides recommen-dations and cautions for each step. Since many variations of temperature, time, processes, cleaning agents and board types are found in the electronics industry, you’ll want to test and verify your own system.

The process steps, recommendations and cautions are based on Bourns Trimpot surveys of SMD users, equipment manufacturers and materials suppliers. Also, comments reflect results of Bourns’ testing. Our findings suggest the following soldering and cleaning processes:

1. SOLDERING – Forced Hot Air, Convection, IR, Vapor Phase (In-Line), Wave (Single and Dual)
2. CLEANING – Solvent, Aqueous, Semi-Aqueous, No-Clean

On the facing page are the common methods, materials and maximum temperature/time parameters for soldering and cleaning processes.

Solder Paste Printing

• Reflow

GENERAL: Use the optimumsolder paste for thepattern, printingprocess, solder pastedensity and solderjoint quality.

RECOMMENDED: Use 8 to 10 mil thickness for solder paste print.
CAUTION: Since solder pasteusually contains a highpercentage of activators,you must ensureadequate cleaning toremove all residues,unless no- clean (lowsolids) paste is used.

Adhesive Application

• Flow (Wave)

GENERAL: The adhesive must hold the SM Component (SMD) in correct orientation upon place-ment and maintain correct trimmer position during physical handling before final solder processing.

RECOMMENDED: To assure positional stability, place a single dot of epoxy under the SMD.
CAUTION: Stability after placement is a direct function of the volume of adhesive used. Use enough epoxy to assure stability through the cure process.
Avoid overflow of epoxy to solder pad and terminal areas.

SMD Placement

GENERAL: Use pick-and-place equipment with vacuum nozzle ID size that allows adequate suction to pick the SMD out of the embossed cavity.
RECOMMENDED: The nozzle inside diameter (ID) should not exceed .100 in.(2.54mm) to ensure adequate suction and part alignment.
CAUTION: Assure parts are placed so that all terminals are equidistant (<4 mils) from the solder pads.
Align terminals with solder belt direction of travel to avoid body shadowing effects during flow soldering.

Adhesive Cure

• Flow (Wave)

GENERAL: Use heat/time curemethod with eitherconvection oven orinfrared radiation.
RECOMMENDED: Cure using thetemperature and times recommendedby the adhesivemanufacturer.CAUTIONUse enough curetime to assurecomplete adhesivetransition from fluidto solid.

Flux Application

• Flow (Wave)

GENERAL: Use the correct fluxto remove surfaceoxides, preventreoxidation andpromote wetting.

RECOMMENDED

• RMA
• No-clean SRB(Synthetic resinbased)
• OA (Organic Acid)(See caution)

CAUTION: Avoid highlyactivated fluxes.Consult factory beforeusing OA.

Soldering And Cleaning Processes

Process Step

| | | | REFLO| W| | | | | FLOW| | | ****

Material

---|---|---|---|---|---|---|---|---|---|---|---|---|---
Hot Air;

Infrared

(Solvent)

| Hot Air;

Infrared

(Semi-Aq)

| Hot Air;

Infrared

(Aqueous)

| Hot Air;

Infrared

(No-Clean)

| Vapor

Phase

(Solvent)

| Vapor

Phase

(Semi-Aq)

| Vapor

Phase

(Aqueous)

| Vapor

Phase

(No-Clean)

| ****

Wave

(Solvent)

| ****

Wave

(Semi-Aq)

| ****

Wave

(Aqueous)

| ****

Wave

(No-Clean)

1. Solder Paste Printing| X| X| X| X| X| X| X| X| | | | |
2. Adhesive Application| | | | | | | | | X| X| X| X|
3. Component Placement| X| X| X| X| X| X| X| X| X| X| X| X|
4. Adhesive Cure| | | | | | | | | X| X| X| X|
5. Flux Application| | | | | | | | | X| | | | Rosin
5. Flux Application| | | | | | | | | | X| | | Rosin
5. Flux Application| | | | | | | | | | | X| | Organic Acid
5. Flux Application| | | | | | | | | | | | X| Synthetic Resin Based
6. Solder (Reflow)| X| X| | X| X| X| X| X| | | | | 63/37 Sn/Pb
7. Solder (Flow)| | | | | | | | | X| X| X| X| 63/37 Sn/Pb
8. Wash (Solvent)| X| | | | X| | | | X| | | | ODS Free
8. Wash (Semi-Aqueous) Based| | X| | | | X| | | | X| | | Terpene, Hydrocarbon
8. Wash (Aqueous) Saponifier| | | X| | | | X| | | | X| | DI H20; Surfacant;
High Pressure Fluids| | | X| | | | X| | | | X| | (See Caution)
Max. Temp. (°C)/Time (Secs)| 235/40| 235/40| 235/40| 235/40| 215/180| 215/180| 215/180| 215/180| 260/5| 260/5| 260/5| |
Min. Temp. (°C)| 215| 215| 215| 215| 215| 215| 215| 215| 215| 215| 215| |

Solder

• Reflow; Hot Air, IRand Vapor Phase

GENERAL: Preheat sufficientlyusing both timeand temperatureto vaporize allsolder paste solvents andmoisture, leavingonly solder andflux as componententers solderreflow phase.

RECOMMENDED: Solder zone profile of 230 °C for 20 seconds.
CAUTION: o not exceed timeand temperature reflow profile of235 °C for 45 ± 5 sec-onds for hot air/IR reflow

Minimize thermal shock by limiting temperature riserate to 3 °C/secand by stabilizing board and components temperature during preheating. Please click here to view our recom-mended lead-free soldering profile

WASH

• Semi-Aqueous

GENERAL: Use semi-aqueousfor nonpolarcontaminants suchas rosin based fluxresidues.
RECOMMENDED: Use terpene orhydrocarbonbased for pre-wash. Use waterfor final wash.
CAUTION: Limit excessivedirect spraypressure to 60 psior below for opti- mum reliability

WASH

• Aqueous

GENERAL: Use aqueous cleaningprimarily for polar contaminants such as organic flux residues.
RECOMMENDED:
Use any of theseaqueous washmaterials:

• Deionized water
• Surfactants
• Saponifiers

CAUTION: Limit excessivedirect spray pressure to 60 psi or below for optimum reliability.Ultrasonics maycause componentdamage or failure.

No-wash is anoption when no-clean (low solids) flux is used for solder operations.

Board Rework Technique

GENERAL Excessive and/or repeatedhigh temperature heatexposure may affectcomponent performanceand reliability.
RECOMMENDED Hot air reflow techniqueis preferred.
CAUTION Avoid use of a solderingiron or wave soldering asa rework technique.

TRIMMING POTENTIOMETERS AND DEFINITIONS

The following terms and definitions have been edited from the Industrial Standard published by the Variable Resistive Components Institute. It is intended to encourage standardization in communica-tion and understanding between the manufacturer and user. The complete standard, including detailed test procedures, is available upon request.

GENERAL TERMS

TRIMMING POTENTIOMETER: An electrical mechanical device with three terminals. Two terminals are connected to the ends of a resistive element and one terminal is connected to a mov-able conductive contact which slides over the element, thus allowing the input voltage to be divided as a function of the mechanical input. It can function as either a voltage divider or rheostat.

WIREWOUND TRIMMING POTENTIOMETER: A trimming poten- tiometer characterized by a resistance element made up of turns of wire on which the wiper contacts only a small portion of each turn.

NON-WIREWOUND TRIMMING POTENTIOMETER: A trimming potentiometer characterized by the continuous nature of the surface area of the resistance element to be contacted. Contact is maintained over a continuous, unbroken path. The resistance is achieved by using material compositions other than wire such as carbon, conductive plastics, metal film and cermet.

RESISTANCE ELEMENT: A continuous, unbroken length of resis- tive material without joints, bonds or welds except at the junction of the element and the electrical terminals connected to each end of the element, or at an intermediate point such as a center tap.

ADJUSTMENT SHAFT: The mechanical input member of a trim- ming potentiometer which when actuated causes the wiper to traverse the resistance element resulting in a change in output volt- age or resistance.
SINGLE-TURN ADJUSTMENT: Requires 360º or less mechani cal input to cause the wiper to traverse the total resistance element.

MULTITURN ADJUSTMENT: Requires more than 360º mechanical adjustment to cause the wiper to traverse the total resistance element.
TERMINAL: An external member that provides electrical access to the resistance element and wiper.

LEADWIRE TYPE TERMINAL: Flexible insulated conductor.
PRINTED CIRCUIT TERMINAL: Rigid uninsulated electrical con- ductor, suitable for printed circuit board plug-in.

SOLDER LUG TERMINAL: Rigid uninsulated electrical conductor, suitable for external lead attachment.
WIPER: The wiper is the member in contact with the resistive ment that allows the output to be varied when the adjustment shaft is rotated.

STOP-CLUTCH: A device which allows the wiper to idle at the ends of the resistive element without damage as the adjustment shaft continues to be actuated in the same direction.
STOP – SOLID: A positive limit to mechanical and/or electrical adjustment.

STACKING: The mounting of one trimming potentiometer adjacent to or on top of another utilizing the same mounting hardware.
THEORETICAL RESOLUTION: (Wirewound only) The theoretical measurement of sensitivity to which the output ratio may be adjusted; the reciprocal of the number of turns of wire in resistance winding expressed as a percentage.

• N = Total number of resistance wire turns.
• N X 100 = Theoretical resolution percent.

INPUT AND OUTPUT TERMS

TOTAL APPLIED VOLTAGE: The total voltage applied between the designated input terminals.
OUTPUT VOLTAGE: The voltage between the wiper terminal and the designated reference point. Unless otherwise specified, the designated reference point is the CCW terminal.
OUTPUT RATIO : The ratio of the output voltage to the designated input reference voltage. Unless otherwise specified, the refer-ence voltage is the total applied voltage.
LOAD RESISTANCE: An external resistance as seen by the Output Voltage (connected between the wiper terminal and the desig-nated reference point.)

ADJUSTMENT TERMS

DIRECTION OF TRAVEL: Clockwise (CW) or counterclockwise (CCW) rotation when viewing the adjustment end of the poten-tiometer.
MECHANICAL TRAVEL — SOLID STOPS: The total travel of the adjustment shaft between integral stops. Continuity must be maintained throughout the travel.
MECHANICAL TRAVEL — CLUTCHING ACTION: The total travel of the adjustment shaft between the points where clutch actua-tion begins. Continuity must be maintained throughout the travel and during clutch actuation.
MECHANICAL TRAVEL — CONTINUOUS ROTATION: The total travel of the adjustment shaft when the wiper movement is unrestricted at either end of the resistive element as the adjust-ment shaft continues to be actuated.
ADJUSTMENT TRAVEL (ELECTRICAL): The total travel of the adjustment shaft between minimum and maximum output voltages.
CONTINUITY TRAVEL: The total travel of the shaft over which electrical continuity is maintained between the wiper and the resistance element.

ELECTRICAL AND OPERATIONAL CHARACTERISTICS TOTAL RESISTANCE: The DC resistance between the input terminals with the wiper positioned to either end stop, or in dead band for continuous rotation potentiometers.

TEST VOLTAGE

Ohms

| Non-Wirewound         Wirewound
Volts DC                Volts DC
.1 to 1.0| 0.1                         0.1
1.0 to 50| 0.3                         0.3
50 to 100| 2.0                         2.0
100 to 1000| 3.0                         3.0
1K to 100K| 10                          10
Over 0.1 megohm| 50                           —

NOTE: The test voltages should never exceed the equivalent of 10% rated power. The minimum voltage to be used is 10 MV.

ABSOLUTE MINIMUM RESISTANCE: The resistance measured between the wiper terminal and each end terminal with the wiper positioned to give a minimum value.
END RESISTANCE: The resistance measured between the wiper terminal and an end terminal when the wiper is positioned at the corresponding end of mechanical travel. Absolute minimum resistance and end resistance are synonymous for continuous rotation trimmers.
TEMPERATURE COEFFICIENT OF RESISTANCE: The unit change in resistance per degree Celsius change from a refer-ence temperature, expressed in parts per million per degree Celsius as follows:

Where:

• R1 = Resistance at reference temperature in ohms.
• R2 = Resistance at test temperature in ohms.
• T1 = Reference temperature in degrees Celsius.
• T2 = Test temperature in degrees Celsius.
  

RESISTANCE-TEMPERATURE CHARACTERISTIC: The difference between the total resistance values measured at a refer-ence temperature of 25ºC and the specified test temperature expressed as a percent of the Total Resistance.

• R2 – R1
• RTC =
• X 100
• R1

Where:

• R1 = Resistance at reference temperature (25 ºC) in ohms.
• R2 = Resistance at the test temperature in ohms.

CONTACT RESISTANCE VARIATION: The apparent resistance seen between the wiper and the resistance element when the wiper is energized with a specified current and moved over the adjustment travel in either direction at a constant speed. The output variations are measured over a specified frequency bandwidth, exclusive of the effects due to roll-on or roll-off of the terminations and is expressed in ohms or % of total resistance.

Figure 1. Contact-resistance-variation measuring circuit

Rt = Test specimen
Output detector bandwidth: 100 cycles to 50 kilocycles Minimum input impedance to output detector: At least 10 times the nominal resistance being tested

NOTE: At the calibration of the decade, terminals 1 and 2 must be coincident. Calibration decade is to be set for the contact-resistance variation (CRV) level of the specified nominal resistance being tested.

TABLE II

EQUIVALENT NOISE RESISTANCE: Wirewound only. Any spurious variation in the electrical output not present in the input, defined quantitatively in terms of an equivalent parasitic, transient resistance in ohms, appearing between the con-tact and the resistant element when the shaft is rotated. The equivalent Noise Resistance is defined independently of the resolution, functional characteristics and the total travel. The magnitude of the Equivalent Noise Resistance is the maximum departure from a specific reference line. The wiper of the po-tentiometer is required to be excited by a specific current and moved at a specific speed.

CONTINUITY: Continuity is the maintenance of continuous electrical contact between the wiper and both end terminals of the resistive element.
SETTING STABILITY: The amount of change in the output voltage, without readjustment, expressed as a percentage of the total applied voltage.

DIELECTRIC STRENGTH: The ability to withstand the application of a specified potential of a given characteristic, between the terminals and all other external conducting members such as shaft, housing and mounting hardware without exceeding a specified leakage current value.
INSULATION RESISTANCE: The resistance to a specified DC volt age impressed between the terminals and all other external conducting members such as shaft, housing and mounting hardware.

POWER RATING: The maximum power that a trimming potentiometer can dissipate across the total resistive element under specified conditions while meeting specified performance requirements.
ROTATIONAL LIFE: The number of cycles obtainable under specified operating conditions while remaining within specified allowable degradation. A cycle is defined as one complete traversal of the wiper over the resistive element in both directions.
LOAD LIFE: The number of hours at which a device may dissipate rated power under specified operating conditions while remaining within specified allowable degradations.
ADJUSTABILITY (OUTPUT RESISTANCE): The precision with which the output resistance of a device can be set to the desired value.

ADJUSTABILITY (OUTPUT VOLTAGE RATIO): The precision with which the output voltage ratio of a device can be set to the desired value.

MECHANICAL TERMS

STARTING TORQUE: The maximum moment in the clockwise and counterclockwise directions required to initiate shaft adjust-ment anywhere in the mechanical travel.
STOP TORQUE: The maximum static moment that can be applied to adjustment shaft at each mechanical stop for a specified period of time without loss of continuity or mechanical damage affecting operational characteristics.
SOLDERABILITY: The ability of the terminals to accept a uniform coating of solder under specified conditions.
WELDABILITY: The ability of materials to be welded together under specified conditions.
TERMINAL STRENGTH: The ability of the terminals to withstand specified mechanical stresses without sustaining damage that would affect utility of the terminals or operation of the trimming potentiometer.
IMMERSION SEALED: The ability of the unit to withstand submer sion in acceptable cleaning solutions used in normal soldering processes without performance degradation under specified environmental conditions.

TRIMMER “ABILITIES” When you are selecting components for a new design, you typically take into account the environmental conditions that the components will need to endure during the lifetime of the instru-ment or device. Designers in the past have often overlooked the environmental extremes of their own production lines, where the conditions may be much more severe than anything encountered in actual end use.

PROCESSABILITY” “Processability” refers to the ability of the unit to withstand the production-line processes associated with the finishing steps on the PC boards. Typically, both SMT and through-hole products are subjected to similar PC board processing operations after prepara-tion for assembly. These operations can generally be summarized as follows:

SOLDERING (SMT)
Four types of equipment are usually associated with SMT soldering:

• IR System — Uses a multi-zone infrared furnace with IR elements heated to a temperature substantially above cham-ber or product temperature. Energy is supplied to the product primarily by IR radiation to reflow solder.Forced Hot Air Convection System — Uses a multizone forced air convection system with heat source panels using IR or other type heating elements. Approximately 85% of the heat-ing is provided by free convection to reflow solder on exposed PC boards.
• Dual Wave System — Utilizes two parallel solder waves. The first is a turbulent wave followed by a laminar wave. The tur-bulent wave is for small, constricted areas, while the laminar wave removes solder projections.
• Vapor Phase System — Provides a single-zone condensation heat source achieved with liquid fluorinated hydrocarbons that have been brought to the boiling point to create a saturated vapor zone. Heat is then released by the fluid’s heat of vapor-ization as the vapor condenses on the product.

SOLDERING (THROUGH-HOLE)

Two types of equipment are usually associated with through-hole soldering:

• Single Wave System — Provides an inclined portion of the solder wave for the PC board to pass over. The PC board is positioned to bring many potential solder joints in contact with the wave simultaneously for a short time for soldering.
• Drag System — Provides for PC boards to be dragged across the surface of the solder pot. Soldered connections are made during this operation.

PC BOARD WASHING
Two types of equipment are usually associated with both SMT and through-hole products.

• Pressure System — Accomplishes cleaning by directing sprays of water under high pressure from multiple nozzles.
• Flooding System — Utilizes a combination of flooding (at normal water pressure) and surfactant action for cleaning).

SOLDERING AND WASH PROCESSES
Figure 1 shows typical profiles any component may see during a soldering and board washing operation. For details of mate-rial and process variables recommendations, see
“Soldering and Cleaning Processes”, page 76.

Figure 1.
Typical temperature profile for board washing and soldering.

Critical profile parameters

• Temperature Shock (°C)
• Maximum Temperature (°C)
• Temperature Exposure (Minimum)
• Temperature Gradient (°C)
• Temperature Shock Decrease in Water (°C)
• Temperature Shock Decrease in Water & Air Pressure (°C)
• Unstable Temperature (see next page)

General Guidelines for Guarding Against Component Damage To minimize temperature shock

• Pre-heat boards to maximum acceptable level
• Reduce time in solder

To avoid heating components above their maximum rated temperature

• Use lowest acceptable solder temperature
• Use maximum allowable conveyor speed
• Limit pre-heat temperature to maximum necessary To limit time of exposure above rated temperature
• Limit time in solder
• After solder operation, cool board to wash temperature before it enters wash

To minimize temperature difference between top and bottom of board

• Apply pre-heat to both top and bottom

To reduce temperature shock on entering the moist environ-ment of the wash

• Use wash/rinse temperature as near component temperature as possible
• Extend time between solder process and wash
• Cool board after solder operation, prior to entering wash

To minimize temperature variations as component travels through moisture

• Minimize number of wash/rinse and rinse/dry cycles
• Use heated air for air knives (to counter evaporative cooling effect)
• Minimize difference between wash and rinse temperature

To minimize exposure to high-pressure water during board wash

• Select trimmer models with pin styles that orient the rotor seal area away from exposure to the high pressure water stream

SUITABILITY

• Settability refers to the ease with which a trimmer can be set accurately to the position that produces the desired circuit con-dition.
• Where the requirement is for obtaining a highly accurate setting the preference is for cermet — because a small incremen-tal adjustment in a wirewound unit does not always produce the expected change in output as the wiper moves off one turn of wire and onto another.
• Setting accuracy is better with a multiturn unit than with a single-turn. This is especially true when the speed of setting is also a requirement as on a production line (Figure 2).

Figure 2.
When accurate setting is required, a multiturn trimmer can generally be set faster than a single-turn.

STABILITY

• Stability refers to the ability of the trimmer to remain at the desired setting. Environmental factors play an important role here: stability may be affected by temperature exposure, thermal shock/ cycling, humidity, and mechanical shock or vibration.
• This is not a matter of concern in most applications, since Bourns trimmers exhibit excellent stability under all specified condi-tions. Stability is most often a concern when cermet trimmers are used in low current “dry” circuits (50uA amps and below). Under these conditions the contact resistance may vary, making the wiper appear unstable.
• This is most noticeable in some rheostat applica-tions. This can be avoided by using a wirewound unit, or choosing a cermet trimmer that has been designed for dry-circuit applications. Bourns applications engineers can assist you on this and other questions.

ACCESSIBILITY

When selecting a trimmer and determining its placement on the board, keep in mind the people who will have to use it. Bourns trimmers are available in a wide variety of sizes, shapes, configura-tions, and placement of adjustment screws. You will usually find a unit on which the access for adjustment will be convenient for the user.

Keep in mind the different requirements for accessibility depending on whether adjustment will be done on the assembly line or in the field; with the board uncovered, in a housing or cabinet, or on an extender. Also consider whether production-line adjustment will be done manually or by robotics. A Bourns applications engineer can advise on special high-speed automatic adjustment features.

USABILITY

• In selecting a trimmer for a specific application, it’s important to be aware that the catalog contains a myriad of facts about each model that can assist you in finding the most suitable choice. For example:
• Contact Resistance Variation (CRV) — Under MIL-R 22097 and MIL-R-39035, the maximum CRV is 3%. All Bourns trimmers meet this standard (3% or 3 ohms, whichever is greater). For ap-plications that demand a more rigorous standard, some Bourns trimmers are rated at 2% or 2 ohms, and many others at 1% or 1 ohm.
• Power Rating — The ambient temperature at which the trim-mer will operate has an important bearing on power rating. Power ratings are usually specified at 70º or 85ºC; at a temperature of 150ºC, the power rating of many trimmers is reduced to zero.
• Temperature Coefficient of Resistance (T.C.) — This speci-fication is a measure of how much the resistance changes with a change in temperature. In many applications a T.C. of ±250PPM/ºC is acceptable. Typical T.C. specifications for cermet models are
  ±100PPM/ºC and ±50PPM/ºC for wirewound models.

RELIABILITY
One of the greatest challenges facing American manufactur-ers in the early ‘90s lies in the area of reliability — a challenge for component manufacturers and equipment manufacturers alike. Bourns has been on the leading edge of this effort, both in the area of instituting new methods and technologies for achieving higher reliability, and bringing an awareness of the need to other manufac-turers.
SURFACE-MOUNTED DEVICES (SMD)

AN EMERGING TECHNOLOGY
Surface mounting of electronic components represents another significant advance in PC board processing. Many U.S. companies have expressed an interest in SMD assembly methods to replace the often troublesome and costly techniques now used with leaded components. Unfortunately, for a number of reasons, this interest has not resulted to date in a major commitment to SMD handling equipment.

There are direct and indirect benefits associated with surface mounting. Since the direct benefits are outgrowths of the indirect ones, some explanation of these interrelated factors is required in order to understand this complex, highly technical and investment intensive subject. Further, a listing of the primary advantages will make additional comments on Japan’s SMD usage and growth un-necessary.

In capsule format, the primary advantages (with comments on secondary benefits) are:

• Lower End-Equipment Cost (positions OEM’s for aggressive pricing to achieve market penetration).
• Superior Product Performance (satisfies user requirements for improved operational performance).
• Improved Product Quality and Reliability (creates confidence factor which easily translates to increased demand or sales).
• Smaller Finished Product Size

(addresses demand for miniaturization).
Cost, performance, quality/reliability and size: how are these factors interrelated and how are they achieved through surface mounting?
A by-product of SMD technology is the downsizing of compo-nents. Size reductions range from 25% to 60%, depending upon the device in question. High PC board densities can be achieved (more components per square inch of real estate; surface mounted units can also be assembled on both sides). PC board material savings alone are substantial. When circuits diminish, external hardware and other materials follow — further savings. Even freight charges are decreased by lighter equipment weight and less packaging.
Surface mounted component prices are forecasted to decline, the result of automated volume production. Volume is directly re-lated to component standardization. By having a few sizes to cover a large range of electrical values and/or parameters, large quanti-ties of a given device can be produced at a much lower per unit cost. Selling prices fall as volume increases. Component quality is also enhanced by eliminating many of the variables associated with short production runs.

Automatic SMD handling equipment, although capital inten-sive, is the single- most effective way to reduce labor costs and increase yields. Typical “pick and place” machines can assemble components 8 to 10 times faster than human assemblers, with virtually no mistakes. Major direct labor reductions are obvious. The combination of improved component quality and “mistake-free” component placement further decreases costs by eliminating the normal rework of auto-inserted boards.

The many advantages of SMD technology will force change upon both electronic equipment manufacturers and component suppliers alike. Worldwide competitive prices and performance pressures will make it happen. Few electronic components will escape its influence, trimming potentiometers being no exception. Bourns is committed to SMD conversion, and we intend to be a leader in surface mounted trimmer devices. Bourns surface mount trimmers begin on page 12.

GENERAL NOTES

Plated-Through Holes: (Ref. MIL-STD-275D).
5.5 Plated-through holes. The difference between the inside diameter of the plated-through hole and the nominal outside diameter of the inserted lead shall be not greater than 0.028 inch (0.71mm) or less than 0.010 inch (0.25 mm). Unless otherwise specified, the hole size shall be the finished plated size after solder coating or fusing. When flat ribbon leads are mounted through plated-through holes, the difference between the nominal thickness of the lead and the inside diameter of the plated-through hole shall not exceed 0.028 inch (0.71 mm).

Specifications are subject to change without notice. Customers should verify actual device performance in their specific applications

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