On January 1, 2006, Knowles completed the transition to using exclusively lead-free sol-der in compliance with European Union en-vironmental directives. This document pro-vides guidelines for soldering to Knowles transducers manufactured since 2005 using lead-free, SAC305 solder. For general in-formation about soldering to Knowles trans-ducers, and for older transducers using lead solder, refer to Knowles Technical Bulletin 4 (TB4), available from the Knowles Electron-ics web site www.knowleselectronics.com.

All receivers and microphones contain heat-sensitive components, and uncontrolled or careless soldering can potentially cause ir-reparable damage. However, it is not diffi-cult to develop a process to solder without overheating transducers as long as certain factors are taken into account (see the “Summary” sidebar). This document con-tains guidelines and background to help you develop a safe soldering process.

The external conductor being soldered to the transducer, which may for example be braided Litz wire or a copper trace in a Kap-ton film, is often called a lead (rhymes with need), which is too easily confused with the soft grey metal lead (rhymes with bed). Throughout this document, the external con-ductor being attached to the transducer will be simply referred to as a wire, regardless of the actual materials involved.

How do I know it’s lead-free?All lead-free transducers have 5-digit model numbers. If the first digit is 2, the remain-ing 4 digits are the model number of the equivalent lead solder model. For example, the microphone model EM-23368-000 is the lead-free version of the EM-3368-000.

Lead-Free SolderHistorically, the solder used for wire at-tachment to Knowles transducers was as much as 40% lead. However, European Un-ion regulations regarding the Restriction of

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Application Note 13E L E C T R O N I C SE L E C T R O N I C S

Lead-Free Soldering to Hearing Aid Transducers

SummaryThe primary factors for lead-free soldering with-out overheating transducers, in order of impor-tance, are:

1.Heat sink. Transducers, particularly micro-phones, can be measurably damaged by a sol-dering iron in less than a second without a proper heat sink. The heat sink should be made of a high heat conductivity, high heat capacity metal and make intimate contact with a large area of the metal housing of the trans-ducer. We recommend the Knowles ET series heat sink.

2.Dwell time. The soldering iron should not remain in continuous contact with the trans-ducer terminal for longer than 3 seconds. We recommend 1 second maximum as normal practice.

3.Cooling time. Transducers should be allowed to cool at least 10 seconds between solder op-erations on the same device. This can be achieved economically with heat sink fixtures that hold several transducers at a time and performing one soldering operation on each transducer in sequence.

4.Temperature. Soldering temperature should not exceed 400°C (750°F) for passive compo-nents or 370°C (700°F) for components with integrated circuits (microphones). Tempera-tures below 340°C (640°F) are not generally recommended since cooler molten lead-free sol-der does not wet as well and could result in a dwell time higher than recommended, or re-duced pull strength. Some experimentation will be required to find the optimum solder temperature to reliably complete the solder operation within the recommended time.

It is important that any process developed for soldering be tested and verified that it causes no change in distortion or sensitivity to the trans-ducer, and results in adequate pull strength and low resistance at the joint.

Hazardous Substances (RoHS) from Waste Electrical and Electronic Equipment (WEEE) have required the change to a lead-free solder. In particular, EU Directive 2002/95/EC on the restriction of the use of certain hazardous sub-stances, including lead, in electrical and elec-tronic equipment, will be enforced throughout the European Community from 1 July 2006. At the time of this writing, information on this regulation is available at www.pb-free.info.

The solder that Knowles Electronics has chosen for RoHS compliance is an alloy of 96.5% tin

(Sn), 3.0% silver (Ag), and 0.5% copper (Cu). This alloy is known as Sn96.5Ag3Cu.5, or ab-breviated simply as SAC305. The behavior of SAC305 solder during wire attachment is dif-ferent from the 60/40 or 63/37 tin-lead solder formerly used at Knowles. In particular, lead solder melts at 183ºC (361ºF), while SAC305 melts at 217ºC (428ºF), 34ºC hotter. Lead-free solder is not a eutectic alloy, so it has a wider “pasty range” where it is partially molten, and has slightly poorer flow and wetting properties. Because of this, recommended iron tempera-

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Quick Reference

Item Requirements Comments Product Examples

Solder SAC305 alloy or equivalent Non-SAC alloys are not recommended

Kester Sn96.5Ag3Cu.5

Flux No-clean, non-RMA grade flux

Avoid excess Kester 959T (liquid), Ke-ster 275 (wire core)

Cored wire Diameter 0.5 mm (.020 in) or smaller

Flux volume in cored wire should be 1–2.2% by weight

Kester core size 58

Heat Sink Large thermal mass, good thermal contact to trans-ducer

Highly recommended Knowles ET Series

Dwell Time 1 s Duration of continuous contact of soldering iron with pad

Cooling Time 10 s Interval between solder-ing operations on a single transducer

Soldering Temperature 340–370ºC (640–700ºF) Active components (mi-crophones, amplified re-ceivers)

EK, EM, TM, TO micro-phones.

340–400ºC (640–750ºF) Passive components (non-amplified receivers)

EF, ED, FC, FH, FK re-ceivers

Soldering Iron Temperature regulated pen-cil, 25 W minimum

ESD safe, grounded Weller WD1001

Soldering Iron Tip 0.2 mm (0.008 in) to 0.8 mm (0.032 in)

Tip style is user prefer-ence

Weller NT1, NT1X, NTH (for WSL iron)

Tip Cleaner Lead-free, no residue Use as necessary to keep tip tinned

Plato TT-95

External wires Pre-tinned with SAC305

tures for lead-free soldering are higher than for lead solder.

The appearance of a lead-free solder bead is different than that of lead solder. Lead solder produces a smooth, mirror-like surface, while lead-free solder is frosty and mottled (Figure 1.)

Lead-free solder sometimes appears very simi-lar to what would normally be considered a cold solder joint with lead solder, so quality docu-mentation regarding appearance of solder joints will need to reflect this.

Lead-free solder joints also tend to be slightly taller than lead solder due to the higher surface tension (and increased wetting angle) of the molten lead-free solder. Because of this, solder

pads will have an unavoidable increase of 20–25% in overall height (Figure 2.)

FluxElimination of lead also changes the chemistry of the soldering process, requiring a flux that is

suitable for lead-free applications. Flux should be a no-clean, non-RMA (Rosin, Mildly Activated) formulation compatible with lead-free solder. No-clean fluxes are non-conductive and non-corrosive, and normally do not require removal of flux residue after soldering. Compared to RMA rosin flux typically used with lead solder, no-clean is slightly less chemi-cally active, very fluid when molten, and evaporates more rapidly.

When liquid flux is used, avoid excess application as it may migrate to the in-side of the transducer and result in dam-age or leave a sticky film. Dipping the

tinned wire in the liquid flux before solder-ing is normally sufficient.

During our development and testing of lead-free soldering processes, Knowles used Kester 0.5 mm (0.020 in) Sn96.5Ag3Cu.5 cored wire solder with 58 core size and 275 no-clean flux (2.2% flux by weight), with very satisfactory results. Comparable products are available from other manufacturers.

Soldering EquipmentThe following basic items are common to all wire soldering operations:

• Heat sink with transducer positioning fea-tures

• ESD-safe, temperature-regulated soldering station

• ESD-safe shop furniture and other equipment

• Flux-cored SAC305 wire solder

• No-clean, non-RMA liquid flux

• Tip cleaner

• Tweezers or other tools for positioning wires during soldering

• Bright workspace lighting

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Figure 1. The bead of lead solder (left) is smooth and shiny, while the lead-free solder (right) appears frosty and mottled.

Figure 2. The lead-free solder bumps on the transducer at the right are higher than the

lead solder bumps.

• Vision magnification such as a stereo micro-scope

Heat sinkThe single most important factor for soldering without damage to transducers is the heat sink. A heat sink dramatically increases the amount of time the soldering iron can contact the sol-dering pad from less than 1 second without a heat sink to about 3 seconds. Heat sinks should be considered a necessary and critical part of any soldering process.

A heat sink is a large mass of metal which con-ducts heat away from the transducer with only a minor temperature increase because of its high heat capacity. A well designed heat sink is made from a large block of metal with notches that contact each transducer on most of its surface area yet expose the solder pads. The contact surfaces must be kept clean of flux resi-due or other contaminants that would interfere with intimate metal-to-metal contact with the transducer.

A heat sink may have additional features to simplify the soldering process, such as the Knowles ET heat sink shown in Figure 3. This

heat sink holds several transducers at once in fixed and oriented positions. It has a spring-loaded top plate for easy loading and unloading. The top plate is removable for easy access for cleaning and the spring applies a firm but gen-tle pressure that assures good contact without

crushing the transducer. To help prevent acci-dental contact between the soldering iron and the receiver case, the heat sink is taller than the transducer terminal board on one side (see Figure 4.) The ET heat sink for most transduc-

ers are available directly from Knowles. Please contact Knowles Customer Service for ordering information.

Soldering station The soldering station should be an ESD-rated model that is designed for intricate electronic hand work. It should have at least a 25 Watt pencil iron with a regulated temperature con-troller. Higher power heaters produce quicker temperature recovery and can improve solder-ing performance. A regulated iron will apply more heating power to maintain soldering tem-

perature when in use, and otherwise idle at its set temperature.

Unregulated irons are not recom-mended because they idle at very high temperatures but cool exces-sively when in use. This will initially overheat the terminal but then re-quire extended contact time. The high idle temperature also shortens tip life. Unregulated irons should not be used to solder Knowles products, as their use can result in damaged transducers and inferior solder joint quality.

Tip design greatly affects the amount of heat delivered to the solder bead,

and appropriate tips should be used. A well tinned, correctly sized and shaped tip will fully melt a tinned solder pad within one second of contact. The size of the soldering iron tip should be in the range of 0.2–0.5 mm (0.008–0.020 in) depending on the size of the

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Figure 3. The heat sink protects 12 transducers and holds them for convenient soldering.

Figure 4. The raised edge of the heat sink protects against accidental contact

between the soldering iron and the trans-

ducer case.

transducer terminal pads. In general, the tip diameter should be no greater than half the dimension of the pad. If it is too large, it will be difficult to position the tip without bridging to adjacent pads or the transducer case. If the tip is too small, it may transfer heat too slowly to melt the solder, and result in overheating of the transducer. The choice of a conical, chiseled or flat tip is left to the preference of the user. Sometimes, a slight bend in the tip may facili-tate soldering.

The tip must be maintained in a very clean and brightly tinned condition to produce maximum heat transfer in minimum time from the iron to the working area. A dirty, oxidized tip will in-crease soldering time and the potential for heat-related damage. The activity of lead-free solder and non-RMA flux may not be enough by themselves to keep the tip clean and tinned, so a lead-free tip cleaner such as Plato TT-95 should be used when necessary.

ESD protectionAll transducers that contain integrated circuits, which includes all microphones, amplified re-ceivers, and active telecoils, are sensitive to electrostatic shock and should be handled ac-cordingly. ESD-safe soldering irons should be used exclusively during soldering. ESD-safe lab benches or surface coverings should be used and grounded wrist straps worn at all times while handling these products.

Soldering ProcessSoldering is not difficult. However, a soldering process for high-volume production that relia-bly delivers a solid mechanical and electrical junction without overheating the components relies on several factors that are unique to the particular application, so there is no universal, rigidly defined soldering process or temperature that can cover all situations. Any high-volume soldering process should be developed by proc-ess engineers familiar with the individual ap-plication, and should be tested and verified that it causes no change in distortion or sensitivity to the transducer, and results in adequate pull strength and low resistance at the joint.

The process factors that we have learned through experience and verified through labo-

ratory experimentation to be critical to solder-ing without overheating transducers are the dwell time, the cooling time, and the soldering temperature.

TimingDwell time is the duration of continuous contact between the soldering iron and the solder pad. Testing has demonstrated that, with a good heat sink, Knowles transducers can withstand 3 seconds of dwell time with a 400ºC iron with-out damage. However, for regular practice we recommend 1 second of dwell time, which is adequate time for a skilled operator with good fixtures to perform a quality solder operation.

Cooling time is the time the transducer has to cool between soldering operations. Performing multiple solder operations on a single trans-ducer in rapid succession can cause as much overheating as a single extended dwell time. Knowles recommends there be at least a 10 sec-ond cooling period between soldering opera-tions.

Best practice to ensure adequate cooling time after each solder operation is to arrange multi-ple transducers, at least 10, on the heat sink as shown in Figure 3. Perform one soldering op-eration on the first transducer, then one on the second, and so on down the line. By the time the operator has returned to the start to per-form the second solder operation, the first transducer has had time to cool.

Soldering temperatureThe measured temperature of a soldering iron tip is typically 10–20°C cooler than the control-ler setting (the soldering temperatures reported in this document are the controller settings on a Weller WSD 81).

Testing at Knowles demonstrated that with good heat sinks and the recommended dwell and cooling times, transducers with no inte-grated circuits (all unamplified receivers) are not damaged by solder temperatures up to 400°C (750°F). Transducers with integrated circuits (all microphones and amplified receiv-ers) are not damaged by temperatures up to 370°C (700°F). Above these temperatures some overheating is possible.

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We found that below 340°C (640°F) the lead-free solder is more viscous in its liquid state and did not wet the wire as well as with higher temperatures. This could result in lower me-chanical bond strength in some percentage of solder operations, so we do not generally rec-ommend using temperatures any lower. How-ever, temperatures as low as 290°C have been used successfully.

Using a low solder temperature is not always the best strategy to prevent transducer over-heating. A cool soldering iron will take longer to complete the solder operation, and may transfer more heat to the transducer. Some experimentation will be necessary to find the optimum combination of soldering temperature and dwell time.

Wire preparationThe end of the wire that is attached to the transducer should be pre-tinned with SAC305 solder. The length of the tinned region should be approximately 0.8 mm (1/32 in) for the larger receivers (CI, EF, and similar), and an appropriately shorter length for the smaller terminals on other transducers. Tinning can be performed using the soldering iron or a solder pot with SAC305 solder.

Some wires will require mechanical or chemical stripping prior to tinning, but in all cases ex-cess insulation and chemical strippers must be cleaned from the wire prior to use. If the wire has insulation, this must be removed and the wire tinned before soldering. The heat required to melt away the solderable insulation could cause damage to the transducer.

The pre-tinned end of the wire should be dipped in a no-clean, non-RMA flux immediately before soldering.

Other tipsKeep the soldering iron clean and lightly tinned. Avoid wiping the soldering iron over the solder pad as this could damage the connec-tion wire underlying the solder. Avoid touching the soldering iron or dropping hot solder di-rectly onto the case of the transducer.

Removal of WiresIf wires must be removed from a transducer, the same precautions and principles listed for soldering apply. Extreme care should be taken to prevent damage to the internal transducer wires which connect through holes in the ter-minal board under the solder bump. Broken wires, internally shorted wires, and wires inad-vertently pushed back into the case can result from poor cleanup techniques. Avoid mechani-cal forces which might pry the terminal board off the case.

In cleaning up, move soldering tip from the cen-ter of the terminal outward, but avoid ground-ing the terminal to the case. Use of heat sinks is especially important to control heat during wire removal operations.

Alternate Methods of Wire Attach-mentOther equipment is currently available to pro-vide wire-to-transducer connections, but has not been discussed. Pulsed power soldering irons, solder reflow equipment, energy dis-charge welding techniques and conductive ad-hesives are just a few of the methods which might be made to work successfully. However, each of these techniques has limitations and disadvantages which must be investigated thoroughly by the user to determine its effect on the transducer and the function and life of the finished product.

Mixing Lead Solder and Lead-Free SolderFor some time after the introduction of lead-free solder in newly manufactured products, there will be a mixture of lead and lead-free products in the field. Thus, hearing aid repair facilities may need to replace a transducer that was originally manufactured with lead solder with a new transducer built with lead-free sol-der. In general, we believe this practice should not cause significant problems. Knowles has done a limited amount of testing with mixed solder materials and found no significant is-sues. However, each facility should perform its own investigation to determine the acceptabil-ity of such practices.

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Whenever a lead-free solder is present, the sol-dering temperature should be in the range specified for the use of lead-free solder. The dif-ferent melting characteristics of the different solder materials will be noticeable in the proc-ess, and some practice may be necessary with the dissimilar materials. The appearance of the solder bead with mixed materials will have many of the characteristics of lead-free solder beads. Based on material from lead-free solder manufacturers and our own limited experience, it seems that the quality and reliability of the completed joints should be good.

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Knowles Electronics, LLC1151 Maplewood DriveItasca, Illinois 61034USA+1 630 250 5100 voice+1 630 250 0575 faxwww.knowleselectronics.com

Knowles EuropeYork Road,Burgess HillWest Sussex, RH15 9TTEngland+44 1444 235432 voice+44 1444 248724 fax

Knowles Electronics, Suzhou#179 SuHong RoadSuzhou Industrial ParkSuzhou, JiangSu 215021China+86 512 6258 8258 voice+86 512 6258 9258 Fax

Knowles Electronics (M) Sdn. Bhd.Plot 104Lebuhraya Kampung JawaGeorgetown, Penang 11900Maylasia+60 4 6437 466 voice+60 4 6437 446 Fax

Knowles Electronics, Japan5/F YK Building 2-16Sangenjaya 2-ChomeSetagaya-Ky, Tokyo 154-0024Japan+81 3 3439 1151 voice+81 3 3439 8822 fax

Micromax Pty Ltd5 Orangegrove AvenueUnanderra NSW 2526Australia+61 2 4271 1300 voice+61 2 4271 8091 faxwww.micromax.com.au

Note: All data and performance specifications in this document are for reference only.