This paper was presented at the CARTS-EUROPE '98 in Nizza DEVELOPMENTS ON LEAD WIRES - NEW MATERIALS AND REVISED SPECIFICATIONS -

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1 This paper was presented at the CARTS-EUROPE '98 in Nizza DEVELOPMENTS ON LEAD WIRES - NEW MATERIALS AND REVISED SPECIFICATIONS - Gerhard Bürstner (Dipl.-Ing. Dipl. Wirtsch. Ing. ) Eckhard Fröhlich (Dipl. Ing.) Feindrahtwerk Adolf Edelhoff GmbH & Co. Am Großen Teich 33 D Iserlohn, Germany Phone: Fax edelhoff@edelhoff-wire.de 1. SUMMARY Manufacturers of electronic components are looking for further developments of their products. Updated technical properties, higher reliability and lower production costs are the main topics for such developments. In regard to leaded components manufacturers have the potential to improve their processes and products by reviewing the technical data of present specifications or by implementing new materials for the design. This paper presents some recommendations concerning coated lead wires. Specific attention is given to the thermal stress of a component. In general: definitions of lead wire specifications should be done in cooperation with the wire manufacturer. The paper summarizes different types of lead wires gives some recommendations about specifications and equipment introduces new materials in regard to thermal aspects Finding the appropriate material and lowering the tolerances are essential key factors for improving

2 present processes. In regard to electronic component`s thermal stress Edelhoff FeNiCuSn wire, which can be tailormade according to manufacturer`s needs. As the copper plating can be varied, the heat transfer to and from the component can be influenced. Laboratory measurements of thermal conductivity were made with serveral different types of FeNiCuSn. The results are shown. In certain applications FeNiCuSn-material can provide better performance in regard to thermal stress than present available CCS products. 2. INTRODUCTION The quality-level of electronic equipment is closely related to the quality-level of its electronic components. Important key factors for optimal function and reliability are: specific component design and well defined specifications for those materials, being used for production. In regard to leaded components further technical developments took place. New specifications for lead wires often were set up by using existing specifications without taking the chance of reviewing them according to present technical standards. Therefore at first general ideas about wires, materials and specifications are given. 3. WIRE MATERIALS, SPECIFICATIONS AND RECOMMENDATIONS According to the technical data of the electronic component itself and according to the production process of the component, the wire has to meet different criteria. Depending on the material, important technical criterias are: Mechanical Properties Processability Electrical / Thermal Properties Solderability Concerning solderability, its coating technologies and properties, more detailled informations were already given by Edelhoff in previous CARTS papers [1] [2] Wire-Materials For electronic lead wire production the use of copper-wire is mostly common. But there is a variety of different copper-types as well as a variety of other materials, which are specificly used for lead wires. The following list shows examples: Copper E-Cu 58; Cu-OFHC; Cu-select-OFHC; CuAg0.1 Bronce CuSn0.6; CuSn5; CuSn6; CuSn8

3 Nickel Silver CuNi12Zn24; CuNi18Zn20 Brass CuZn15; CuZn20; CuZn30; CuZn36 Nickel Ni 99.8 Nickel-Iron Ni42Fe CCS (Copper Cladded Steel) CCS 30; CCS 40 Steel C4D; C20D 3.2. Requirements on Mechanical Properties Mechanical properties of the above mentioned materials can be found in specific literature[3]. Requirements on mechanical properties occur from the component`s production process itself, and from the special application, how and where the component is used. Quite often the specification is mainly made according to the end-use of the component, although slight modifications would meet the end-use parameters as well, and would bring significant benefits in the component`s productionprocess itself. Fig. 1 shows different types of leaded components. Fig. 1 Types of Bended Lead Wires Besides tensile strength and elongation there are other factors like: Surface Properties and Abrasion Strength which influence the production process of the component as well as its end-use [2] Recommendations on Wires Specific production-procedures for producing reliable components require appropriate specifications and appropriate equipment Specifications with Lower Tolerances Wires for electronic components are produced according to official standards (DIN, ASTM, BS, etc.), company standards and/or other specifications.

4 According to such standards you will find wire-related data like: Metallurgical Analysis Dimensions and Tolerances for Diameter Surface Conditions Tensile Strength and Elongation Electrical Properties Tin Plating Classifications Solderability Properties Lower tolerances of specific parameters quite often enable manufacturers to increase the yield of their process. This influences both: technical and economical factors as well. Examples for such reductions are: Wire Diameter from ± mm to ± mm Tensile Strength ± 20 N/mm² instead of ± 35 N/mm² Variation of Breaking Elongation within a Spool: ± 2,5% Reduction of the Helix-Phaenomenon Lowered tolerances have to become valid for total production lots and not only for specific spools within this lot. Therefore both - producer and wire supplier - should check present specifications. Together they have the ability to define, how present specifications should be revised in an appropriate manner Appropriate Equipment Wire should be produced according to manufacturer`s specific needs. Depending on the type of wire, or depending on the speed of the production-unit, where wires are produced, or depending on customer`s demands wires are delivered on spools, in drums or in pay-off packs. In regard to wire feeding the use of drums or pay-off packs define the minimum level. Wires on spools are prefered more and more due to their superior surface quality, better handling etc.. Further improvements can be achieved by using appropriate unwinding systems. Quite often special attention is given only to the equipment, which bonds the wire to the component. Minor attention is given to the right feeding of the wire to the machine. Brush-rings and flyers are a minor investment for feeding wires from spools. Motor-driven unwinding units should be state of the art by feeding wires to fast-running production units. Motor-driven, properly designed unwinding units enable manufacturers for instance, not to exceed specific tension within the wire. This becomes important, if the wire has to meet close tolerances within the component`s production-process. Regarding wires in general: specific attention should be given to the wire properties, wire packaging, the feeding and processing. All these factors determine the output of a component production unit - qualitywise and quantitywise. A close collaboration between wire producer and wire user will cover all

5 this Electrical and Thermal Conductivity of Lead Wires Lead wires have to fulfill several technical requirements: Conducting Current to the Passive Component itself Fixing the Component on the PCB Preventing a Heat Balance In this paper a major focus will be laid on the thermal conditions of lead wires. Electrical and thermal conductivity are related to each other according to the Wiedemann - Franz Law. According to that materials with high electrical conductivity have high thermal conductivity as well - and opposite. In room temperature there is a linear relation between electrical and thermal conductivity. Fig. 2 shows some data for different materials. Fig. 2 Electrical and Thermal Conductivity of Lead Wire Materials Heat Impact to Passive Components In general electronic components should be protected against heat stress. In reality there are several occasions, where heat occurs and where the component`s properties or even its reliability could be influenced: during the production process of the component itself - when the leads are connected to the body of the component during soldering the component onto the PCB during operation of the electronic device Welding Process

6 Fig. 3 shows the welding of leads to a capacitor. Fig. 3 Lead Wire Welding Two electrodes weld within a specific time leads to the capacitor. This is mostly done by using the resistance welding process. Depending on the type of capacitor, temperatures above 105 C (polypropylen film) or 140 C (polyester film) must not be exceeded. During this welding process the leads has a function as a heat sink by transporting heat away from the component The Soldering Process Components are connected to the PCB by different ways. SMD - and THT - technologies, wave soldering process and/or reflow process are state of the art for single or double sided PCBs. Fig. 4 shows the wave soldering principle. Fig. 4 Wave Soldering Process [4]. During this assembling process the leads have a function as a heat sink as well. Wires with lower thermal conductivity will keep thermal stress better away from the component than wires with higher conductivity. Wires with longer leads function more as a heat sink than those with short leads. In the very end a compromise in design / material / costs must be found again. Mechanical properties like mechanical strength and elongation have to be taken into account as well as electrical and thermal properties. Technical factors have to be seen as well as commercial ones.

7 Heat due to Energy Losses Heat due to energy losses cannot be avoided and therefore heats the component up during operation. Wires of leaded components can function as a heat sink again, because they lead the heat away from the component. The heat then is transferred by radiation away from the lead wire; a certain percentage of heat comes to the PCB. Wire-material, length of the leads etc. influences this heat balance again. 4. NEW MATERIALS IN REGARD TO THERMAL CONDUCTIVITY There are three main types of coated lead wires being used in the electronics: the copper wires the steel wires the copper cladded steel wires (CCS) Fig. 5 shows some comparisons in regard to specific factors. Wire Type Thermal / Electrical Conductivity Strength Price Copper ++ o o Steel CCS Fig. 5 Wire Characteristics 4.1. FeNiCuSn Wires In regard to conductivity, strength and price newly designed multiplated wires like FeNiCuSn could give superior properties. Fig. 6 shows FeNiCuSn wire with its different platings. Fig. 6 FeNiCuSn Wire

8 Concerning the design of the FeNiCuSn wire there are the following general ideas: Steel is used as core material due to its high tensile strength. Nickel is used as a barrier-layer with excellent adheasion to copper and steel. Copper is used due to its excellent conductivity - the plating thickness can be varied according to application needs. Tin is used as a layer coating due to its excellent solderability. Fig. 7 shows the figures according to Fig. 5. Wire Type Thermal / Electrical Conductivity Strength Price FeNiCuSn Fig. 7 Wire Characteristics of FeNiCuSn Fig 8. shows the physical properties of different FeNiCuSn wire types with copper layers between 2 and 10 micron. Fig. 8 Technical Data FeNiCu/02-10 Wires (q=20 C) The physical properties of the FeNiCuSn wires are between those from steel and CCS Thermal Conductivity of FeNiCuSn Wires Edelhoff produced a variety of different wire types - especially FeNiCuSn wires - and measured the thermal conductivity. Therefore the leads were dipped with a specific speed of 10 mm/s into a tin bath with a temperature of 235 C. After finishing the dipping process the temperature/time curves

9 were measured at a defined measuring point on the lead wire. Fig 9 shows the heat up diagrams of various wire types after a certain time and by using leads of 15 mm length ( Staku 30 corresponds with CCS 30, Staku 40 with CCS 40). Fig. 9 FeNiCuSn Wire with 15 mm Leads

10 Fig. 10 FeNiCuSn Wire with 20 mm Leads Fig. 11 shows in comparison the data for FeNiCuSn wires with 25 mm long leads.

11 Fig. 11 FeNiCuSn Wire with 25mm Leads Copper, CCS 40 (Staku40) and CCS 30 (Staku30) have the highest thermal conductivity. The thinner the copper layer is, the lower is the thermal conductivity. Therefore materials could be designed specificly according to the application. Lower or higher copper plating could become necessary due to thermal aspects or due to welding aspects, when the leads are connected to the component itself. The thickness of the copper layer has impact on the welding process and on the tinflow during this process. Component manufacturer and wire supplier should find the specific wire design due to the special needs. Others curves can be supplied showing the heating rate for specific leads with specific lengths and coating thicknesses, if the ends of those leads have contact with solder baths of 250 C. Fig. 12 gives an example for FeNiCuSn6 with different lead lengths x. Fig. 13 shows the 3 dimensional heat up diagram for FeNiCuSn6 as a function of time and lead length x by using a solder bath with 250 C.

12 Fig. 12 Heat Up Diagram for FeNiCuSn 6

13 Fig. 13 3D Heat Up Diagram for FeNiCuSn 6 References: [1] Bürstner, G. and Fröhlich, E., The Hot-Dipped Tinning Process For Lead-Free Electrical Connections; in CARTS-USA `97, pp. 174ff. [2] Bürstner, G. and Fröhlich, E., Electroplating versus Hot-Dipped Tinning - A Comparison of Application-Experiences, in CARTS-EUROPE `97, pp 17ff. [3] ASTM, EN, DIN [4] from Scheel, W. : Baugruppentechnologie der Elektronik, Verlag Technik, Berlin 1997, p. 284.