连接器设计指引课件.ppt

INTRODUCTION TO CONNECTOR DESIGN FUNDAMENTALSSection 1:Contact Interfacel Surface Roughnessl Electrical Propertiesl Mechanical PropertiesSection 2:Engineering Materials l Plasticsl Metalsl PlatingsSection 3:Contact Normal Force and Contact Geometry l Relationship to Hertz stressl Effects on contact performanceSection 4:Connector Design Overviewl Engineering approach to design processl Customer and manufacturing requirementsl Design FMEAl Concept development approachSection 5:Connector Testingl Purposel LLCR measurementl Common types of tests and purposesSection 6:Recommended Areas for Further TrainingSection 1:Contact InterfaceThe first 3 sections will deal with some of the basic underlying theory for connectors.The contact interface is the heart of the connector,so we will look at this area first.In order to understand what happens at the contact interface,it is necessary to first look at the contact surface from the microscopic point of view.The surface is not a smooth surface,regardless of whether it is on the rolled surface of the material or the stamped edge.An analogy between the contact surface and a hilly terrain can be used to help visualize the topology of the contact surface.The contact surface is made up of a numerous hills and valleys.The high points of the hills where electrical contact is made with the opposite mating surface are called asperities or a-spots.a-spots where electrical connection is madecontact acontact bSection 1:Contact InterfaceThe current flowlines must bend and constrict to go thru these a-spots.Thus,the terminology constriction resistance is used for the resistance resulting from the current having to flow only thru the a-spots when it reaches the mating interface.This is often also called contact resistance,but contact resistance is actually constriction resistance plus the resistance of any thin oxide films or other contaminants at the interface.An analogy might be to think of it as a large group of people moving down a hallway and then suddenly encountering a small exit to leave the building.There will occur a bunching up people at the entry way,reducing the flow of people(i.e,restricting the number of people who can pass thru this exit area at one time.current flowlinesSection 1:Contact InterfaceThe formula for constriction resistance,Rc,for a single a-spot isRc =kr r/awhere r=resistivity of the materiala=contact spot diameterk=a parameter dependent on the mode of deformation and geometric factorsAdditionally,the variable a is related to hardness,H,and normal force,FN,in the following mannera (FN/H)1/2 Thus,as normal force increases or hardness decreases the a-spot size increases.In actuality,there are numerous a-spots at any contact interface,so the constriction resistance is actually a combination of a macroconstriction determined by the overall distribution of contact spots and a parallel resistance of microconstrictions due to the individual spots.Thus,Rc becomes Rc=RM +Rm =r r/D+r r/nae whereD =diameter of the distribution of contact spots(apparent contact area)ae=effective size of the individual spotsSection 1:Contact InterfaceHowever,it has been shown that a distribution of contact spots behaves in the same manner as a single spot of diameter,De,where De is an effective diameter that represents the total a-spot area.Thus the constriction resistance can be represented by the formula:Rc =kr r/DeSo what does all this tell us in a nutshell?Basically that there are 4 factors that significantly effect the value of constriction resistance:1)resistivity of the material2)normal force3)hardness of the material4)geometry of the contact areaHowever,with contacts there is another factor that must be considered.Contacts are for the most part always plated.So how does this effect the previous discussion?Well,since the plating thickness is very small in the 15 to 200 microinches,we would expect that the bulk of the constriction to occur in the base metal and the deformation of the a-spots to occur in the plating.base metalplatingSection 1:Contact InterfaceThus,we would expect the constriction resistance to be a function of the resistivity of the base metal and the hardness of the plating.For example,tin plated brass has a much lower constriction resistance than pure brass or pure tin(note:the resistivity of brass and tin in Wcm x 104 are 7.5 and 16.9,respectively,and the hardness in lbs/in2 are 172,000 to 185,000 and 18,000,respectively).Also,note that as the thickness of tin increases the constriction resistance also increases,due to some of the constriction occurring in the tin,instead of the brass.However,what we are really interested in is contact resistance,which is made up of both constriction resistance and film resistance,as mentioned earlier.Film resistance is a function of the type of plating used and environmental exposure.This discussion will be postponed until Section 3,when engineering materials are covered.Section 1:Contact InterfaceWe will now move into a more detailed discussion of the mechanical aspects of the contact interface.The mechanical aspects are concerned with friction and wear.Friction and wear are actually two different ways to describe what happens at the a-spots when the interface is disrupted by an applied stress.The discussion will focus on a single a-spot,since the argument can easily be expanded to multiple a-spots.Friction,as we know,is the force that opposes the relative motion of two surfaces in contact under a shear stress.Ff =m Fnwhere Ff=friction force,m=coefficient of friction,and Fn=force holding the two surfaces together(in the case of the connector this is the contact normal force)For contacts,the coefficient of friction can range from 0.05 to 1.Low values of coefficient of friction reflect situations where the contact area is covered by some chemically bonded film(such as oxides),adsorbed films(such as water or organic),or intentionally applied lubricants.High values of coefficient of friction reflect the effects of plastic deformation of the a-spots and the creation of metallically bonded junctions of higher shear strength than the base metal.Shear forces in this latter case result in fracture occurring away from the surface,resulting in wear debris.Section 1:Contact InterfaceWhat happens at the a-spots is that some them can plastically deform under small loads due to their physical size,which results in localized work hardening and cold welding(creation of a bond between two metallic surfaces that occurs when they are in intimate contact).When the cold welding joint is stronger than the cohesive strength of the base metal,fracture occurs away from the junction,resulting in a wear particle and metal transfer.If the plastic deformation is not as severe,less work hardening and cold welding will occur and the disruption of the junction will occur at or near the surface,i.e.little wear or metal transfer occurs.In the picture below“a”represents the first case and“b”represents the second case.a:large plastic deformation at a-spotb:small plastic deformation at a-spotwear particle(adhesive wear)No appreciable wear(burnishing wear)Section 1:Contact InterfaceNow if the transferred metal particle at“a”breaks loose there is a third wear mechanism that occurs,which is called abrasive wear.These broken particles are harder due to work hardening and thus will gouge at both contact surfaces during movement,resulting in increased wear rates at the contact surface.The coefficient of friction value,along with the lead-in geometry,affects the connector mating force and the wear characteristics(burnishing,adhesive,and abrasive)affect the durability of the connector contact interfaces.In summation for this section,we have found out that surface roughness,contact normal force,and the geometry of the surfaces in contact are the key parameters effecting the contact interface.Surface roughness effects the number and size of a-spots created.The normal force effects the total contact area at the interface,and geometry effects the area over which the asperities will be distributed.Section 2:Engineering MaterialsThere are 3 basic materials used in connector design:engineering plastics;conductive,non-ferrous,spring metals;and plating material,both noble and non-noble to coat the metals.The first materials we will discuss are the engineering plastics used in connectors.PlasticsPlastics are one of the basic components used in making connectors.They are used over other materials,because they are reasonably high strength,dimensionally stable dielectric material that can be molded into intricate,thin walled shapes at a low cost.Plastics can be divided into two groups:thermosets and thermoplastics.Thermosets are plastics that take a permanent shape after heating due to cross-linking of molecules.Thus,thermosets cannot be recycled as additional heating will cause the plastic to degrade rather than soften or reflow.An analogy would be to compare thermosets to hard boiling an egg.Once an egg has been hard boiled,additional heating will not return the egg to a liquid state,but only cause it to burn if enough heat is applied.Thermoplastics,on the other hand,harden at lower temperatures and soften and become moldable at high temperatures.Some degradation will occur when thermoplastic is recycled,but generally most thermoplastics can be recycled without any serious consequences,if mixed as a percentage with virgin material.An analogy to butter could be used to explain thermoplastic behavior.As the butter is heated it softens and allows you to mold it into a different shape.When it cools it again becomes hard.This process can be repeated,with only minor degradation unless the butter is overheated.This difference in behavior between the two groups is the main reason why thermoplastics are the predominant group used in the connector industry.Section 2:Engineering MaterialsPlastics(continued)It is important to realize when designing with plastics that their properties do not allow them to be treated as inert,isotropic,rigid material.Additionally,there are different classes within the thermoplastic group,each with distinct structures that effect their mechanical and physical properties.Thermoplastics can be grouped into 3 classes by their polymer structure:amorphous,crystalline,and liquid crystalline.Polymer chains without any organized structural regions are known as amorphous.Typical examples are ABS,polycarbonate,PVC,and polystyrene.Polymer chains that have regions of organized structure,which behave like crystals,connected by amorphous regions are known as crystalline plastics.Typical examples are nylon,polyester(PBT,PET),polypropylene,and polyethylene.Finally,polymer chains that are stiff,rod-like structures that are organized in large parallel arrays in both the melted and solid states are known as liquid crystalline polymers(LCP).Section 2:Engineering MaterialsPlastics(continued)Some of the property differences between the 3 classes of plastics are shown in the following two tables:AmorphousCrystallineLiquid Crystallineflexible,weaker,less impactstiff,strong,impact resistantstiff,strong,impact resistantresistant,soften gradually&flow easily,high melt tempsoften gradually&continuouslycontinuously resistant to heat,creep,&flow easily,high melt temp,chemicalsresistant to heat,creep,&chemicalslow viscosity,warpage,&shrinkageSection 2:Engineering MaterialsPlastics(continued)General relative polymer propertiesPropertyAmorphousCrystallineLiquid CrystallineSpecific gravitylowerhigherhigherTensile strengthlower higherhighestTensile moduluslowerhigherhighestDuctility,elongationhigherlowerlowestResistance to creeplowerhigherhighMaximum usage templower higherhighShrinkage and warpagelowerhigherlowestFlowlowerhigherhighestChemical resistancelowerhigherhighestSection 2:Engineering MaterialsPlastics(continued)The physical and mechanical properties of these plastics can also be modified with the addition of fillers,fibers,and other chemical compounds.When the mechanical properties are improved,the modified plastic resin is called a reinforced resin.When the additive does not significantly improve the mechanical properties,but does affect the physical nature of the material,the modified plastic resin is called a filled resin.Some properties of plastics that should be remembered during the design process are:1)Under loading,plastic material will creep(geometry change under fixed load).The rate of creep increases nonlinearly with increased temperature.2)Plastics shrink as they cool and the shrinkage rate varies with the type of plastic.Wall thickness,flow direction,and molding conditions will all effect the amount of shrinkage that occurs.Bow and sink marks occur due to some sections cooling slower and shrinking more than other sections.3)Most plastics used in the connector industry are reinforced resins.Glass fibers are the normal fill material used.Thus,fiber orientation in the part is important as the strength of the part will vary greatly with orientation.4)Knit lines form where two fronts of flowing plastic meet.These areas are not as strong as other areas,so it is important to control the location of knit lines.5)Regrind material can be remolded with virgin material.Since there is some degradation of material properties,it is important to determine the acceptable amount of regrind.Section 2:Engineering MaterialsPlastics(continued)6)Plastics are affected by various solvents.Some plastics are more solvent resistant than others,so it is necessary to determine what solvents the plastic will be exposed to in its expected application in order to determine acceptable plastics that can be used in the design.7)Plastics also absorb moisture to varying degrees.Water absorption will affect mechanical and electrical properties,as well as,dimensions.8)Plastics vary in their degree of thermal stability.Some plastics change dimensionally more than others at higher temperatures.Some lose their physical properties faster with increasing temperatures.The temperatures at which these changes are significant can often be in the range of temperature the connectors may see either during processing,transporting,or in operation.Thus,this should be carefully considered during the design process.9)The molding process leaves internal stresses in the plastic part.These stresses may result in warpage,not present in the as molded condition,occurring in the part during later assembly operations,such as solder reflow.Thus,it is important during the design process to consider if these possibilities exist.Section 2:Engineering MaterialsPlastics(continued)A few basic rules to remember when designing with plastics are:1)Avoid undercuts whenever possible.Keeps the mold tooling simpler and more cost effective.2)Sharp corners should be avoided.Sharp corners,particularly interior corners,cause poor flow patterns,reduce mechanical properties,increase tool wear,and increase molded-in stresses.If possible,inside radii should be equal