2007年8月1日CONNECTOR DEGRADATION MECHANISMS连接器失效(退化)机构目录6.连接器失效(退化)机构6.1简介6.2腐蚀A腐蚀对接触电阻的影响B腐蚀膜的形成1.环境腐蚀2.毛孔腐蚀3.腐蚀爬行4.接触腐蚀C腐蚀与对应电镀系统的关联1.金2.钯3.钯-镍4.银5.锌-铅&锌6.镍D防止腐蚀策略1.名贵电镀系统a.增加电镀厚度b.加润滑剂c.毛孔堵塞d.喷金e.环境防护(隔离)f.从开始和整个使用期加足够常态力6.1简介Intrinsic degradation mechanisms 内在(固有)退化机构Active degradation mechanisms 活性退化机构failure criteria失效标准6.2腐蚀A.腐蚀对接触电阻的影响Rcontact= Rconstriction+ RfilmEquation 6.1.6.2腐蚀B腐蚀膜的形成环境腐蚀主要是空气中的成分与金属的化学反应. 接触腐蚀是导体接触界面微运动的结果.1.Environmental Corrosion环境腐蚀Environmental corrosion is primarily an issue with noble metal plating systems, especially in the case of pre-plated stock.noble metal finishes were described as a system which consists of a preciousmetal surface, a nickel underplate and the contact base metal. Each performs differentfunctions. The precious metal surface, in particular gold, provides an inherently corrosionresistant surface. The nickel underplate performs several functions to ensure that the nobilityof the surface plating is retained. 详细重复如下:reduce sensitivity to porosity减少多孔性reduce corrosion migration减少腐蚀移动reduce diffusion of base metal constituents减少基材成分扩散improve contact finish durability提高接触表面的耐久性(寿命)With the exception of gold, environmental corrosion may occur directly on the surface plating,even though the surface plating is a noble metal. While gold does not corrode, other noble platings, primarily palladium and palladium-nickel alloy, are corrosion resistant, but not fully corrosion proof.6.2腐蚀除了表面腐蚀,还有打底电镀层腐蚀和基材腐蚀.很多时候打底电镀层腐蚀和基材露在外面,如表面电镀质量不好或镀得薄.还有边缘,边界棱角的暴露. 以及电镀前在放置时的暴露受到腐蚀.磨损同样暴露打底电镀层或基材.根据腐蚀流程的动力学原理可将环境腐蚀分为孔腐蚀和腐蚀爬行.孔腐蚀中腐蚀部分只在腐蚀起始位置. 腐蚀爬行中腐蚀会从起始扩大到其它位置.2.Pore Corrosion孔腐蚀Pore corrosion occurs in situations where small openings in the surface plating allow corrosiveelements (e.g. oxygen, chlorine, nitrogen dioxide, hydrogen sulfide, sulfur dioxide) in thesurrounding atmosphere to come into contact with the underplate and/or base metals. Oncethe opening is filled with corrosion products, no further corrosion will occur. See Figure 6.2.Pore CorrosionFigure 6.2.6.2腐蚀A film of corrosion may exist in the immediate area around the initiation site, appearing as rings around the initiation site, However, if the dominant mechanism is pore corrosion, the films beyond the initiation site will be relatively thin. Figure6.3 is an example of a surface exhibiting pore corrosion as the dominant environmental corrosion mechanism.Figure 6.3.Surface of a gold/nickel/phosphor-bronze coupon after exposure toa Class NOTE ass II MFGenvironment exhibiting porecorrosion as dominant mechanismwith sometransition to corrosioncreep evidenced by thin film “rings”surrounding the initiationsite.The effect of pore corrosion on interface resistance depends on whether or not the contactarea occurs within an area containing pore corrosion products and on the ability of the contactto penetrate the corrosion film if it does exist in the contact area. The ability to penetrate the corrosion film depends on the normal force, contact geometry, and wipe characteristics.3.Corrosion Creep腐蚀爬行This degradation mechanism can cause a large area to become covered with relatively thick corrosion films.Figure 6.6.Note that the corrosion creep is most severe on the gold plated sections andless severe and in some cases non-existent in the nickel and tin plated areas. It is a characteristic of corrosion creep that migration of the corrosion products is accelerated on noble surfaces.4. Fretting Corrosion 接触腐蚀Fretting corrosion is primarily an issue with non noble plating systems and unplated contacts. It may also occur in plating systems incorporating thin (less than 10 microinch) noble platings, as the surface plating may be quickly worn away leaving the underplate or base metal as the primary contact surface.Figure 6.8.Fretting corrosion resulting fromcyclic micromotion of contactinterface NOTE ace on nonnoblesurfaces.Figure 6.9.At some point, typically between 500 and 1000 cycles,the normal force becomes insufficient to consistently penetrate the oxide layer, and resistancewill rise quickly with additional cycles.C. Corrosion Concerns with Specific Plating Systems腐蚀与对应电镀系统的关联1.Gold金金本身完全不受腐蚀．Gold provides a true noble surface, Any variation in contactresistance due to corrosion of gold plated contacts is due to exposure of the underplate or base metal.The intrinsic degradation mechanisms associated with gold are porecorrosion and corrosion creep. As stated, these mechanisms can become active only if underplate or base metal is exposed. 可由以下原因引起:*多孔性(特别薄时) *边缘暴露*非(名贵)电镀相邻区别*磨损Note:镀金层薄,打底层或基材暴露,接触磨损发生．２．钯钯虽不完全不受腐蚀,但极难被腐蚀. 因此它引起腐蚀的原因和金相似.只是它还容易有小裂纹. *多孔性(特别薄时) *边缘暴露*非(名贵)电镀相邻区别*磨损*产生小裂纹(露下层)3.钯-镍钯-镍(一般80%钯20%镍)可防止微裂纹但防腐蚀性没纯钯好.*随着镍比例增加防腐蚀性变差----Æ非名贵电镀**多孔性(特别薄时) *边缘暴露*非(名贵)电镀相邻区别*磨损4.银Silver tarnishes rapidly in most environments. the primary corrosionconcern is the corrosion of the silver itself*与普通污染物容易反应产生腐蚀层.*多孔性(特别薄时) *边缘暴露*非(名贵)电镀相邻区别*磨损5.锌铅和锌非名贵电镀.容易产生氧化层.但氧化层产生能很好防止下面的金属继续被氧化.主要腐蚀是接触腐蚀.6.镍非名贵电镀.容易产生氧化层.象锡,氧化层产生能很好防止下面的金属继续被氧化.但镍氧化层很硬,不象锡那么软.主要腐蚀是接触腐蚀.它做打底层很好.但一般不做表层,除非已决定表层用大的常态力.D防止腐蚀策略1.名贵电镀系统a.增加电镀厚度.但费用贵b.加润滑剂.可以覆盖暴露的打底或基材层,还可以减少摩擦因而减少磨损.还可以防止接触腐蚀,薄金电镀易产生接触腐蚀c.毛孔堵塞d.喷金(由于薄,只能存在于前若干次插拔中)e.环境防护(隔离) . 插配好的端子远少腐蚀于非插状态. HOUSING防护(隔离). Receptacle or socketcontacts often are far less affected by a corrosion environment than pins.f.从开始和整个使用期加足够常态力. 可更好穿过既有腐蚀层,又可防止微运动,防止接触腐蚀.g.周期(赫兹)压力2.非名贵电镀系统a.加润滑剂. prevent fretting corrosionb.机械顺从. 减少微运动可能.Figure 6.13 illustrates a design incorporating complianceto reduce the likelihood of fretting motions. Small tangentialforces being applied to the contact interface will beabsorbed by the horizontal spring.c.高的常态力Prevented Fretting motions3.镍打底镍打底能有效提高名贵电镀或镀锌表面的防腐性.*减少基材暴露可能,镍比基材防腐性好. *限制腐蚀爬行,使暴露基材的腐蚀不易爬到名贵电镀面. *提高电镀层硬度,减少磨损.*在基材与镀层间作为障碍物,防止基材因金属化合作用而扩散到电镀面降低电镀面防腐性A永久装置卸荷时原点偏移-------因为负荷超出弹性极限,金属弹性变形后再产生塑性变形. While a given deflection will induceplastic deformation in a contactbeam, so long as the deflectionfrom the original (pre yielded)rest position of the beam is thesame, the normal force will remainthe same. This can be seen inFigure 6.14.Figure 6.14 illustrates an example in which the design deflection is intended to be 0.010 inch, at which point the stress within the spring is within the elastic limit. The design normal force at this deflection is 100 grams. However, during mating the contact spring is displaced 0.015 inch, which induces stress beyond the elastic limit of the spring material, and the spring deforms plastically. If the final displaced position remains at 0.015 inch from the original rest position of the spring (i.e. before it was plastically deformed), the normal force will be at the level attained when the spring was first deflected to 0.015", which is 130 grams in this example. This will be true for any subsequent mating which results in a 0.015" deflection from the original rest position and which does not induce further yielding.•B应力释放(松弛)应力释放------通过金属内部缺陷结构的重新组合,弹性应力转化为塑性应力. 例如双臂端子,加荷一段时间卸荷后,间隙会增大.应力释放率随着温度的上升而升高. 应力释放率-------单位时间内的应力释放.它与常态力也有关系.C常态力丧失的结果常态力控制接触界面实际接触面积的数量.As the area in contact decreases, resistance will increase. Equation 6.3. relates interface resistance to the contact area.R = k p {(1/nd) + (1/D)}Equation 6.3.R = Interface resistancek = Constant that is a function of materials usedn = Number of asperities微观(不可见)接点数d = Average asperity area微观(不可见)接点平均面积D = Apparent contact area接触[表观]面积常态力上升, n上升,d上升.刚加载,n,d很小, 1/nd大,起主导作用,下降快,电阻下降快;到一定载荷时, 1/nd很小.1/D主导,电阻趋向常数.卸栽时,由于asperities应力释放,弹性变形小,常态力显著减少,电阻趋向稳定,并不显著上升.只到将要界面分离时,电阻突然跃升.Conclusions to be drawn about the effects of loss of normal force, based on the contact Interface model and supported by the heat age study, are as follows:–Loss of normal force, unless quite substantial, has little direct effect on resistance.–Loss of normal force will allow a contact interface to be more easily displaced.–If displaced, resistance will increase substantially only if the interface is re-established in a region with a non conductive film.Based on these conclusions, the primary concern with loss of normal force is that stresses Which may not have induced motion at the original normal force levels will now produce motion at the interface. These stresses are typically temperature cycling and vibration. Once motion is initiated, if corrosion exists outside the contact area, a shift into the corroded area will obviously cause an increase in resistance. Another concern is that the motion itself can induce fretting corrosion and subsequent increases in resistance.D防止常态力丧失的对策1.永久设置*设计时加大弹性余量.*加入可调或导向设计,如斜坡,倒角.2.应力释放(松弛)不能避免,只能从选材考虑提高. 或是选用好的电镀表面减少所需的最少常态力.把磨损作为退化机构不是基于磨损本身,而是因为磨损破坏电镀层.A磨损机构分为缓和(适量)磨损和磨伤磨损.对于一个既定界面,是缓和(适量)磨损还是磨伤磨损取决于常态力.常态力小于某一值,为缓和(适量)磨损. 当大过一定值时,就变为磨伤磨损.两个表面的(相对)硬度影响磨损程度. 破裂(裂纹)浅,只是接近表层,是缓和(适量)磨损;破裂(裂纹)深,渗入到表层底下,是磨伤磨损.B影响磨损的因素*常态力上升,磨损上升*表面硬度上升,磨损下降*表面覆盖物有助磨损下降. *表面粗糙度*几何形状与移动距离.C磨损的设计考虑减少磨损的设计,往往会有其它负面影响. 如减少常态力,机够不稳定,接触磨损增加,及穿透腐蚀膜更难. 设计成平壮界面,难穿透腐蚀膜及排除杂质微粒. 因此应平衡考虑.D防止磨损的对策*加适量润滑剂*加厚电镀层＊提升表面硬度，加镍打底．6.5其它磨损机构A金属B聚合物C润滑剂Thank You!。
