【作者】 蒋礼；

【导师】 周孑民；

【作者基本信息】 中南大学 ， 热能工程， 2009， 博士

【摘要】 高密度电子封装集成化,使得焊点可靠性评估和检测更加困难,而封装无铅化,又给焊点可靠性研究带来新的问题。无铅焊点的可靠性问题中,热可靠性问题占了大部分,其热可靠性问题是源于热损伤。因此,对无铅焊点的热损伤研究成为热点问题。怎样检测无铅焊点热损伤是研究者面对的重要课题。本文是在热载荷条件下基于电测理论及方法的单个无铅焊点热损伤的基础研究。主要工作及结论如下：1、无铅焊点的热损伤理论研究本文描述单个无铅焊点热损伤电测方法的概念,并分析电测方法的优缺点和可行性。基于弹塑性热应力理论,结合无铅焊点的热载荷实际情况,分析了热对焊点的影响、焊点热损伤和失效的特征,针对焊点热损伤不同机理分析讨论了热蠕变和热疲劳损伤,将其受损截面和孔洞等效为有效损伤承载截面面积,基于能量守恒揭示了无铅焊点的损伤变量与剪切应变间的关系,为讨论损伤变量与电阻应变奠定了理论基础。从金属导体电阻的本质出发,结合固体裂纹等效理论揭示了电阻应变与裂纹扩展的定量关系。基于“有效承载横截面面积”模型和损伤力学导出无铅焊点损伤变量与电阻应变的关系,并进行了模拟实验。研究结果表明,不同载荷下的焊点的电阻应变特性反映了相应损伤变量的变化规律。通过损伤变量建立了焊点的电阻应变与焊点的热蠕变和热疲劳的塑性应变的定量关系,为实现在线电阻测量无铅焊点热损伤的方法提供理论依据。2、无铅焊点特性测试系统研制以无铅焊点电阻及电阻应变与焊点损伤变量的关系为理论依据,采用电子技术、单片机技术和电子测量技术,研制了通过监测无铅焊点电阻描述其损伤过程的测试系统。该系统由上位机(PC)、下位机(单片机)、控制和测试装置的软硬件、测试平台等组成。为了保证测试系统的精度采取了如下的措施：选择精密元器件,应用多数据采集平均后计数,改四探针法为五点差分法等,使得测量电阻精度达到了微欧级。将无铅焊点在热载荷下的温度、时间、电阻、电阻应变等参量,以曲线的方式显示在PC机的界面上,它们反映了焊点的热损伤状态和失效过程。自制温度控制仪,采用Pt100铂电阻作为测温元件,测量分辨率达到0.125℃；实现了电加热炉功率的调节,并辨识系统工作模式,自动记录循环次数。3、电阻应变描述的无铅焊点热损伤实验研究在无铅焊点的恒温剪切蠕变实验中,室温(25℃)和高温(125℃)实验结果表明,两者的电阻应变特性曲线均可分为减速应变区域、线性(稳定)应变区域和加速应变区域,分别对应金属蠕变损伤的第一阶段、第二阶段与第三阶段；电阻应变反映了焊点的热损伤,只需测试焊点在一段时间内少数几个时刻的电阻应变,便判定服役焊点所处的损伤程度；两者不同之处在于高温时的电阻应变大于室温时的电阻应变,表示高温时焊点的损伤大于室温时的损伤,并且使用寿命较室温时短。在无铅焊点的热疲劳实验中,温度范围从40℃到125℃,热循环加剪切应力蠕变疲劳实验结果表明：电阻应变变化过程反映焊点的热疲劳损伤过程；在循环温度范围内,高、低温端的不可逆电阻应变趋势相似,也有减速变化区域、线性(稳定)变化区域和加速变化区域,很好地反映了热蠕变的变化特性,但高温的不可逆损伤略强于低温的不可逆损伤；对焊点在循环的断裂前一两个循环周期研究,发现失效均发生在循环上升阶段。实验结果与理论关系吻合,证明此方法适合作为无铅焊点可靠性检测方法。4、基于电阻应变的无铅焊点热损伤与失效检测应用研究应用电测无铅焊点损伤理论,探讨了无铅焊点厚度的尺寸效应,温度与电阻应变迟滞回线和焊点失效判据,结论如下：(1)无铅焊点厚度的尺寸效应在剪切蠕变条件下,通过试验和有限元仿真的方法,对横截面积为1mm2的不同厚度的矩形焊点(Sn3.5Ag)研究。结果表明,当焊点厚度为0.25mm时,不仅电阻应变最小,而且蠕变性能最优。该无铅焊点的厚度尺寸效应,为其制作工艺提供了理论依据。(2)温度与电阻应变迟滞回线在无铅焊点的热蠕变疲劳中,通过分析和研究温度与电阻应变的关系表明：它们具有类似于材料力学的应力应变迟滞回线的特性,该迟滞回线反映无铅焊点的损伤变化和积累程度,可逆电阻应变范围对应于可逆损伤变化,不可逆电阻应变范围对应于不可逆损伤积累,焊点的失效取决于不可逆损伤积累程度。温度与电阻应变的迟滞回线为无铅焊点热损伤的检测提供了新的检测方法。(3)无铅焊点的失效判据对大量无铅焊点热载荷下的电阻应变特性进行了分析研究,结果显示,每条电阻应变特性曲线均存在线性区域与加速区域的临界点,该临界点与热损伤变量的临界点对应,试验测得的临界电阻应变是0.05左右,过临界点之后焊点热损伤急速积累很快断裂失效,加速变化至失效的时间约占焊点使用寿命的20%-30%；40℃的电阻应变曲线临界点滞后于125℃的电阻应变曲线临界点,滞后时间约占焊点使用寿命的7.5%；在实际检测中临界点作为失效的判据；这些技术参数丰富了无铅焊点的可靠性检测内容。更多还原

【Abstract】 Because of the high integration level of electronic packaging, the reliability of solder joints is difficult to evaluate. Meanwhile, lead-free packaging brings new problems to the research of this topic. For lead free solder joints, thermal reliability is a major problem due to root thermal damage. The measurement of this thermal task has become a crucial issue nowadays.This project is focused on the study of single lead free solder joints (the specimen length and width 1.0mm, SnAgCu and SnAg solder paste), by detecting micro-resistance change through thermal load electrical measurement. Key conclusions as follows:1. Theoretical research of lead free solder joints thermal damageThe electrical test method for singular lead free solder joint was described, and analyzed in this dissertation. Based on elastoplasticity thermal stress and the actual thermal load, several features of lead free solder joints have been studied, including the thermal damage and fatigue property. Among different mechanisms of solder joint thermal damage, the thermal creep and fatigue damages were discussed; the damage cross section and voids were equivalent to damage cross section. The relationship between the damage variable of lead free solder joints and shear stress-strain was revealed, which established the theoretical model for damage variable and resistance strain.By nature of metal conductor resistance, according to solid fracture theory, the quantitative relationship was revealed for the resistance strain and the crack expanding. Founded on "effective bearing cross section surface" model and Damage mechanics, the formula between damage variable and resistance strain was derived for lead free solder joints. This formula were analyzed and discussed under different thermal load. Generally, the quantitative relationship was set up by damage variable for resistance strain of solder joint and plastic strain of thermal creep and fatigue. Theoretical foundation has been laid down for damage variable and resistance strain, which support the design of using resistance to survey thermal damage of lead free solder joints.2. Test system for lead free solder jointsBased on previous knowledge, the test system for lead-free solder joints has been manufactured using single-chip computer and electrical measurement technology; the overall damage process was monitored by detecting resistance change of lead-free solder joints. In order to ensure accuracy of the test system, several measures were adopted, such as precision electron components, data averaging, improved five probes difference method. Physical variables of lead-free solder joints were displayed on PC screen in temporal curves, as temperature, resistance, resistance strain, which indicate the property of fracture mechanism and resistance strain under various thermal loads. Self-regulating temperature control apparatus, using Pt100 platinum resistance as the component of measurement temperature, increases the resolution up to 0.125℃; the power of resistor oven was adjusted by the software, the system work model was designed, and the cycle numbers were recorded automatically.3. Investigation of thermal damage of lead free solder joint with resistance strainIn shear creep experiment of lead free solder joint with ambient temperature, the resistance strain curve of solder joint was divided into three sections (deceleration, linear and acceleration) which correspond to the first, second and third stage of creep failure. Therefore, the damage degree of solder can be determined by the resistance strain of temporal checkpoints in stretchIn shear creep experiment of lead free solder joint at high temperature, the change of electrical resistance strain generally follows the creep discipline. The process trend curve of high temperature shear creep is similar to ambient temperature. However, the resistance strain of high temperature is larger than that of ambient temperature, which indicates a more serious damage degree. Consequently, the viable time of solder joint at high temperature is shorter than ambient temperature. In addition, as far as fracture surface of solder joint was concerned, a let of cracks were shown on the broken surface of solder joint.In the thermal circulation and shear creep experiment of lead-free solder joint, the varying property of thermal creep fatigue was successfully represented by resistance strain of lead free solder joint. Few cycles before breaking in the thermal circulation process of lead free solder joint were closely investigated. The broken fracture occured in rising phase of circulation, and the range was found. Under situation of serious damage, such as cycling down, the damage degree recovered properly, the broken fracture happened in next rising circulation.The theoretical relationship of is verified by the experimental results in the investigation. The method described is an effective approach as detection method for the reliability of lead free solder joints.4. The reliability application research of lead free solder joints with resistance strainTo apply electrical measure damage theory of lead free solder joints, the thickness effect, the hysteresis loop between temperature and resistance strain and the failure criterion of lead free solder joints were investigated, the conclusions as follow:(1) The thickness effect of lead free solder jointIn shear creep, the highness was investigated for 1mm2 rectangle Sn3.5Ag solder joint, the quantitative relationship between resistance strain and thickness of solder joint shown, when the thickness is about 0.25mm, its resistance strain is the least; meanwhile the creep property is top-notch, which represents the highest reliability of lead-free solder joints.(2) The hysteresis effect between temperature and resistance strain for leadfree solder jointIn thermal creep fatigue, the property of hysteresis loop between temperature and resistance strain for lead-free solder joints is similar to that for material mechanics. This property reflects the actual damage status of lead-free solder joints, where the irreversible resistance strain range matches the irreversible damage accumulation;(3) The failure criterion of lead free solder joint There are long-drawna lineation change zone and accelerated non-linear exponential change zone for the resistance strain curve of lead free solder joint. The period from exponential change to failure is about 20%-30% of the total useful time of lead free solder joints, the critical point of resistance strain matches along with the critical point of damage. The failure develops rapidly after the point. The curve critical point of resistance strain at 40℃lags behind at 125℃, the hysreresis time is about 7.5% of the total useful time. It is because the plastic strain happened primarily in holding high the temperature, the solder joint is more easy damaged in high temperature zone.更多还原