【作者】 徐波；

【导师】 钱乙余；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2006， 硕士

【摘要】 电子组装业有铅钎料禁用期限日益临近。行业内包括材料、设备、生产等各环节的厂商都在加快无铅制程导入的步伐。无铅化过程中,表面组装的焊接工艺至为重要,而随着熔点较高的新型钎料陆续应用,焊接过程的冷却速率也逐渐成为被关注点。无铅钎料熔点较Sn-Pb共晶提高30~40℃,焊接温度相应提高。炉温的提高对元件和电路板构成挑战,焊接出炉温度也相应提高,钎料液相线上时间相对延长。较快的冷速可以控制出炉温度,从而一定程度的控制焊点内部组织以及界面化合物的厚度,提高焊点质量。本文基于实际的回流焊生产工艺,研究冷却速率对无铅焊点质量的影响。主要研究两种无铅焊膏在不同冷速下焊点微观组织和力学性能的变化。实测冷速在-4℃/S~-6.5℃/S之间时形成的无铅焊点具有以下特点:微观组织细化,金属间化合物Ag3Sn和Cu6Sn5呈细颗粒状在钎料中弥散分布,使焊点断裂为韧窝断裂模式,可以起到类似复合材料的原位增强作用。在钎料和Cu盘的界面,化合物厚度较小,且呈大波浪形态,容易缓解应力集中的问题,焊点的力学载荷最大。快冷下界面IMC的强度较焊点内部钎料高,故焊点内部通常成为断裂面;当冷速小于-1.5℃/S时,组织粗化,内部Ag3Sn粗大而尖锐。界面的Cu6Sn5形貌尖锐,且厚度较大,焊点在力学测试时这成为裂纹萌生点,力学载荷最小。界面IMC的形貌和厚度对焊点强度起决定性作用,这也是冷速影响焊点强度最根本的原因。对于无铅回流炉,必须尽可能减小最后一个加热模块和冷却模块之间的距离。使强制冷却在焊点凝固以前起作用,这样才能真正提高焊点质量。更多还原

【Abstract】 The legislation to ban the use of Pb-based solders will become effective im-mediately, which provide a driving force for enterprises to accelerate Pb-free process. It’s found that reflow soldering plays an important role in Surface Mount-ing Technology ,moreover, cooling rate in reflow soldering profile is getting more and more attention after the use of high-melting-point solders.The melting point of Pb-free solder is 30℃~40℃higher than Sn-Pb eutectic solder. The increase of temperature in reflower becomes a challenge of Print Circuit Board (PCB) and components. As a result, the Time Above Liquid (TAL) of solder joints becomes longer, therefore, fast cooling in reflow soldering is used for controlling the PCBA temperature , improving the microstructure of joints and decreasing the thickness of intermetallic compound , consequently, high quality products can be obtained. How cooling rate affects the quality of soldering joints in lead-free process was studied in this paper. The experiments were based on practical industrial production and it focused on the effect of cooling rate on microstructure and mechanical properties.When cooled at -4~-6.5℃/S, the microstructure of joints were refined, the IMC of Ag3Sn and Cu6Sn5 phases dispersed in eutectic network in joints which presented spherical particles. The fracture of these joints after tensile failure presented dimple mode. Furthermore, the thickness of IMC was thin and it presented gentle incline morphology , as a result, it got high mechanical properties. Actually, the strength of IMC on the interface was higher than the inside of joints under fast cooling condition, so the inside of joints crack first ; On the contrary ; joints cooled at a speed less than -1.5℃/S got rather coarse microstructure, including coarse Ag3Sn phase and IMC on the interface. These joints behaved relatively weak mechanical properties.In a word, the thickness and morphology of intermetallic compound is the determinant factor of strength of joints which explains how cooling rate affects the mechanical properties of solder joints.It should pay much attention to the distance between the last heating zone and the first cooling zone of lead-free reflower. It should decrease the distance as更多还原