Patty was walking to her office as she saw Professor Ulf Gabrielson pass by. "What an impressive older gent, " she thought. He had been a contender for a Nobel Prize a few decades ago and still came into the office most days. He was spry 85 years old, or so.
在学术界，让帕蒂印象深刻的一件事是，人们对年长的教授几乎是敬而远之。少数 70 岁以上的教授全职从事教学和研究工作，本科生和研究生热切地向他们寻求技术、职业，甚至是个人事务方面的建议。
她和罗伯在艾毅大学的整个经历都很顺利。他们很快都将获得博士学位。现在，帕蒂就不会再为没有博士学位而当教授感到不自在了。罗布毕业后在常春藤大学找到了一份科学家研究员的工作。虽然这个职位的薪水比他在工业界的薪水低，但他也喜欢这里的学术氛围和灵活的工作时间安排。
走进办公室后，帕蒂坐下来回复邮件，看到一封来自迈克-麦迪根（Mike Madigan）的邮件。
信中写道："科尔曼教授，您和您的团队能否就无铅和锡铅混合焊料的组装制定一份建议清单？鄙人学生迈克。"
现在是学期之间的休息时间，帕蒂有了一些时间；而且这项任务听起来很有趣。她立即给罗布和皮特发了一张纸条，建议他们先做一些研究，然后过几天再聚在一起。
和往常一样，日子过得很快。"团队 "很快就来到了帕蒂的办公室。
"嘿，罗伯现在你是科研人员了，办公室比帕蒂好吧？"皮特调侃道。
"是啊，没错！我现在比举世闻名的科尔曼教授还要出名。"罗布反驳道。
"好的，好的！我们开始工作吧！"帕蒂笑着说，罗伯和皮特也跟着笑起来。
“What strikes me the most, in gathering information for this little assignment, is that, with all of the concern about the reliability of lead-free solder, so many people have assembled products with mixed technology and seem so cavalier about it,” Pete offered, seriously.
"我同意。"罗布说。
“Think about what a mixed solder alloy system this is. Four metals (tin, lead, copper and silver) in uncontrolled amounts throughout the solder joint,”Pete went on.
“So, you can’t really say what the melting temperature or mechanical properties are. They will vary throughout the solder joint,” Patty added.
“That’s all true, but we have to be a little practical. Many people have assembled with such mixed alloy systems with success – for a long time now,” Pete suggested.
“Look at this work by Bradley et al with NEMI on thermal cycling,” Pete continued.
三人看着皮特打印出来的 achart。
Figure 1.Bradley's data suggests thatmixed tin-lead and lead-freethermal cycle reliability is about equal to tin-lead solder reliability.
"不出所料，锡铅焊料比无铅焊料落后。但有趣的是，混合焊料和锡铅焊料的结果大致相同"。Patty 指出。
"数据显示了典型的情况。SAC 焊料的热循环性能更好。如果我们有跌落冲击数据，锡铅焊料和混合焊料可能会更好。
三人在短时间内查看了其他一些数据...
“I think we agree that, for non-mission-critical electronics, mixed alloy systems can work. But, I think we also agree that, for something like a fighter jet or medical device, it shouldn’t even be considered,” Patty stated.
罗布和皮特喃喃地表示同意。
"好吧，看来这些数据支持锡铅焊料和无铅焊料混合使用。但是，工艺如何呢？帕蒂问道。
"如果同时使用 SAC 焊料，我们就不能使用锡铅焊接温度，因为我们可能无法完全熔化 SAC 焊料。请看我找到的这些图片（见图 2）。
图 2.使用锡铅和无铅混合焊料时，应确保回流焊温度足够高，以熔化无铅焊料。
"哇！"皮特说。"看看这糟糕的晶粒结构！你不需要是冶金天才，也能看出这个焊点很薄弱"。
"我同意。但是，如果温度偏低，使用具有良好助焊剂的焊膏确实很有帮助。还记得马里奥-斯卡尔佐在这方面所做的工作吗？我有他的实验照片（见图 3），"帕蒂说。
Figure 3. Although advisable to have the peak reflow temperature higher than the melting point of the lead-free solder (typically 219C or so), a good flux can aid in the melting of the lead-free solder at lower temperatures. The result, on the right, is at 208C.
我记得那部作品。他在我参加的一次研讨会上做了报告。他能让 SAC305 焊球在 208 摄氏度的锡铅焊料中熔化。他使用了一种非常好的助焊剂来帮助这一过程，"Rob 补充道。
"Pete 补充说："正如我们以前看到的那样，首先要有好的焊膏。
帕蒂走到白板前做了总结。她写道
- Mixed lead-free and tin-lead electronics assembly can work for non-mission-critical applications.
- There should always be some discomfort as a four metal alloy system of varying composition is created.
- The resulting solder joint will have varying mechanical properties and melting temperatures
- 预期热循环效果比 SAC 焊料差
- 跌落冲击可能比 SAC 焊料更好
- The reflow profile should be such that the peak temperature is higher than the melting point of the SAC solder.
- As always, work with your solder paste supplier to obtain the best solder paste for this challenging application.
在他们都审阅了这份名单并认为没有问题后，皮特建议他们都去打九个高尔夫球洞。
干杯
罗恩博士
