GOALI: Assessment of Solder Joint Quality in Microchip Packages Used in Printed Circuit Boards by means of Laser Generated Sound Waves

微芯片的设计和制造是一项价值数十亿美元的业务。几乎没有任何电子设备不包含微芯片。这些微芯片通过隐藏在芯片和电路板之间的焊点与印刷电路板互连。通常情况下，这些焊点具有难以检查的制造缺陷，因为它们被隐藏在视图中。这些缺陷必须在电路板完全组装之前及早发现。然而，传统的检测工具来检测这些缺陷是非常不令人满意的。该奖项旨在支持基础研究，以提供知识，从而能够开发出上级现有工具的低成本和高效的检测工具。这种新工具有可能（1）显著提高芯片的产量，（2）缩短新产品开发周期，（3）降低使用芯片的设备的成本。这一工具的商业化将创造就业和财富，并为各国政府带来税收收入。研究目标是建立激光阵列束产生的超声波强度与束中光纤数量、光纤长度以及每根光纤携带的激光能量之间的关系。另一个目的是建立每个焊接缺陷（如裂纹，开放，枕中头和焊盘凹坑）对所产生的超声波的影响（在接收到的时域信号的能量方面）。为了实现这些目标，将开发一种有限元分析方法，并用于预测当纤维数量、纤维长度和能量密度变化时由阵列产生的超声波（纵向、剪切或兰姆波）的强度。模型预测结果将用于指导在芯片中产生超声的阵列子系统的实验设置。测量系统将包括阵列子系统、将激光能量馈送到阵列束中的脉冲激光器以及用于拾取芯片表面上的振动信号的激光干涉仪。由干涉仪在测试芯片表面上的不同点处拾取的振动信号将与从无缺陷芯片获得的参考信号相关，以确定焊料或芯片缺陷的存在。

The design and manufacture of microchips are a multi-billion dollar business. There is hardly any electronic device that does not contain microchip. These microchips are interconnected to the printed circuit board with solder joints which are hidden between the chip and the board. Often times these solder joints have manufacturing defects which are difficult to inspect because they are hidden from the view. These defects must be detected early before the board is fully assembled. However, traditional inspection tools to detect these defects are very unsatisfactory. This Grant Opportunity for Academic Liaison with Industry (GOALI) award supports fundamental research to provide the knowledge to enable the development of a low cost and efficient inspection tool that is superior to the existing tools. This new tool has the potential to, (1) significantly increase the yields of chips, (2) reduce new product development cycle time, and (3) reduce the costs of devices that use chips. Commercialization of this tool will create employment and wealth, and generate tax revenues for governments.The research objective is to establish the relationship between the strength of the ultrasound generated by the laser array bundle and the number of fibers in the bundle, length of the fibers, and laser energy carried by each fiber. Another objective is to establish the effects of each solder defect (such as crack, open, head-in-pillow, and pad cratering) on the generated ultrasound (in terms of energy of the received time domain signals). In order to accomplish these objectives, a finite element analysis method will be developed and used to predict the strength of the ultrasound (longitudinal, shear, or Lamb waves) generated by the array when the number of fibers, fiber length, and energy density are varied. The model predicted results will be used to guide the experimental set-up of the array sub-system for generating ultrasound in the chip. The measurement system will include an array sub-system, pulse laser which feeds laser energy into the array bundle, and laser interferometer for picking up the vibration signals on the chip surface. The vibration signals picked up by the interferometer at different points on the test chip surface will be correlated to the reference signals which are obtained from a defect-free chip to determine the presence of solder or chip defects.