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。几乎没有不包含微芯片的电子设备。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，这些焊点有制造缺陷，很难检查，因为它们隐藏在视野之外。这些缺陷必须在电路板完全组装之前及早发现。然而，传统的检测工具对这些缺陷的检测效果很不理想。这一学术与工业联系机会(GOALI)奖支持基础研究，以提供知识，使开发一种低成本、高效率的检查工具优于现有工具。这一新工具有可能(1)显著提高芯片的产量，(2)缩短新产品开发周期时间，(3)降低使用芯片的设备的成本。该工具的商业化将创造就业和财富，并为政府创造税收。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、光纤长度和每根光纤携带的激光能量之间的关系。另一个目标是确定每个焊料缺陷(如裂纹、开口、枕头凹陷和焊盘凹陷)对产生的超声波(根据接收到的时域信号的能量)的影响。为了实现这些目标，将发展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，阵列产生的超声(纵波、横波或兰姆波)的强度。模型预测结果将用于指导芯片中产生超声的阵列子系统的实验设置。该测量系统将包括阵列子系统、向阵列束馈送激光能量的脉冲激光器和用于拾取芯片表面振动信号的激光干涉仪。干涉仪在测试芯片表面不同位置拾取的振动信号将与从无缺陷芯片获得的参考信号相关，以确定是否存在焊料或芯片缺陷。

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.