A conformal coating is a protective chemical material that can be applied as a thin layer on printed circuit board assemblies (PCBAs). The purpose of conformal coating is to prevent the interaction between external environmental factors and the electrical assembly. However, since conformal coating materials have different mechanical properties than electronic components, their use may induce additional mechanical stresses on the board. This study focuses on the effect of having a different coefficient of thermal expansion (CTE) for the conformal coating material from the solder joints of ball grid array (BGA) packages on printed circuit boards (PCBs). Typical lead-free solders have a CTE of less than 25 ppm/°C, while coating materials have a much higher CTE. For example, polyurethane has a CTE around 193 ppm/°C. This CTE mismatch generates stresses in different directions on the solder joints, especially when the assembly is subjected to temperature cycling during its service. In this effort, four samples of PCB assemblies that have BGAs were used in temperature cycling tests. The sample size is nine modules, with each module having at least 272 exposed BGA joints. Three of the samples were coated by a layer of polyurethane and one baseline sample was not coated. Coating thickness was selected to be the experimental design factor. Thus, these three samples, which had different thicknesses (30, 80, and $130~\mu \text{m}$ ), were evaluated and compared against each other and against the noncoated sample. By comparing the four samples, it was found that a thick coating material significantly reduced the lifetime for BGA solder balls by initiating early solder cracks, which propagated and caused complete separation in the solder joint.

中文翻译：

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保形涂料对球栅阵列焊点长期可靠性的影响

保形涂层是一种防护性化学材料，可以作为薄层施加在印刷电路板组件（PCBA）上。保形涂层的目的是防止外部环境因素与电气组件之间的相互作用。但是，由于保形涂层材料的机械性能与电子部件不同，因此，保形涂层材料的使用可能会在板上引起额外的机械应力。这项研究的重点是使保形涂料的热膨胀系数（CTE）与印刷电路板（PCB）上的球栅阵列（BGA）封装的焊点不同。典型的无铅焊料的CTE低于25 ppm /°C，而涂层材料的CTE则高得多。例如，聚氨酯的CTE约为193 ppm /°C。这种CTE失配会在焊点上产生不同方向的应力，尤其是在组件在使用过程中经受温度循环的情况下。为此，在温度循环测试中使用了四个具有BGA的PCB组件样品。样本大小为九个模块，每个模块至少具有272个裸露的BGA接头。其中三个样品涂有一层聚氨酯，而一个基线样品未涂。选择涂层厚度作为实验设计因素。因此，这三个样品的厚度不同（分别为30、80和 样本大小为九个模块，每个模块至少具有272个裸露的BGA接头。其中三个样品涂有一层聚氨酯，而一个基线样品未涂。选择涂层厚度作为实验设计因素。因此，这三个样品的厚度不同（分别为30、80和 样本大小为九个模块，每个模块至少具有272个裸露的BGA接头。其中三个样品涂有一层聚氨酯，而一个基线样品未涂。选择涂层厚度作为实验设计因素。因此，这三个样品的厚度不同（分别为30、80和 $ 130〜\ mu \ text {m} $ ）进行评估并相互比较，并与未涂层的样品进行比较。通过比较这四个样品，发现较厚的涂层材料会通过引发早期的焊缝裂纹而大大缩短BGA焊球的使用寿命，焊缝会扩展并导致焊点完全分离。