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Column-grid-array (CGA) technology could lead to a highly reliable package design

机译：列栅阵列（CGA）技术可能会导致高度可靠的封装设计

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An analytical stress model is developed for a short cylinder (beam) with clamped and offset ends. The offset is caused by an external lateral force that has to be determined from the known offset. It is envisioned that such a beam can adequately mimic the stresses and strains in a column of a column-grid-array (CGA) solder joint interconnection. Such an interconnection is characterized by an elevated stand-off height of the solder joints compared to the joints in a widely used today ball-grid-array (BGA) system. In a simplified and conservative analysis the ends offset in a CGA joint can be determined beforehand as a product of the known thermal mismatch strain between the IC package and the printed circuit board (PCB), and the position of the joint with respect to the package mid-cross-section. The objective of our analysis is to use the developed model to demonstrate that the application of CGA technology enables one not only to significantly relieve the thermally induced stresses in the solder material, but to do that to an extent that the induced stresses remain within the elastic range. The analysis is limited therefore to elastic deformations. It should be emphasized that while the classical Timoshenko short-beam theory seeks the beam's deflections caused by the combined bending and shear deformations for the given loading, an inverse problem is considered here: the lateral force and the induced stresses are sought for the given end offset. In short beams this force is larger than in long beams, since, in order to achieve the given displacement (offset), the applied force has to overcome both bending and shear resistance of the beam. The carried out analysis and the numerical data indicate that by employing beam-like CGA solder joints one could possibly manage to remain within the elastic range, i.e. to avoid inelastic strains and, hence, low cycle fatigue conditions. If this is achievable, the fatigue lifetime of the solder material will increase dramatical- y, because the low-cycle fatigue condition will be replaced by the linear accumulation of elastic damages. The numerical example indicates that the stand-off of the CGA joint should be rather large to make the shearing stress low compared to the normal bending stress. The height-to-diameter ratio of the CGA joint should be increased to the level of about 12¿¿¿13. In such a situation the Bernoulli beam model, could be used instead of Timoshenko model for stress evaluations. The further increase in the stand-off height over this ratio, even if it is technologically achievable, is not advisable, since this will not lead to an appreciable further stress reduction. Future work will include, but might not be limited to, the finite-element-analysis (FEA) computations and experimental evaluations (such as, e.g., shear-off testing and/or temperature cycling) of the induced stresses in, and the fatigue lifetime of, typical BGA and novel CGA joints. We would like to point out that a solder joint in isolation is neither reliable nor unreliable, and that reliability has meaning only in the context of interconnections either within package or outside of package onto PCB. For this reason the future work should include also better understanding of how to translate the stress relief in a single joint into the improved reliability of an interconnection as a whole, thereby leading to a highly-reliable solder joint interconnections and, since these interconnections are the bottle-neck of the today's IC packaging technologies, to a highly reliable package design as a whole. Particularly, for BGA/CGA package assemblies, it is critical to determine the effective offset values for the peripheral joints.
机译：针对具有夹紧端和偏置端的短圆柱（梁），建立了分析应力模型。该偏移是由必须从已知偏移确定的外部横向力引起的。可以预见，这样的光束可以充分地模拟列栅阵列（CGA）焊点互连的列中的应力和应变。与当今广泛使用的球栅阵列（BGA）系统中的焊点相比，这种互连的特征在于焊点的支座高度增加。在简化和保守的分析中，可以预先确定CGA接头中的端部偏移，这是IC封装和印刷电路板（PCB）之间已知的热失配应变以及接头相对于封装的位置的乘积中截面。我们分析的目的是使用开发的模型来证明，CGA技术的应用不仅可以极大地减轻焊料中的热感应应力，而且可以使感应应力保持在弹性范围内。范围。因此，分析仅限于弹性变形。应该强调的是，尽管经典的Timoshenko短梁理论在给定载荷下寻求由弯曲和剪切变形的组合所引起的梁的挠度，但这里要考虑一个反问题：在给定的端部寻求侧向力和感应应力抵消。在短梁中，此力要比长梁中的力大，因为为了实现给定的位移（偏移），施加的力必须克服梁的抗弯强度和抗剪强度。进行的分析和数值数据表明，通过采用梁状CGA焊点，一个人可能可以设法保持在弹性范围内，即避免非弹性应变，从而避免了低周疲劳条件。如果可以做到，则焊料材料的疲劳寿命将大大增加，因为低周疲劳条件将被弹性损伤的线性累积所取代。数值示例表明，与正常弯曲应力相比，CGA接头的间距应较大，以使剪切应力较低。 CGA接头的高径比应增加到大约12 ?? 13的水平。在这种情况下，可以使用伯努利梁模型代替Timoshenko模型进行应力评估。即使在技术上可以达到，也要使支撑高度进一步超过该比例，这是不可取的，因为这不会导致明显的应力进一步减小。未来的工作将包括但不限于有限元分析（FEA）计算和所产生的应力和疲劳的实验评估（例如，剪切测试和/或温度循环）。典型的BGA和新型CGA接头的使用寿命。我们想指出的是，孤立的焊点既不可靠也不可靠，并且可靠性仅在封装内部或封装外部与PCB互连的情况下才有意义。因此，未来的工作还应包括更好地理解如何将单个接头中的应力消除转化为互连整体上更高的可靠性，从而导致高度可靠的焊点互连，并且由于这些互连是当今IC封装技术的瓶颈，将高度可靠的封装设计作为一个整体。特别是对于BGA / CGA封装组件，确定外围连接的有效偏移值至关重要。

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