應用於電子散熱之散熱片優化減重技術

Abstract

本論文主要從溫度場及流場二方面來探討應用於電子散熱之散熱鰭片優化減重技術。在溫度場的考量上，主要探討如何用不同的散熱片截面，達成減重目標但仍能維持相同的散熱效能。在流場的考量上，主要探討如何利用鰭片模組的幾何型狀，切掉一部份的鰭片面積來降低散熱片的風阻，以提升散熱片流道內的風速，藉此提升熱交換係數來補償減少的散熱面積，達成減重的目標。 一般散熱片截面的設計，主要採用矩型截面的散熱片，製造成梯形截面形狀的目的，不只可以減重以降低成本，並可以增加模具的強度及提升量產的損耗壽命。然而梯型截面的角度與散熱效能的關係，尚未有完整的無因次解析解。本文為了建立一個快速評估效能損失與梯型截面角度的關聯性，藉由提出一個新的參數rt－鰭片尖端te 對鰭片根部tb 的厚度比，成功推導出一個無因次的微分方程式，並完成了解析解的推導!可以快速估算出矩型鰭片和梯形鰭片截面角度的相對關係，以求得性能損失和材料節省的相互關聯性。由於採用新的參數rt，所導出的無因次微分方程式迴避了三角函數的複雜性，大量簡化了相關的參數，而得出對應的解析解方程式。 另一種優化減重方案為散熱片的底板，可以把原本長方型截面的底板改為梯型底板，在數學模型的的合理假設前提下，採用一面積比的概念 rA -底板上面的鰭片的總面積 Afin 比上底板上的面積 Abase, 可以假設底板的上面有一相對放大倍數的熱傳係數ｈ來代表散熱能量，底板下面由於無風流可以假設為絕熱狀態，如此可以採用上文所導出的方程式來做優化減重的各項運算。 第三種有效的優化減重方案為交錯平行四邊形散熱片設計- IPFM（Interleaved Parallelogram Fin Module）。藉由結合單數鰭片（1,3,5..）為長方型及雙數鰭片(2,4,6..)為平行四邊形的特殊設計，可有效的降低散熱片的阻抗而提升約10～15％的風量。雖然IPFM的散熱面積少於傳統散熱片，但由於提升的風量，造成了鰭片間距中風速的提升而提高了熱交換係數h，補償了損失的散熱面積而達成了優化減重的目標。 本論文針對這三種方案做了詳細的研究，並成功推導出梯型鰭片的無因次解析解，並應用在底板的優化減重方面，以及推導出IPFM的切角與壓降相對於風量的關係，讓工程師在散熱片的減重及優化的設計程序，不須經由冗長而複雜的計算，而能快速獲得答案。本文提供一個新的思維，利用內文中的各項推導方法可以快速估算材料重量節省及性能損失的相對關係，達成散熱片優化減重的目標。

In this paper, it is focused on the application of electronic heat sinks to explore the optimization techniques of weight saving from two aspects: temperature field and flow field. In consideration of the temperature field, different kinds of fin profiles are studied to achieve the goal of weight saving; yet still maintaining the same heat dissipation performance. In consideration of the flow field, a different kind of geometrical shape of the heat sink module for reducing the flow resistance is studied, so as to enhance the heat exchange coefficient h to compensate the reduced fin surface area. Usually, the heat sink design mainly adopts the rectangular profile for easy calculation in design stage. However, changing to trapezoidal profile is not only able to reduce the weigh and cost; but also increases the tooling strength that will extend the tooling life expectancy. Nevertheless, the relationship between the profile angle and the heat dissipation performance has not yet been fully studied, and no dimensionless analytical solution has been proposed. In order to establish a rapid correlation between the loss of performance and the angle of trapezoidal fin profile, a novel dimensionless ratio of the fin tip to fin base thickness rt is defined, and a dimensionless analytical solution can be successfully derived. With this analytical solution, the correlation between rectangular and trapezoidal profiles about performance loss and material saving can be easily obtained. Another weight saving optimization target is the base of heat sink. If the original rectangular cross-section base is changed to trapezoidal cross-section, the goal for material saving can be achieved with little loss of performance. By reasonable assumption of the mathematical model, and applying the concept of fin surface area ratio rA – fin surface area Afin to the base area Abase, it can be assumed that the top face of base has a relative magnified heat transfer coefficient; and the bottom side of base can be assumed as adiabatic due to no airflow on that face. Hence, the derived equations for trapezoidal fin profile can be applied to the optimization of the weight saving of the heat sink base with minor redefinition of some parameters. The third effective approach for weight saving optimization is to apply IPFM (Interleaved Parallelogram Fin Module) technology on heat sink design. With the combination of odd number (1,3,5 ..) fins for rectangular shape and even numbers (2,4,6 ..) fins for parallelogram shape , this special design can effectively reduce the heat sink flow impedance; and about 10 ~ 15% more of the air flow rate can be obtained. Despite the much less surface area of the IPFM heat sink than that of the traditional heat sink, the thermal performance is about equivalent due to higher convection coefficient h resulting from a higher fin channel velocity. By applying this technology, the goal for heat sink weight saving can also be achieved. In this paper, these three approaches are studied in detail. The dimensionless analytical solution of the trapezoidal fin profile is successfully derived for weight saving optimization; and this solution can be applied to heat sink base weight saving evaluation with minor alteration of some parameters. The relationship between angle of the IPFM and the pressure drop are successfully deduced. These will contribute to avoid from the lengthy and cumbersome design calculations, and help engineers in heat sink weight saving optimization processes.