彎曲測試中電路板形變對覆晶球柵陣列構裝力學效應之影響研究

Effects of the PCB Deformation on the Mechanics of FCBGA Components under Bend Test

鄧文鈞(Wen-Chun, Teng)

指導教授 ： 陳永樹

元智大學/工程學院/機械工程學系/碩士(2007年)

摘要

隨著科技之蓬勃發展，功能複雜之高科技電子產品，已深入日常生活中，相對地對於產品可靠度要求也逐漸提高。如何找出影響可靠度之因素，進而掌握設計關鍵以提高電子產品的生命週期，為各方研究之重點。而四點彎曲測試，則是廣為用於電子元件做為取代耗時之熱循環可靠度測試(ATC)方法。因此，本研究乃針對四點彎曲測試下，電路板形變量對於覆晶球柵陣列構裝元件受力大小之影響，分別從實驗、理論分析與有限元素分析等方面著手探究，以期深入瞭解此可靠度測試方法之理論基礎。
理論分析方面，根據雙邊引腳封裝形式，分別比較運用彈簧及樑模擬引腳時之受力情形。前者之數學模型較為簡便，但僅能承受軸向力，因此提出能承受剪力、軸向力及彎矩影響的樑來取代彈簧以更適切地模擬引腳，唯四點彎曲中元件僅受到純彎矩作用，因而外力中剪力之影響可不考慮。元件受力產生形變後，因無法對錫球之應變量有即時之掌握，因此在實驗過程中運用LVDT將電路板形變曲線量出，再利用電路板之形變數據代入有限元素分析軟體中推算錫球之應力、應變值，進而可以計算錫球之疲勞壽命。
實驗中發現，元件對於電路板有某種程度上的強化作用，可局部降低電路板所受應力。此外，試驗時針對電子元件朝上與朝下相互比較，發現元件朝下時，對於電路板的強化效果不如朝上者。而研究亦呈現其他實驗參數與應力變化之關連性，例如在相同下壓量下，跨距增加時錫球所受應力之降低；與固定跨距，在下壓量增加時，錫球所受應力增加之關係等。理論分析結果發現，運用彈簧或樑之假設與實驗、分析所得數據尚有差距，但其趨勢則顯示理論模型是可接受的。而找出相關誤差原因並提出改善方法，皆有助於提高理論的準確性。然而本研究所闡釋可靠度評估模式之理論基礎，對於相關測試數據與產品可靠度之關連性之掌握，將會有實質之助益

關鍵字

彎曲測試，形變，疲勞壽命，覆晶球柵陣列構裝，LVDT，電路板，可靠度

並列摘要

With the fast growing technologies, the hi-tech electronic products with complicated functions have already accompanied people in their daily life. Therefore, the reliability of products is also required to be raised correspondingly due to its close links with people. How to test the designed product for its life cycle so as to improve the reliability have attracted many of the researchers to think about all possibilities. Among them, the four-point bend test is widely used as an alternative of the time consuming accelerated thermal cycling test(ATC) in the reliability assessment of electronic products. Therefore, to have thorough understanding about the influence of the printed circuit board deformation on the mechanics of the FCBGA component during the four point bend test(FPBT) is focused in this study. The investigation includes works in experiment, theoretical calculation and finite element analysis.
In the theoretical calculation, the mechanics model for the solder joints is proposed with spring and beam respectively. However, the former can only bear the axial force except the shear force and bend moment, although it is simpler in the mathematical formula. Consequently, the beam theorem is introduced for better representing the physics of solder joint. Also, in the four-point bend test, the component is subjected to pure bending only. Hence, influence of shear force has been ignored. During the four point bend test, an LVDT is utilized to measure the deformation of the PCB. This deformation data of the PCB is then taken as the input loading condition in the finite element analysis. The corresponding stress and strain in each of the solder balls are then obtained. And the fatigue life of solder ball is also calculated based on these information.
In the experiment when comparing the deformation between the bare PCB and the PCB with component, it is found that the component can strengthen the PCB and result in the decrease of the strain on the PCB. Also, when the component is placed facing downward, it has much less strengthening effect on the PCB than that with the component placed upward. Moreover, different span and downward displacement all have different effects on the stresses of solder balls. The relations between these test parameters with the solder stress are all addressed. When applying theoretical models of spring and beam in checking the stresses, it is notices that there still exists some differences with that of experiment and analysis results. But the similar trend in the outcome also indicated that the theoretical model is adoptable quantitatively. To improve the accuracy of the theoretical model by finding out the influencing factors of error can help in realizing the theoretical prediction of product life. Nevertheless, the methods as described in this study are believed to have improved a step further in the reliability assessment of electronic products during the designing stage.

並列關鍵字

Bend Test, Deformation Curve, Fatigue Life, FCBGA, LVDT, PCB, Reliability

參考文獻

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