利用低成本高分子基板結合覆晶封裝技術之高頻及機械特性之探討

Abstract

本實驗主要探討低成本高分子基板RO3210應用於覆晶封裝結構之高頻特性的研究。設計在 GaAs 晶片上的CPW 傳輸線成功地以熱壓法與 RO3210 基板做覆晶結合。通過適當的設計，高頻特性可被成功地提升。從DC 到67 GHz 的S參數量測中，反射損失(reflection loss, S11) 及 介入損失(insertion loss, S21) 皆有很好的表現（-20 dB 及 -0.8 dB）。此結果與我們的模擬結果相互符合。此外，封裝在RO3210高分子基板上的主動式元件（m-HEMT）較封裝前的gain 僅少了2 dB，呈現優秀的特性 。在這些結構中，被填入的underfill及glob top也同時被探討。在可靠度測試中：TCT (-55 °C to 125 °C, 600 cycles)及高溫高濕測試(8 5% RH/ 85 °C, 500 hours)，填入了glob top 材料後的覆晶結構在經過200個溫度循環測試後，由於高熱膨脹係數的glob top與高分子基板之間的熱應力而徹底壞掉，可觀察到破壞發生於金凸塊與高分子基板的界面。但是，填入underfill的覆晶結構，可成功渡過高溫循環測試，並有效地減低應力和提升可靠度。

This work studies the selection and evaluation of new board materials RO3210 that enables flip chip packaging in microwave applications. A coplanar transmission line structure on GaAs chip were mounted on a RO3210 substrate using flip chip Au-to-Au thermal compression method. By using suitable layout optimization and compensation design, the S parameters performance of this structure was greatly improved. From DC to 67 GHz measurements, the resultant reflection loss S11 and insertion loss S21 exhibited excellent performance of -20 dB and -0.8 dB respectively. These results agreed well with the EM simulation data. Meanwhile, the flip chip bonding of in-house fabricated m-HEMT active devices on RO3210 also displayed excellent gain performance by small deviation -2 dB. Besides, the influence of encapsulant (underfill/ glob top) was also investigated. When epoxy resin encapsulant was injected into flip chip structure, the frequency band of the devices shifted to low side. To extend the use of this encapsulation method to high reliability and harsh environmental conditions, thermal cycling (-55 °C to 125 °C, 600 cycles) and humidity test (85 % RH/ 85 °C, 500 hours) were carried out. The effect of board properties such as coefficient of thermal expansion (CTE) upon the reliability performance was investigated. Glob top encapsulation which found widespread acceptance in electronics assembly for low frequency packaging consumer products (wire bonding type packaging) showed poor performance as a result of large CTE difference between the board and encapsulant (completely failed at 200 cycles). Meanwhile, packages with underfill exhibited excellent bump interconnection reliability. It is well-understood that underfill materials can redistribute the stresses and hence enhance the reliability. Overall, both electrical and reliability tests demonstrated the feasibility of using RO3210 as microwave packaging substrate up to U-band applications.