Investigation of Joule heating effect and thermomigration in flip-chip solder joints under current stressing
|關鍵字:||覆晶銲錫接點;電遷移;熱遷移;紅外線熱像儀;焦耳熱效應;flip-chip solder joints;electromigration;thermomigration;thermal infrared microscopy;Joule heating effect|
|摘要:||由於可攜式電子元件越做越小，銲錫接點的尺寸也因此縮小的特別快；也造成了各個銲錫接點內的電流密度快速增加。隨著電流的增加，造成銲錫接點內嚴重的焦耳熱效應(Joule heating effect)，接點內的溫度也快速上升。因此，真實的量測到銲錫接點內之溫度分布是很重要。但因為銲錫接點被矽晶片，高分子填充物(underfill)和基板所包圍；要直接量測到銲錫接點內的溫度是很困難的。本篇論文的主要主軸是利用紅外線熱像儀(thermal infrared microscopy)在不同的通電條件下，直接量測銲錫接點內的溫度分布情形。也因此可以發現銲錫接點內的一些熱特性，如溫度增加，溫度梯度(thermal gradient)和熱點(hot spot)溫度。我們主要是利用共晶錫銀(SnAg3.5)銲錫接點來做溫度分布的量測，發現在銲錫接點內有個熱點的存在；其一是在銲錫接點內部電流密度集中區(current crowding)，另一個則是在晶片端銲錫接點的邊緣靠近高分子填充物的地方。在通以電流密度1.06 □ 104 A/cm2下，銲錫接點內的平均溫度是150.5 □C；而兩個熱點的溫度分別是161.7 □C 和167.8 □C。另外，我們亦研究under-bump-metallization (UBM)厚度的效應對銲錫接點內溫度分布的影響。由實驗結果可以發現，較薄的UBM厚度會在銲錫接點內產生較高的熱點溫度。
在電遷移(electromigration)的過程中，焦耳熱效應永遠扮演著很重要的角色。在不同階段的電遷移過程中，空孔(voids)的生成與延伸可能會影響到銲錫接點內的焦耳熱效應。所以，我們利用凱文結構(Kelvin bump probes)和紅外線熱像儀來探討在電遷移的過程當中，空孔的生成與延伸和焦耳熱效應的關係。我們觀察到空孔的生成在銲錫接點內的電阻上升到原來的1.2倍時；而當電阻增加時，空孔也隨之延伸。此外，隨著通電時間的增加造成銲錫接點內的電阻也上升，因而發現銲錫接點內的溫度也隨之上升。而在通電的後期，銲錫接點因為電組上升和焦耳熱效應的影響，使得接點內的溫度快速上升。
由於鋁導線在加速電遷移測試的過程中是主要的發熱源，它所造的焦耳熱效應讓銲錫接點內產生了溫度不均勻的現象；在覆晶封裝銲錫接點內創造了極大的溫度梯度。因此，我們利用交流電(alternate current)來減去通電時的電遷移效應，直接觀測銲錫接點內的熱遷移效應(thermomigration)。因為在交流電的驅使下銲錫接點內不受電遷移效應影響；而且所造成的焦耳熱效應是與通以直流電是相同的。我們利用共晶錫鉛(eutectic SnPb)和錫銀銲錫接點來研究熱遷移效應，並利用聚焦式離子束(focus ion beam)在銲錫接點內產生標記點(marker)來直接量測熱遷移通量(thermomigration flux)。研究發現當共晶錫鉛銲錫接點在9.7 × 103 A/cm2電流密度下且加熱溫度為100 °C時，可以產生很高的溫度梯度大約2143 °C/cm。在熱遷移效應測試的前後，我們使用聚焦離子束來產生標記點，來計算銲錫的熱遷移速率。我們可以得到銲錫熱遷移通量3.3×1013 atoms/cm2和鉛的熱傳送值26.8 kJ/mole。
另外，在無鉛銲錫熱遷移研究方面，本實驗是利用共晶錫銀銲錫接(SnAg3.5)在加熱板溫度為100 °C時，施加0.57安培的交流電，做熱遷移效應的觀察。在此電流下可以觀察到銲錫接點內有大約2829 °C/cm的溫度梯度產生。實驗結果發現在熱遷移的效應下，錫是往銲錫接點內較熱端即晶片端來移動。我們同時量測到錫的熱遷移通量和熱傳送量(molar heat of transport)分別是5.0×1012 atoms/cm2 和1.36 kJ/mole。|
To meet the miniaturization trend of portable devices, the dimensions of the solder bumps continue to shrink, causing the current density in each solder joint to increase abruptly. With the increase of applied current, the temperature increased rapidly due to Joule heating. Therefore, temperature measurement in the solder joint becomes a important issue. Because the solder joints are completely surrounded by an IC chip, underfill and a substrate. Thus it is difficult to measure the temperature distribution around the solder joints. This dissertation focuses on the measurement of the temperature distribution in the solder bump at various stressing conditions by thermal infrared microscopy. It can also help us to explore the thermal characteristics in the solder joint, such as temperature increment, temperature gradient and hot spot temperature. We used eutectic SnAg3.5 solder joints with typical dimensions to measure the thermal characteristics in solder bump. Two clear hot spots are observed in the bump. One is located at the region with peak current density, and the other one is at the bump edge under the current-feeding metallization on the chip side. Under a current stress of 1.06 □ 104 A/cm2, the temperature in the two hot spots are 161.7 □C and 167.8 □C, respectively, which surpass the average bump temperature of 150.5 □C. In addition, effect of under-bump-metallization (UBM) thickness on the hot spots is also examined. It indicates that the hot-spot temperature in the solder bump increases for the solder joints with a thinner UBM. During electromigration test, Joule heating effect in the solder bump plays an important role. For this reason, the Joule heating effect at various stages of electromigration of flip-chip Sn3.5Ag solder joints was investigated under a current of 0.5 Amp at 100 °C. During various stages of electromigration, voids may form and propagate. Thus Joule heating effect may vary at different void sizes. To verify the void nucleation and propagation on Joule heating effect during electromigration process, the solder bump was stressed for different lengths of time and then examined by Kelvin bump probes and infrared microscopy. We found that voids started to form at approximately 1.2 times of the initial bump resistance. Then the voids propagated when the bump resistance increased. In addition, the temperature of the solder joints is also increased with the increase of bump resistance. In the last stage, the temperature of the solder bump increased rapidly due to the dramatic increase in the bump resistance and local Joule heating effect. Joule heating in the silicon chip generates a thermal gradient in a flip chip solder joint. Since Al traces serve as the major source of heat during accelerated electromigration tests, high current stressing also produces a non-uniform temperature distribution, creating a large thermal gradient in a flip chip solder joint. Therefore, we used alternate current (AC) to the joint to decouple the thermomigration from electromigration effect, since there is no electromigration effect under the AC stressing. Yet the AC produces the same amount of Joule heating as the direct current dose. Eutectic SnPb and lead-free solders have been adopted by the microelectronics industry. To measure the thermomigration rate directly, markers fabricated by focus ion beam are employed. The thermomigration flux of Pb is measured to be 3.3×1013 atoms/cm2, when the solder bump was stressed by 0.55 Amp at 100 □C. With the known thermal gradient, the molar heat of transport of Pb can be obtained as 26.8 kJ/mole. About the thermomigration in lead free solder joint, it is found that Sn atoms migrated toward the hot end. The thermomigration flux and molar heat of transport are measured to be 5.0×1012 atoms/cm2 and 1.36 kJ/mole, respectively, when the solder bump are stressed by 0.57Amp at 100 □C.
|Appears in Collections:||Thesis|