日期： 2 0 1 1 . 0 5 . 02 指導老師：林克默、黃文勇博士學生：陳立偉

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1STUT 太陽能材料與模組實驗室112/04/19

日期： 2 0 1 1 . 0 5 . 02指導老師：林克默、黃文勇博士學生：陳立偉


2STUT 太陽能材料與模組實驗室112/04/19

Outline

1. Introduction

2. Reliability test plan3. Power cycling4. thermal shock5. Vibration testing6. Conclusions


1. Introduction

A Directive, endorsed by the European Parliament, on “Waste from electrical and electronic equipment” can be expected which could include a ban on lead based soldering in electronics in the near future.

The Japanese electronics industry has already indicated they will phase-out lead in consumer electronics in the coming years, which could have worldwide implications.

112/04/19 STUT 太陽能材料與模組實驗室 3


Earlier in the project it was decided to target alloys with melting points in the range 190-220°C, and ternary alloys of Sn, Cu, and Ag, with small additions of Sb and Bi were investigated. It was recognised that other alloys such as eutectic SnBi58, SnAg3.5, or SnCu0.7 could also be considered, but not for a wide application range.



The scope of this paper is confined to the assessment of the reliability of mainstream electronics surface mount reflow assemblies based on the ternary SnAg3.8Cu0.7 (Tm = 217°C) solder alloy. In the following SnAg3.8Cu0.7 is referred to as SAC, while SPA denotes Sn62PbAg2.



2. Reliability test plan

• According to Ref. [4] and the IPC-SM-785 guidelines for accelerated reliability testing of surface mount solder attachments, the most common reliability threat comes from stress-relaxation based (thermal) fatigue damage.

• However, in Ref. [5] is indicated that environmental temperature variations do not represent inservice conditions,



• Other tests such as thermal and mechanical shock and vibration fatigue In Ref. [6] is explained that the fast thermal mechanical excursions without enough hold time to induce creep and stress relaxation effects in the solder joints.



3. Power cycling

3. 1. Test setup

• A 64 channel multiplexer system was built for in-test (not continuous) monitoring of electrical continuity and resistance. Each standard single layer FR4 PCB board contained two daisy chained circuits of four 1206 100 Ohm resistors with Sn100 metallisation and one daisy chained 100 gullwing leads, 0.65 mm pitch. MQFP plastic package with a copper leadframe and Sn85/Pb15 metallisation.



• Each batch underwent a number of power cycles; i.e. 0, 1000, 3000 or 5000 cycles and consisted of different board metallisation and solders (see Table 1.

• SAC-X where X is a proprietary elemental addition. The same all-round flux (which combined high-speed printability with low void formalion) and reflow conditions were chosen for the lead-free boards, while a standard reflow profile for the SPA boards was used.





• An IR camera and thermocouples were used to assess the temperature distribution over the board and components as power was applied. 50 W power dissipation in the 18 MQFP dice was required to reach a temperature of 100°C in the leads. The 1206 resistors reached a maximum temperature of 100°C for 60 W of applied power.



• The temperature cycle was selected to go from ambient to 100°C with a dwell time of 15 min to allow sufficient stress relaxation. This selection was based on guidelines for accelerated testing for a consumer product use environment [7]. Ramp up and cool down rates were about 25℃/min, which resulted in a 30-min cycle. Fig. 1 shows the actual solder joint temperature profiles for both MQFP100 sides and a 1206 resistor.





3.2. Test results

• MQFPs, which did not experience wirebond failures had no electrical resistance increases over 5000 cycles No electrical failures were seen for the 1206s up to 4000 cycles, but between 4000 and 5000 cycles, four boards, including SPA, indicated resistance increases and interrupts.







• Before ageing the SPA resistor joint on OSP-Cu showed a eutectic structure with the Pb phase finely dispersed in the Sn phase. The SAC on immersion Sn solder body (before power cycling) showed a Sn-rich matrix of large ß-Sn phase dendritic globules. with a dispersed fine eutectic microstructure of Cu6Sn5 and Ag3Sn and a Cu6Sn5 intermetallic layer at the pad/solder interface. This type of microstructure was evident in all SAC resistor joints before ageing.







• At 1000 cycles, cracks had developed in all 1206 resistor joints. SPA on OSP-Cu resistor joints after 1000 power cycles showed that cracks initiate under the component termination and progress along the grain boundaries between the Pb-rich and Sn-rich areas of the joint, indicating intergranular cracking. Coarsening of the microstructure is also evident and the crack propagates through the coarsened region.







4. thermal shock

• Thermal shock testing was carried out whereby the rapid temperature changes induced shear and tensile stresses in the solder joints. For this relatively short environmental test twin baths were used and consisted of cycling between -20°C and +125°C, with 3 min dwells at both temperature extremes and a cycle time of 7 min . Boards were cycled at 1000, 2000 and 3000 cycles.







5. Vibration testing

• frequency band between 100 and 2000 Hz for up to 60 mm per axis. No electrical, mechanical or microstructural degradation was observed. Longer test durations, up to 8 h per axis in x and y direction (see Fig. 15) still did not result in significant solder joint damage.

• Random vibration testing for 6 h in the axis direction showed daisy chain resistance increases of up to 10 times the original values.





For the final tests

1. after assembly

2. after 6 h random vibration (RV) in z direction (between 50 and 2000 Hz)

3. after 500 thermal shock (TS) cycles (cycled between -40°C and 125°C with dwell time 15 mm and cycle time 30 mm)

4. combined random vibration (2) and thermal shock(3)





• In correspondence with previous test results with eutectic Sn37Pb solder, the random vibration test on its own has no mechanical degradation effect on the SPA assemblies, but it has some fatiguing effect on the SAC assemblies. The combined effect of thermal shock and random vibration showed that SAC has slightly better fatigue resistance then, but comparable strength to that of SPA.



6. Conclusions

•初步可靠性的測試結果，表示使用結合各種元件與基板金屬化與 SAC焊料有可能取代傳統的鉛基系回流焊接。

•現有回流焊接設備無必需的重大修改，但必須修改最低回流焊溫度至 225℃

•振動測試對於元件與基板的接合度具有強大的破壞性。結合熱衝擊和振動試驗可以加速疲勞壽命，但還要需進一步的研究。



• 熱衝擊和隨機振動試驗表明，耐疲勞性之間的差異，含鉛和 SAC 焊點無顯著差異，但電路板的金屬表面影響也會影響其焊接效果。


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