Wettability SnPb and Lead-free

Transcription

1 Wettability SnPb and Lead-free Miloš Dušek 1), Ivan Szendiuch, Jindrich Bulva, Michal Zelinka 2) 1) Materials Centre National Physical Laboratory,Teddington., Middlesex, TW11 OLW,UK 2) Brno University of Technology, FECC, Dept. Microelectronics, Brno, Udolní 53, CZ Abstract Lead free soldering implementation process needs some knowledge s about behavior of lead-free materials where fundamental seems to be wettability. Without good wettability the process performance can t be convenient technical and economical too. Material selection for soldering process has to be compatible for having the best results in characteristics, which are important for solder joint formation, and for final solder joint reliability. This includes optimal selection of fluxes, air/nitrogen atmosphere, solders alloy compositions, surface treatments of components and PCBs and other factors. This paper presents some facts from this area. Key words : solders and soldering, lead-free, wettability, inerting, flux, surface treatment Introduction Wettability of soldered surface is one of the main aspects for solderability classification. It is property indicating ability to create uninterrupted layer of soldering material on surface when contacting melted solder. To create reliable joint it is necessary to reach good wettability on contact pads as well as on component. Wetting curve (Fig. 1) expresses behavior of solder during process. a b c d e f Force d e a c Buoyancy Level f Time b Figure 1: Wetting curve

2 For melted solder wettability classification are two main factors: - wettability level - wettability speed Wettability level sets how far the melted solder is spread on surface. Wettability speed is speed of melting and spreading. Wettability level can be defined as force F max. Wetting speed is period of time T during which force reaches two thirds of its (Fig. 2).. Wettability is influenced by surface s properties and also solders alloy proportion. With transition to lead-free solders appears a question how it take effect on wettability. Paper dealing with differences between lead-free and standard lead solders shows this: The results show that of lead-free alloys are similar, but significantly different to that of SnPb. Lead-free alloy has higher force F max but longer period to reach its two thirds. It is because at temperature 250 C has lead-free solder, because of its higher claims, higher viscosity.wetting time decreases with increasing temperature for all alloys. Good wettability is less temperature dependant for SnPb alloy. So-called Tombstoning, thus rise of the soldered component during reemelting process to one side is much more numerous events for lead-free alloys. Number of failures of this type increase with minimizing of the soldered component. Weight of the larger component is enough to its keeping in the right position. +ve curve Maximum F Force (mn) f 0 -ve 2/3 of F max F 1 = 2sec T = time to 2/3 force t 0 Time (seconds) Figure 2: Wettability curve So we can say that lead-free solders usually have smaller wettability angle. On the other side there is bigger risk of Tombstoning. This unwelcome effect can be eliminated by increasing melting temperature. But it also increases thermal stress.

3 2.0 SnPb SnAgCu SnAg SnCu Solder Temperature ( C) Figure 3: Wettability of copper coupons for lead-free alloys in air 2.0 SnPb SnAgCu SnAg SnCu Superheat ( C) Figure 4: Wettability of copper coupons for lead-free alloys in air The times for copper coupons for four different solder alloys in air are shown in Figure 3. Figure 4 compares times in accordance with superheat temperatures. Superheat temperature is defined as the difference between the soldering temperature and the solder alloy melting point. Figure 4 shows that there is not so high difference in times at any given superheat temperature between the three leadfree alloys and the SnPb solder. Fig. 5 shows comparison of force for lead-free alloys in three different temperatures.

4 2s [mn] C 30 C 20 C SnAgCu (217) SnAg (221) SnCu (227) Solder Alloys Figure 5: Comparison of force for lead-free alloys Subsist three options to improve : increasing soldering temperature increasing flux activity reducing oxygen concentration To achieve acceptable, a balance between flux activity and nitrogen inerting is inevitable. Testing in oxygen free environments suggests favorable results for lead free solder alloys. Lead free alloys have better characteristics when tested in nitrogen compared to air. With narrower process windows associated with lead-free soldering technologies the use of nitrogen inerting will play an increasingly important role in attaining satisfactory soldering. Nitrogen inerting can largely compensate for the poor solderability experienced when using lead-free solders. Developments in flux chemistry may partially offset the need for inerting. 2.0 Copper in Copper in nitrogen Solder Temperature ( C) Figure 6: Wettability of copper coupons with SnAgCu solder in air and in nitrogen

5 In order to achieve good yield and acceptable reliability when using SnZn solder alloys it will be necessary to use nitrogen inerting with oxygen levels better than 100 ppm. For SnAgCu, SnCu and SnAgBi solder alloys an improvement in can be achieved with nitrogen inerting at oxygen levels of ppm. However, a level of 5000 ppm oxygen is recommended as an ideal compromise between product quality and cost. Inerting to 50 ppm oxygen when using SnAgBi and SnCu solders will provide a benefit when soldering particularly difficult assemblies e.g. multi-layer boards. Conclusion There are many factors influencing wettability in soldering process. The right choice of solder material and flux is only first step to achieve good results. It is characteristic the lead-free solders usually have higher angle. On the other side there is bigger risk of Tombstoning. Increasing melting temperature can eliminate this unwelcome effect. But it also increases thermal stress. Nitrogen and Nitrogen/hydrogen promote and angles are smaller than in air. Slightly better results were achieved in Nitrogen/Hydrogen atmosphere, especially in reproducibility. Not big differences in SnAgCu alloys with 3,5 and 4 % of Ag was found. Wetting angle hysteresis can make a problem on NiAu chem. in some applications - IMC modifies de characteristics. Wettability is significantly influenced by surface s properties in correlation with solder alloy proportion. With transition to lead-free solders appears a question how it take effect on wettability. The results show that of tested lead-free alloys are mutual similar, but significantly different to that of SnPb. Lead-free alloy has higher force but longer period to reach its two thirds. It is because at temperature 250 C has lead-free solder higher surface tension and higher viscosity too. Wetting time decreases with increasing temperature for all alloys. Good wettability is less temperature dependent for SnPb alloy. Acknowledgment This paper is created with support of research project of the Czech Ministry of Education in the frame of Research Plan MSM MIKROSYT Microelectronic Systems and Technologies and Grant project GACR GA Development of Electronic Assembly Technologies for 3D Systems References [1] Zelinka, M.: Reliability investigation of lead-free solders, Diploma thesis 2003, Brno University of Technology, FEEC, Dept. Microelectronics, Brno/Teddington [2] Dusek, M., Nottay, J., Hunt, Ch.: Compatibility of Lead-free Solders with PCB Materials, NPL Report MATC(A) 89, August 2001 [3] Hlavac, R.: Accelerated Thermal Cycling for Lead-free Alloys Reliability Assessment, Technical University of Brno, Faculty of Electrical Engineering and Computer Science, National Physical Laboratory, Teddington, 2001