Effect of cold spraying on residual stress and mechanical properties of friction stir welded 2219 aluminum alloy joint
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摘要: 为了改善铝合金搅拌摩擦焊接头残余应力与力学性能,在4 mm厚2219铝合金搅拌摩擦焊接头上表面进行冷喷涂试验,研究分析了冷喷涂前后接头残余应力与力学性能的变化. 结果表明,焊态接头纵向残余应力呈不对称“M”形分布,残余应力峰值位于前进侧靠轴肩附近;冷喷涂后,接头残余应力大幅度降低,残余应力峰值由186 MPa降低至43 MPa.涂层沉积厚度约200 μm,涂层与基体界面产生了较大的塑性变形,基体界面附近组织晶粒得以细化.由于冷喷涂过程的喷丸效应,接头上表面显微硬度平均提高了25 HV,作用深度约1 mm.接头拉伸性能也获得明显改善,抗拉强度提升6.3%,断后伸长率提升78.6%,焊态与冷喷涂态的拉伸试样均在接头前进侧的热影响区附近发生断裂,符合在低硬度区或弱结合面产生裂纹并扩展的弱区断裂的特征.Abstract: In order to improve the residual stress and mechanical properties of friction stir welded aluminum alloy joints, the cold spraying process was carried out on the upper surface of 4 mm thick friction stir welded 2219 aluminum alloy joint. The changes of residual stress and mechanical properties of the joint before and after cold spraying were studied. The results show that the longitudinal residual stress of the as-welded joint presents an asymmetric “M” shape distribution, and the peak of residual stress was located at the advancing side near the shoulder outer. After cold spraying, the residual stress of the joint was greatly reduced, and the peak of residual stress was reduced from 186 MPa to 43 MPa. The thickness of the coating is about 200 μm, and the large plastic deformation occurred at the interface between the coating and the substrate. What’s more, the grains near the interface are refined. Because of the shot peening effect during cold spraying, the microhardness of the upper surface of the joint was increased by 25 HV on average, and the affected depth was about 1 mm. The tensile properties of the joint were also significantly improved, with the tensile strength increased by 6.3% and the elongation increased by 78.6%. Both as-welded and coated tensile specimens fractured near the heat affected zone at the advancing side of the joint. It is consistent with the characteristics of fracture in the weak zone.
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0. 序言
随着电子元器件封装密度的增加,陶瓷球栅阵列(CBGA)和陶瓷柱栅阵列(CCGA)因其高密度的面排布引脚形式,在航空航天等高可靠性领域产品中得到了广泛应用[1-2]. CBGA和CCGA封装器件分别通过陶瓷管壳上的焊球和焊柱实现与PCB基板的组装互连,由于氧化铝陶瓷管壳(热膨胀系数为6.5 × 10−6/℃)和PCB基板(热膨胀系数为18 × 10−6 ~ 21 × 10−6/℃)的热膨胀系数差了近3倍,这种差异会在温度变化过程中产生剪切应变而导致裂纹萌生,进而引发焊点失效,因此温度循环成为了封装器件可靠性评估的关键手段. CCGA封装是在CBGA封装的基础上,用柱栅阵列代替了球栅阵列,增加互连引脚的距离,大大缓解了热膨胀系数不匹配带来的焊点失效问题,提高了焊点的可靠性,成为大尺寸产品封装的更优选择[3-5].
在温度循环过程中,焊点的界面显微组织会发生变化,包括金属间化合物(IMC)的成分及厚度等[6-8],界面的显微组织会影响焊点的可靠性,焊点的抗剪强度是反映其可靠性最直观的方式,因此分析温度循环过程中焊点的显微组织与抗剪强度的演变关系对揭示CCGA封装焊点的失效机理及建立可靠性评估依据具有重要的参考价值.
文中以CCGA484封装器件为研究对象,分析温度循环过程中焊点的界面显微组织演变与抗剪强度的对应关系,研究温度循环过程中焊点的失效机理,为CCGA封装的发展及应用提供理论指导.
1. 试验方法
1.1 原材料及试验样品制备
试验中选用高温共烧氧化铝陶瓷外壳,型号为CLGA484,镀层为Ni/Au,焊盘直径为ϕ0.8 mm ± 0.05 mm,焊盘间距为1.27 mm,板级封装用PCB板上的焊盘直径为ϕ0.8 mm ± 0.05 mm,焊柱采用ϕ0.51 mm × 2.21 mm的Pb90Sn10普通高铅焊柱,植柱和组装到PCB板上采用的锡膏均采用Sn63Pb37.
试验样品制备过程为:丝网印刷锡膏→植柱→真空回流焊接→清洗→PCB板喷印锡膏→CCGA器件与PCB焊盘喷印锡膏光学对位→真空回流焊接,完成板级封装后的试验样品如图1所示,将组装到PCB板的试样与未组装的CCGA器件同时进行温度循环试验,前者用于观察不同温度循环次数下焊柱的外观形貌,后者用于焊点的金相分析和剪切力测试.
1.2 温度循环条件
对CCGA484试验样品进行温度循环试验,试验条件按照美军标MIL-STD-883. 温度循环曲线如图2所示,温度范围为−65 ~ 150 ℃,循环周期为50 min,高低温保温时间均为15 min,升降温速率相同,试验过程中,每隔100次温度循环取出进行形貌观察、剪切力测试和金相分析,共进行500次温度循环.
1.3 抗剪强度测试
选用抗剪强度作为CCGA焊点的可靠性评估依据,试验设备采用专门的微焊点强度测试仪(DAGE4800),剪切速度均为0.4 mm/s,由于CCGA484器件的焊柱间距较小,试验过程中需要铲去周围的焊柱,保证剪切工具在行进时不会接触其它材料.
2. 试验结果与分析
2.1 焊柱宏观形貌演变分析
由于陶瓷管壳和PCB板的热膨胀系数差别较大,这种热失配会在温度变化过程中产生剪切应变,宏观表现为焊柱发生塑性变形.
温度循环过程中焊柱形貌如图3所示. 从图中可以看出,温度循环次数达到400次时,焊柱在反复热冲击作用下开始发生明显的塑性变形,且表面变得更加粗糙,焊点位置伴随有轻微的颈缩现象,500次后焊柱的扭曲程度进一步加剧,但肉眼还未观察到焊点开裂现象.
由于焊柱在长时间的高温、恒压力作用下,即使应力小于屈服强度也会慢慢发生蠕变变形[9]. 在温度循环的升温及高温保温阶段,陶瓷的热膨胀系数大于PCB基板,焊柱发生倾斜,在温度循环的降温及低温保温阶段,焊柱恢复至初始状态后向相反方向偏移,在反复的升温降温过程中,焊柱蠕变变形逐渐累积,达到宏观可见的扭曲状态,而焊点钎料的强度要略大于焊柱,因此在焊点处会有颈缩现象产生.
2.2 焊点显微组织分析
在焊点形成过程中,钎料与焊盘金属在短时间的高温作用下扩散生成硬脆的IMC层,实现焊柱与基板之间的电气和机械连接,但是在长时间的温度循环过程中,扩散作用导致IMC层厚度逐渐增加,其成分也会发生相应的变化,由于IMC的热膨胀系数与钎料相差较大,因此过厚的IMC会对焊点的可靠性产生不利的影响.
不同温度循环次数下的CCGA焊点界面显微组织如图4所示,在温度循环前,高铅焊柱与Ni焊盘界面观察不到明显的IMC层,因为Ni相对稳定,其界面反应层与铜相比是相当薄的,所以观察不到,随着循环次数增加,界面出现不同颜色对比度的中间层,且厚度逐渐增加,根据Ni-Sn二元相图可知,Sn-Pb钎料与Ni焊盘扩散反应生成的界面IMC从Ni侧依次包括Ni3Sn,Ni3Sn2和Ni3Sn4,具体的化合物成分取决于Sn与Ni的相对浓度.
采用EDS分析界面IMC成分,不同温度循环次数下测试IMC成分的位置如图5 ~ 图7所示,对应不同位置的成分如表1所示. 从图中可以看出,100次温度循环时,界面点1主要为偏析的富锡相,点2处Ni与Sn的原子比接近3∶4,结合Ni-Sn二元相图可知,应为Ni3Sn4化合物,与已有的研究一致[8];在200次温度循环后,界面点1处仍为Ni3Sn4相,靠近Ni焊盘侧的点2处Ni与Sn的原子比接近3∶2,推测为Ni3Sn2相;500次温度循环后,在Ni与Ni3Sn2相之间的点2处,Ni与Sn的原子比接近3∶1,应为Ni3Sn相,因此推测随着温度循环次数增加,从焊柱到Ni焊盘之间依次生成的IMC为富锡相→Ni3Sn4→Ni3Sn2→Ni3Sn. 分析IMC形成过程,认为在温度循环前,富锡相与Ni通过元素相互扩散,反应生成极少量Ni3Sn4化合物层,Ni3Sn4化合物层的生成阻挡了Sn与Ni的扩散反应,Ni与Ni3Sn4化合物层中微量的Sn继续发生扩散反应,生成Ni含量更高的Ni3Sn2化合物相,之后,Ni3Sn2化合物层进一步阻挡Ni3Sn4化合物层中Sn与Ni的扩散反应,生成Ni含量更高的Ni3Sn化合物相. 统计不同温度循环次数下界面IMC厚度,如图8所示,两者基本呈指数为1/2的幂函数增长关系,符合扩散控制机制.
界面IMC层存在离子键或共价键,所以往往具有硬脆特性,与基板和焊柱的线膨胀系数差别较大,随着温度循环次数增加,硬脆的IMC层厚度会逐渐增加,因此焊点界面处会产生较大的应力集中,在反复热应力作用下会萌生不同方向的细微裂纹,如图7所示,推测随着温度循环次数继续增加,应力集中导致微裂纹沿着剪切应变方向逐渐扩展,直到覆盖整个焊点,导致基板与焊柱之间发生断裂失效.
表 1 不同温度循环次数下焊点界面的成分分析Table 1. Component of CCGA solder joints at different thermal cycling times循环次数 界面点 质量分数w(%) 原子分数a(%) Pb Sn Ni Pb Sn Ni 100 点1 13.43 76.39 10.18 7.35 72.99 19.66 点2 4.97 69.37 25.66 2.3 55.9 41.8 200 点1 9 64.83 26.17 4.2 52.75 43.05 点2 4.91 51.99 43.1 1.93 35.63 62.44 500 点1 8.77 45.44 45.79 3.51 31.77 64.72 点2 8.24 28.16 63.6 2.92 17.44 79.64 2.3 抗剪强度测试结果与分析
焊点的力学性能是评估其可靠性的最直观方法之一,采用DAGE4800微焊点强度测试仪测试不同循环次数下焊点的抗剪强度,如图9所示. 随着温度循环次数的增加,CCGA封装焊点的抗剪强度呈现下降趋势,到500次温度循环结束,抗剪强度相对下降了15.6%,且下降的速率逐渐增大. 结合上文界面显微组织分析可知,长时间高温会促进界面元素相互扩散,依次生成Ni3Sn4,Ni3Sn2和Ni3Sn多种IMC化合物层,且IMC层厚度逐渐增加,这些化合物与Sn,Pb的晶格常数和晶格结构存在较大差异,具有较高的熔点,呈现硬脆特性,因此在反复塑性变形过程中会产生应力集中,容易萌生裂纹而导致焊点失效,所以焊点的力学性能随着IMC厚度增加而逐渐下降,与已有的研究结果一致[10]. 对抗剪强度Rτ与温度循环次数n之间的关系做曲线拟合,得到下式,即
$${R_\tau } = 682.25 - 0.06\;n - 2.77 \times {10^{ - 4}}{n^2}$$ (1) 根据技术指标要求,焊柱的最小剪切力为5.6 N,由式(1)推算可得,当温度循环次数大于550次时,焊点的抗剪强度将不满足要求.
3. 结论
(1) 温度循环超过400次时,CCGA器件焊柱开始发生明显的塑性变形.
(2) CCGA封装器件的焊点随着温度循环次数增加,从Ni焊盘侧依次生成的IMC层成分为Ni3Sn→Ni3Sn2→Ni3Sn4,且IMC层厚度逐渐增加.
(3) CCGA封装器件焊点的抗剪强度随着温度循环次数增加呈下降趋势,且下降的速率逐渐增大,到500次温度循环结束,抗剪强度相对下降了15.6%,这是由于硬脆的IMC层厚度增加,在变形过程中导致应力集中而引发焊点失效.
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表 1 2219铝合金的化学成分(质量分数,%)
Table 1 Chemical compositions of 2219 aluminum alloy
Si Fe Cu Mn Mg Zn Ti V Zr Al 0.3 0.3 6.5 0.6 0.05 0.1 0.15 0.1 0.15 余量 表 2 冷喷涂工艺参数
Table 2 Cold spraying process parameters
温度
T/℃压力
P/MPa喷涂距离
d/mm喷涂速度v/(mm·s−1) 送粉速度ν0/(g·min−1) 500 3 30 50 80 表 3 冷喷涂前后接头的拉伸性能
Table 3 Tensile properties before and after cold spraying
状态 抗拉强度Rm/MPa 断后伸长率A(%) 焊态 316 3.5 冷喷涂态 336 6.5 -
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