CN112117205B - 一种锡基钎料封装焊点的制备方法 - Google Patents
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Abstract
本发明属于钎焊材料技术领域,更具体的涉及一种锡基钎料及其制备方法。本发明锡基钎料由Sn0.3Ag0.7Cu钎料和Ag颗粒构成SAC‑15Ag复合颗粒钎料。本发明采用晶圆键合机,运用TLP连接技术制备了Cu/SAC‑15Ag/Cu 3D封装焊点,通过调节键合时间和键合压力的工艺参数来提高焊点剪切强度,获得最优的工艺参数,改善了钎料的可靠性。当键合时间为30min,键合压力为1MPa时,抗剪强度达到了47.86MPa。SAC‑15Ag无铅钎料可在较低温度下键合,焊点由Cu3Sn和Ag3Sn相构成,这两种相熔点均高于400℃,因此能够满足低温键合、高温服役的要求。
Description
技术领域
本发明属于钎焊材料技术领域,更具体的涉及一种锡基钎料封装焊点的制备方法。
背景技术
第三代宽禁带半导体,如SiC和GaN,由于其优异的性能,特别是稳定性及高温下优异的电性能,已成为传统Si半导体的潜在替代品。SiC芯片能够在温度高达600℃的环境下稳定工作,SiC半导体高温服役的趋势对封装方法及材料提出了巨大挑战。
近年来,许多研究集中在探索新的3D封装技术和可靠的无铅耐高温焊料,其中低温瞬态液相连接(TLP)被认为是最有前途的方法之一。TLP键合过程中,可以在低温下通过高熔融基底(例如Cu,Ag基底)和低熔点夹层(例如Sn,In)之间的扩散反应获得金属间化合物(IMC)焊点。由于IMC的重熔温度比初始焊料高得多,TLP键合焊点通常具有优异的耐热性能,有望实现“低温键合,高温服役”的目标。目前,已有许多高温封装系统被提出,如Cu-Sn和Ag-Sn系统,其研究主要集中在IMC的生长行为,空隙的形成和消除,以及IMC层的延展性改善,但是键合工艺对锡基钎料封装焊点的影响方面并没有深入的研究。
Sn0.3Ag0.7Cu是一种较为优良的无铅钎料,其优点是塑性较好,熔点低,可焊性好,强度高,价格低廉,具有良好的润湿性及抗热疲劳性等。但是低银Sn0.3Ag0.7Cu钎料离共晶点较远,钎料在凝固过程中由于过冷度较大,导致最早析出的初晶长时间处于高温下而长成较为粗大的晶粒,导致该系无铅钎料力学可靠性较差,限制其广泛应用。通过在钎料凝固过程中纳米Ag颗粒为β-Sn相的凝固提供形核质点,使钎料的基体组织均匀细化,从而提高焊点力学性能。并通过优化键合时间和键合压力的工艺参数,进一步提高焊点力学性能。
发明内容
本发明是提供一种锡基钎料封装焊点的制备方法,提高焊点的剪切强度的同时降低孔隙率,本发明通过控制键合工艺参数,申请人惊奇的发现当设定键合参数为温度260℃、压力为1Mpa、时间为30min时焊点的剪切强度高达47.86Mpa,孔隙率最小仅为0.24%。
本发明是通过如下技术方案实现上述目的的,一种锡基钎料封装焊点的制备方法,包括如下步骤:
1)以纯铜为上下基板,对其进行研磨抛光吹干,以去除氧化物和油污;
2)在Sn0.3Ag0.7Cu(SAC)锡膏中加入质量分数为15wt.%的纳米Ag颗粒(7~8nm),充分混合搅拌混合均匀,得SAC-15Ag焊膏。
3)采用模具将获得的SAC-15Ag焊膏均匀涂覆在下铜板上,然后将上铜基板置于其上形成三明治结构;
4)采用晶圆键合机制备焊点。
优选的,采用TWB-100晶圆键合机制备焊点,键合参数:温度为260℃、时间为10s-120min、压力为0.1MPa-5MPa。进一步优选时间为30min、压力为1Mpa,申请人惊奇的发现当设定键合参数为温度260℃、压力为1Mpa、时间为30min时焊点的剪切强度高达47.86Mpa,孔隙率最小仅为0.24%。
与现有技术相比,本发明具有如下优点:
(1)本发明发现最优工艺参数下的焊点剪切强度高达47.86MPa,远满足于绝大部分应用场合。
(2)本发明中的锡基焊点由Cu3Sn和Ag3Sn相构成,这两种相熔点均高于400℃,因此能够满足低温键合、高温服役的要求。
(3)本发明晶圆键合机操作简单,无需复杂操作与培训,降低人工费用。
附图说明
图1为Cu/SAC-15Ag/Cu 3D封装焊点键合过程示意图。
图2为不同键合时间Cu/SAC-15Ag/Cu3D封装焊点显微组织形貌图,其中(a)10s,(b)1min,(c)10min,(d)30min,(e)60min,(f)120min。
图3为SAC-15Ag-10s、SAC-15Ag-30min和SAC-15Ag-120min断口界面的XRD分析图。
图4为键合时间对Cu/SAC-15Ag/Cu 3D封装焊点剪切强度的影响规律图。
图5为不同键合时间Cu/SAC-15Ag/Cu 3D封装焊点断口表面图:(a)10s,(b)1min,(c)10min,(d)30min,(e)60min,(f)120min。
图6为不同键合压力Cu/SAC-15Ag/Cu 3D封装焊点显微组织形貌图:(a)0.1MPa,(b)0.5MPa,(c)1MPa,(d)3MPa,(e)5MPa。
图7为键合压力对Cu/SAC-15Ag/Cu3D封装焊点剪切强度的影响规律图。
具体实施方式
原料和设备:Sn0.3Ag0.7Cu锡膏,Ag颗粒(7-8nm),铜块若干,TWB-100晶圆键合机,德国蔡司Supra55型扫描电子显微镜,UTM5305型电子万能试验机。
本试验选用纯铜(99.99%)为上下基板,用线切割机制备出规格分别为10mm×10mm×4mm(长X宽X厚)和12mm×12mm×4mm(长X宽X厚)的铜块若干,并对其进行研磨抛光吹干,以去除氧化物和油污。在Sn0.3Ag0.7Cu锡膏中加入锡膏15wt.%的纳米Ag颗粒(7~8nm),充分混合搅拌两小时以获得相对均匀的SAC-15Ag钎料膏。将获得的SAC-15Ag焊膏均匀涂覆在尺寸为12mm×12mm×4mm的下铜板上,然后将尺寸为10mm×10mm×4mm的上铜板置于其上形成三明治结构(如图1所示)。采用TWB-100晶圆键合机制备焊点,键合温度为260℃,键合时间为10s-120min,键合压力为0.1MPa-5MPa。
试样经5%硝酸酒精腐蚀后,采用德国蔡司Supra55型扫描电子显微镜(SEM)对试样焊点的组织及断口进行观察分析,并用能谱仪(EDS)对焊点组织成分进行分析。通过X射线衍射(XRD)分析,确定断口界面的主相以及Sn是否被完全消耗。采用UTM5305型电子万能试验机在室温下对焊点进行剪切试验,每组参数测试三个试样,求平均值以减小误差。试验过程中的拉伸速率为0.02mm/min.
实施例
一、不同键合时间下的显微组织演变
Cu/SAC-15Ag/Cu 3D封装焊点在不同键合时间(10s,1min,10min,30min,60min,120min)下组织如图2所示,键合温度为260℃,键合压力为0.5MPa。焊点高度约为30μm。本发明将焊点分为界面扩散反应区(Sn-Cu)和原位反应区(Sn-Ag)两部分。根据EDS结果可知,键合时间为10s(图2a)时,界面扩散反应区靠Cu侧的界面IMC层为Cu3Sn,与其相邻的为扇贝状Cu6Sn5。原位反应区生成不规则椭圆形Ag3Sn相,残留大量未反应的钎料及Ag颗粒,并存在少量孔洞,孔隙率为1.75%。键合时间为1min(图2b)时,不规则椭圆形Ag3Sn转变为块状,由于键合时间短,中间仍残留大量未反应的钎料,孔隙率急剧增大为20.99%。键合时间为10min(图2c)时,Cu和Cu6Sn5相进一步反应生成Cu3Sn相,原位反应区Ag3Sn组分的含量增大,孔洞逐渐减少,孔隙率减小为4.69%。键合时间为30min时(图2d),界面扩散反应区形成一层致密且连续的IMC层,Ag3Sn相组织连续,微量Cu3Sn相扩散到原位反应区,孔洞继续减少,此时孔隙率最小仅为0.24%。键合时间为60min时(图2e),界面扩散反应区萌生柯肯达尔孔洞,孔隙率增大为0.70%。键合时间为120min时(图2f),界面IMC厚度进一步增加,原位反应区Cu3Sn相组分含量增加,在Cu3Sn与Ag3Sn两相之间产生大量孔洞,界面反应区萌生裂纹,此时孔隙率为1.37%。
随着键合时间延长,Sn颗粒通过毛细作用熔化并填充相邻Ag颗粒之间的所有间隙。随后,液态Sn与被包围的Ag粒子发生反应,逐渐形成固体Ag3Sn相。在接下来的Sn-Ag反应中,由于体积收缩,形成了收缩孔洞,如图2c。焊点在持续施压下,孔洞又逐渐减少,如图2d。另一种是亚微米大小柯肯达尔孔洞,主要位于界面扩散反应区,如图2d-f所示。主要由于Cu和Sn原子在界面上的扩散速率差异所致,这种不平衡的扩散机制导致了原子级空穴的产生[Hollow nanostructures based on the Kirkendall effect:Design andstability conside-rations.2005,093111(86)]。键合时间的增加加速了柯肯达尔效应的发生,并促使柯肯达尔孔洞聚集长大,形成较大的孔洞,极大影响了焊点的可靠性。
二、界面IMC层演变机理
键合时,随着键合时间的增加,界面层发生Cu-Sn元素之间的扩散和反应,钎料首先与Cu元素发生冶金反应,在互连界面处,溶解到钎料中的Cu元素,在界面处形成局部饱和的平衡状态,在此处形成扇贝状Cu6Sn5,并伸向焊料内部。借助Cu-Sn二元合金相图可知[Alloy Phase Diagrams Version3.1992],Cu和Cu6Sn5相之间并非完全稳定结构,在键合温度260℃下,两相会发生反应生成相对平坦的Cu3Sn相,反应方程式(1)如下:
Cu6Sn5+9Cu→5Cu3Sn式(1)
各相物理参数见表1[Experimental determination of fatigue behavior oflead free solder joints in microelectronic packaging subjected to isothermalaging.2016;56:136–147]。
表1各相摩尔质量与密度
Cu原子与Cu6Sn5相反应生成Cu3Sn相的前后体积变化,如方程式(2)所示:
计算结果表明,Cu6Sn5转化为Cu3Sn后,体积缩小约4.38%。键合时间从1min到10min时界面IMC层厚度变小,如图2b-c。
此外,Cu6Sn5的热电性能都不如Cu3Sn,如表2所示[Thermal and electricalproperties of copper-tin and nickel-tin intermetallics.1992,72(7):2879-2882]。
表2Cu6Sn5和Cu3Sn的热电性能
Cu3Sn相组分的增加有利于焊点力学性能的提高,但需要足够的时间使所有Cu6Sn5转变为Cu3Sn。Cu/SAC-15Ag/Cu 3D封装焊点键合时间10s、30min和120min的剪切断口XRD分析如图3,Sn和Cu6Sn5相的衍射峰仅在键合时间10s焊点中出现,表明Sn未耗尽,Cu6Sn5没有完全转变为Cu3Sn。键合时间达到30min后,Sn已耗尽,Cu6Sn5完全转化为Cu3Sn,同时Ag3Sn相组分不断增大。
随着反应的进行,界面IMC层厚度增加,阻碍Cu元素向钎料中的溶解。这是由于界面扇贝状Cu6Sn5间的山谷成为元素扩散的通道,当大量Cu元素在某一山谷扩散时,会使山谷处IMC迅速长大。当生长到一定程度后,已形成的较厚IMC阻碍了Cu原子进入钎料中,改变了Cu在该位置的扩散速率。因此,在Cu侧界面上形成了层状界面IMC,从而增加了界面IMC层的厚度,且界面IMC的形貌也由初始的扇贝状转变为层状。并且可以从图2a-c观察到,Cu6Sn5曲率逐渐减小,这可能是由于晶粒粗化所致。
三、焊点的剪切强度和断口
Cu/SAC-15Ag/Cu 3D封装焊点不同键合时间的剪切强度变化如图4所示。结果表明,随着键合时间的增加,剪切强度呈先升高后降低的趋势。键合时间为10s时最低,仅为14.63MPa。此时,原位反应区存在大量未反应完全的钎料,Ag3Sn相组织不连续,同时还存在孔洞,极大影响了其剪切强度。键合时间为30min时,其剪切强度达到峰值为45.27MPa。这是由于经过充分反应后,界面扩散反应区形成的一层致密且连续的IMC层使焊点剪切强度提高,另外原位反应区孔洞减少,Ag3Sn相组织连续,均会提高剪切强度。键合时间超过30min后,界面IMC层过厚,界面扩散反应区萌生柯肯达尔孔洞及裂纹,原位反应区也产生了孔洞,降低了焊点剪切强度。
图5为不同键合时间下的Cu/SAC-15Ag/Cu3D封装焊点剪切断口SEM形貌。键合时间为30s时(图5a),在剪切断口上观察到许多韧窝形状的钎料基体和微量Ag3Sn相,这表明焊点在原位反应区处发生了韧性断裂,焊点内部大量的钎料基体极大影响了其力学性能。当键合时间为1min时(图5b),可以观察到台阶型Cu6Sn5出现在剪切断口处,表明断裂在界面扩散反应区与原位反应区交界处。键合时间为10min(图5c)的剪切断口处可以观察到明显的冰糖状Cu3Sn相,断口呈典型沿晶断裂,其断裂机制为脆性断裂,断裂发生在界面扩散反应区。当键合时间为30min时(图5d),可以观察到块状Ag3Sn上有明显的河流花样剪切痕迹,可知断裂发生在界面扩散反应区与原位反应区交界处。这是由于Ag3Sn和Cu3Sn的晶体结构均为正交晶系,且具有相似的晶格参数,但Cu6Sn5的晶体结构为六方晶系,和Ag3Sn晶格参数差异较大,因此在Cu6Sn5与Ag3Sn的界面处会形成一些缺陷,由于晶格不匹配而减弱了界面结合,造成界面的结合力减弱。键合时间达到60min(图5e)后,界面IMC过厚,而且在断口中可以观察到Cu3Sn相,可知断裂发生在界面扩散反应区。键合时间为120min(图5f)时,依旧断裂在界面扩散反应区,且有裂纹产生。
实施例2
键合压力对Cu/SAC-15Ag/Cu 3D封装焊点组织及力学性能的影响
一、不同键合压力下的显微组织演变
图6为键合温度260℃,键合时间30min,不同键合压力(0.1MPa,0.5MPa,1MPa,3MPa,5MPa)下Cu/SAC-15Ag/Cu 3D封装焊点组织形貌。键合压力为0.1Mpa时(图6a),焊点组织主要由Cu3Sn、Cu6Sn5和Ag3Sn组成,且Cu3Sn相和Cu6Sn5相之间有少量Ag3Sn相,这是由于压力过小,Ag原子与Sn原子互相扩散所致。原位反应区存在明显的收缩孔洞,孔隙率为27.47%,显著减少了焊点面积。键合压力增加到0.5MPa时(图6b),Cu6Sn5相完全转化为Cu3Sn相,收缩孔洞大量减少,孔隙率仅为0.24%。键合压力适当增大到1MPa后(图6c),孔洞几乎消失。当键合压力增加到3MPa时(图6d),压力过大,金属间化合物和晶粒尺寸逐渐增大,受应力集中影响,导致高应变区晶粒粗化,沿晶界交界面和弱晶界萌生裂纹[Experimental determination of fatigue behavior of lead free solder joints inmicroelectronic packaging subjected to isothermal aging.2016;56:136–147]。小尺寸的孔洞在整个焊点中随机分布,裂纹沿孔洞传播,并且出现了少量柯肯达尔孔洞,孔隙率为4.74%。键合压力为5MPa时(图6e),界面扩散反应区出现大量柯肯达尔孔洞,孔隙率为6.51%,裂纹大量生长,导致焊点失效。
二、焊点剪切强度
Cu/SAC-15Ag/Cu 3D封装焊点在不同键合压力下的剪切强度变化如图7所示。结果表明,随着键合压力的增加,剪切强度呈先升高后降低的趋势。键合压力为1MPa时,其剪切强度达到峰值为47.86MPa。这是由于此时焊点裂纹和孔洞较少,Ag3Sn相组织连续,IMC之间结合紧密,均有利于提高焊点剪切强度。键合压力5Mpa时,最低为36.78MPa。此时,焊点原位反应区裂纹大量萌发生长,界面扩散反应区出现大量柯肯达尔孔洞,裂纹和孔洞处极易发生断裂,急剧降低焊点剪切强度。
本发明研究了在焊接温度为260℃,键合时间和键合压力对Cu/SAC-15Ag/Cu 3D封装焊点组织及力学性能的影响。通过实验分析,得出了如下结论:
(1)随键合时间延长,界面IMC厚度呈现增加的趋势,孔洞呈先增加后减小再增加的趋势。焊点界面IMC由Cu6Sn5相逐渐向Cu3Sn相转变,原位反应区Ag3Sn相由不规则椭圆状逐渐变成块状。键合时间很短时(1min),原位反应区形成大量收缩孔洞。键合时间为30min时,界面扩散反应区形成一层致密且连续的IMC层,原位反应区孔洞减少,Ag3Sn相组织连续,微量Cu3Sn相扩散到原位反应区。孔洞和裂纹随键合时间延长进一步增加。
(2)剪切强度随着键合时间的增加,呈先升高后降低的趋势。键合10s时原位反应区仍存在大量未反应完全的钎料,同时还存在孔洞,剪切强度最低仅为14.63MPa。键合时间为30min时,界面扩散反应区形成致密且连续的Cu3Sn层,与原位反应区Ag3Sn相结合强度高,孔洞减少,剪切强度达到峰值为45.27MPa。断裂机制从韧性断裂转变为脆性断裂,Cu3Sn和Ag3Sn的界面结合强度高于Cu6Sn5和Ag3Sn界面结合强度。
(3)键合压力为0.1Mpa时,焊点组织主要由Cu3Sn、Cu6Sn5和Ag3Sn组成,存在明显的收缩孔洞。且Cu3Sn相和Cu6Sn5相之间有少量Ag3Sn相,这是由于压力过小,Ag原子与Sn原子互相扩散所致。随键合压力增加,Cu6Sn5相逐渐向Cu3Sn相转变,原位反应区孔洞呈先减少后增加的趋势,裂纹萌发并大量生长。
(4)随着键合压力的增加,剪切强度呈先升高后降低的趋势。键合压力为1MPa时,焊点裂纹和孔洞较少,Ag3Sn相组织连续,IMC之间结合紧密,剪切强度达到峰值为47.86MPa。键合压力5Mpa时,焊点原位反应区裂纹大量萌发生长,界面扩散反应区出现大量柯肯达尔孔洞,裂纹和孔洞处极易发生断裂,剪切强度最低为36.78MPa。
上述实施例为本发明较佳的实施方式,但本发明的实施方式并不受上述实施例的限制,其他的任何未背离本发明的精神实质与原理下所作的改变、修饰、替代、组合、简化,均应为等效的置换方式,都包含在本发明的保护范围之内。
Claims (2)
1.一种锡基钎料封装焊点的制备方法,包括如下步骤:
1)以纯铜为上下基板,对其进行研磨抛光吹干,以去除氧化物和油污;
2)在Sn0.3Ag0.7Cu锡膏中加入纳米Ag颗粒,加入量为Sn0.3Ag0.7Cu锡膏的15wt.%,充分混合搅拌混合均匀,得SAC-15Ag焊膏;
3)采用模具将获得的SAC-15Ag焊膏均匀涂覆在下铜板上,然后将上铜基板置于其上形成三明治结构;
4)采用TWB-100晶圆键合机制备焊点,键合参数:键合温度为260℃、键合时间为30min、键合压力为1Mpa。
2.根据权利要求1所述的制备方法,其特征在于:所述纳米Ag颗粒的粒径为7~8nm。
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