CN101409243B - 粗铜丝引线键合的实现方法 - Google Patents
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Abstract
本发明涉及粗铜丝引线键合的实现方法,该方法包括:铜球结晶时保护、劈刀的设计优化、芯片铝层厚度检测、关键工艺参数的优化匹配。铜球结晶时保护采用高纯度,含氧量小于1ppm的N2、H2保护气体;劈刀的形状设计成外角度改小、内角度改大,这样可以产生较强的尾焊,避免粗铜丝引线键合作业时断丝。采用85%的磷酸溶液检测芯片铝层厚度,确保粗铜丝引线键合时铝层厚度满足大于4μm的要求,避免作业过程中出现弹坑;工艺参数上选择使用初始压力,避免弹坑的产生。本发明既能明显降低成本,又能提高产品的可靠性。
Description
技术领域
本发明涉及一种半导体封装技术领域,尤其是涉及半导体封装中使用的粗铜丝引线键合的实现方法。
背景技术
金丝引线键合是电子工业中应用最广泛的引线键合技术,但随着高密度封装的发展,铜丝引线键合日益引起人们的关注.,铜丝引线键合具有很多优势:(1)价格优势:引线键合中使用的各种规格的铜丝,其成本只有金丝的1/10。(2)电学性能和热学性能:铜的电导率为[0.62(μΩ/cm)-1,比金的电导率[0.42(μΩ/cm)-1]大,同时铜的热导率也高于金,因此在直径相同的条件下铜丝可以承载更大电流,使得铜引线不仅用于功率器件中,也应用于更小直径引线以适应高密度集成电路封装;(3)机械性能:铜引线相对金引线的高刚度使得其更适合细小引线键合;(4)焊点金属间化合物:对于金引线键合到铝金属化焊盘,对界面组织的显微结构及界面氧化过程研究较多,其中最让人们关心的是″紫斑″(AuAl2)和″白斑″(Au2Al)问题,并且因Au和Al两种元素的扩散速率不同,导致界面处形成柯肯德尔孔洞以及裂纹。降低了焊点力学性能和电学性能,对于铜引线键合到铝金属化焊盘,研究的相对较少,Hyoung-JoonKim等人认为在同等条件下,Cu/Al界面的金属间化合物生长速度比Au/Al界面的慢10倍,因此,铜丝引线键合的可靠性要高于金丝引线键合。
铜丝引线键合技术是目前国际上正在进行开发研究的一种用于微电子器件芯片与内引线连接的新技术。目前细铜丝引线键合(铜丝直径φ0.8mil~φ2.0mil)已经逐步开始推广,但粗铜丝球焊因存在诸多的技术难点,目前还未应用。如果粗铜丝引线键合能够推广使用,降低成本的优势更加明显,铜丝引线键合技术在半导体器件封装上的普及面将更广。
发明内容
本发明所要解决的技术问题是针对上述现有技术现状而提供一种粗铜丝引线键合的实现方法,以降低成本,减缓脆性金属化合物的形成,提高键合强度。
本发明解决上述技术问题所采用的技术方案为:粗铜丝引线键合主要的技术关键点在于:铜球结晶时保护、劈刀的设计优化、芯片铝层厚度检测、关键工艺参数的优化匹配。
铜球结晶时保护。
粗铜线引线键合过程中,使用N2、H2混合保护气体。设计一种装置,固定在焊接设备上,调节好位置,将N2、H2混合保护气体接上,并调整好N2、H2混合保护气体的流量,N2、H2混合保护气体的流量决定了铜球的冷却速度,冷却速度是铜球硬度的决定性指标,冷却速度越快,则球的硬度越大,在压焊时,芯片所受的力就越大,极易压伤芯片铝层下的硅,造成芯片上的第一焊点被拉起。
提供合适含H2比例的高纯度N2、H2混合保护气体、调整好气体装置的位置,调整好N2、H2混合保护气体的流量以防止铜球的氧化。使用时,要求保护气体纯度含氧量小于1PPM;露点小于-40℃;H2的体积比比例5%~10%,流量0.5l/min;
氧化:按直观的验证方法:粗铜线引线键合工艺N2、H2保护就是保护铜球,但实际操作过程中,如果N2、H2气体只保护球,实际上铜球表面极易产生氧化层,其产生机理是:在常温和高温下,主要是化学性质活泼的氧分子与铜球表面的铜产生化学反应,此种反应速度较慢。主要的氧化产生于:氧离子与铜的接触,在打火的过程中,打火杆与铜线间进行放电,产生的火花,是对空气中的氮气与氧气离子化的结果,而氧离子的化学活动性较氧分子活泼得多,为防止铜球的氧化,其关键点是防止氧分子被离子化,所以要求保护混合气体对打火路线保护好,在高压打火时,被离子化的是氢分子,这样就可保证铜球表面产生的少量氧化层被还原。
劈刀的设计优化
因粗铜丝硬度大,延展性小在生产线的作业过程中,易断丝,无法连续作业。设计特殊规格的劈刀,使之保证生产线顺利作业。有三个方法:一是对现有劈刀的角度进行修正,将劈刀的外角度改小,或内角度改大,在脚位上焊接时,不会将铜线切断;另一种方法是将劈刀嘴的结构进行变更,劈刀嘴结构中,有几个重要之处,阳角、阴角的大小确定,劈刀头锥度的选择,劈刀材料的选择以及表面光洁度的选择。对劈刀的结构进一步优化,通过调整劈刀的外角,克服了框架上焊线无尾丝问题,现将劈刀内角微结构的样式调整,使铜球受力更均匀;第三种方法是劈刀加工过程中,劈刀边锋部倒圆角。都可以纠正此缺陷,确保脚位上的拉力足够。
本发明在粗铜丝引线键合作业时采用如下规格的劈刀:
H:孔径是102μm~175μm、CD:倒角直径是152μm~215μm、CA:内倒角孔角是120°~150°、T:头部直径是356μm~465μm、FA:端面角是4°;
芯片铝层厚度的检测。
弹坑:压焊时输出能量过大,使芯片压焊区铝垫受损而留下小洞。具体实验方法如下:将压焊后的芯片在不加热的王水中腐蚀24小时,然后在高倍显微镜下观察,会发现芯片压焊区有针孔大的小洞,颜色呈彩色。
失铝:压焊时输出能量过大,使芯片压焊区铝垫撕裂。将压焊后的芯片在不加热的王水中腐蚀24小时,然后在高倍显微镜下观察,会发现芯片压焊区有铝层脱落现象,颜色呈黑色。
制造大圆片时,因为蒸铝工艺上的问题或处于成本的考虑会导致一批大圆片出现压焊区铝层结合不致密或太薄。当压焊时劈刀在铝垫上振荡就比较容易将铝垫撕裂(产生失铝)或留下空洞(产生弹坑),导致产品失效。
实验得出铝层厚度大于4μm的芯片,才可用粗铜线引线键合。
芯片本身铝层厚度是否达到粗铜线工艺标准。各芯片厂家铝层的成份不同、结构上也存在差异性,根据其蒸镀的速度不同,铝层的致密性及铝层厚度也会发生较大变化。
铝层致密性及厚度的测量设备价格较贵,一般的封装厂都没有这种检测设备,为此,通过一套铝层厚度检测方法,将芯片放置在纯度大于85%的磷酸溶液,根据铝层的溶解时间与标准样本的比较,计算出铝层的厚度。
关键工艺参数的优化
铜球硬度:这是一个较为复杂的问题,与许多因素相关,现将相关量全部列出来:铜线材质、打火温度、电流、球的大小、球的冷却速度、冷却气体的混合比例等,铜球硬度是根据以上多种因素相互作用的结果,
工厂所用芯片的镀层原来是为金丝引线键合(细铜丝引线键合)而设计的。粗铜球比金球要硬,所以在粗铜丝引线键合的时候会遇到类似于弹坑等现象。这个是因为粗铜球焊接的功率和压力比金球要大,而芯片的镀层相对又比较薄。根据这个情况,在进行粗铜线引线键合的时候,可以适当提高温度和时间,像功率和压力此类参数不宜增加太大。
解决这个问题有一个原则,就是先调整参数使内焊点打不粘,然后逐步增加功率,压力以及压焊时间,直到金球剪切力达到工艺要求为止。不要过分追求过大的剪切力,而追求金球剪切力的CPK值。在实际批量生产过程中遇到压焊区铝垫面积大小不等的各类芯片,压焊区面积大小不同,则解决弹坑与失铝的办法也不尽相同。
铝层厚度4.5μm,粗铜丝φ3mil~5mil,优化后的工艺参数:
打火电流:250mA~360mA、打火时间:800μs~1800μs、打火尾丝:100mil~120mil、初始功率:20/80W~50/150W、初始压力:350/400克~900/800克、初始时间:2/2ms、焊接功率:65/150W~90/200W、焊接压力:80/200克~250/600克、焊接时间:10/15ms~10/20ms;
本发明即按照优化后的工艺参数,进行芯片与粗铜丝引线键合的操作。
与现有技术相比,本发明的优点在于:与现行的金丝引线键合技术相比,不仅可以节省黄金、降低成本,而且可以减缓脆性金属化合物的形成,提高键合强度,采用铜丝键合新工艺不但能降低器件制造成本,而且其互连强度比金丝还要好。因而,在今后的微电子封装发展中,铜丝引线键合会成为主流技术。铜丝与传统的晶片上铝金属化焊区的键合,可降低成本,使高产、细间距封装的焊丝更牢固和坚硬。
附图说明
图1是保护气体装置的立体示意图1。
图2是保护气体装置的立体示意图2。
图3是喷嘴安装示意图1。
图4是喷嘴安装示意图2。
图5是喷嘴安装位置尺寸图。
图6是劈刀外形图示。
图7是劈刀的仰视图(放大)。
图8是劈刀头部纵向局部剖视图(放大)。
图9是实验流程图。
具体实施方式
以下结合附图实施例对发明作进一步详细描述。
本发明的主要技术关键点在于:铜球结晶时保护、劈刀的设计优化、芯片铝层厚度检测、关键工艺参数的优化匹配。
铜球结晶时保护
保护气体的装置由四部分构成:与设备固定的保护气体装置的固定脚1、第一中间连接段2、第二中间连接段3、保护气体装置的喷嘴4,喷嘴的口径大小要求φ2mm。
保护气体装置的固定脚1固定在设备焊头部位,此固定脚材料是铝合金;第一中间连接段2与保护气体装置的固定脚1相连,并且两者之间相连有一弹簧,第一中间连接段2的材料是铝合金;第二中间连接段3与第一中间连接段2相连,第二中间连接段3的材料是绝缘胶木;保护气体装置的喷嘴4与第二中间连接段3相连,喷嘴4的材料是铝合金。
使用方法:焊接过程增加N2、H2保护气体,对气体纯度及氢气混合比以及保护气体的流量有较严的要求。保护气体纯度含氧量小于1PPM;露点小于-40℃;H2体积比比例5%~10%,流量0.5l/min。
上述的装置在安装时,其中需特别关注的是劈刀5、打火杆6、保护气体的喷嘴4三者间的距离、角度以及打火杆表面的洁净度,喷嘴口必须与烧球平面平行,同时离劈刀口h约2mm,如图5所示。打火产生的火焰线条要与喷嘴方向平行,并且在其内圈0.2毫米以内。这样就可保证铜球得到有效的保护。
图3、图4为喷嘴安装示意图,但采用图4的喷嘴安装示意图与图3的喷嘴安装示意图比较有以下几方面的优点:劈刀装卸方便,在作业时,穿铜线方便。
图3的喷嘴安装方法,在安装劈刀时,都需对打火杆、劈刀、喷嘴的位置进行微调,调整的难度增加,不便于作业,需耗费的时间较多。
采用图4的设计模式,只需一次性地将喷嘴外套7固定好,装卸时,与金丝引线键合调整方式一样,不增加维修人员与操作人员工作难度。只要铜丝能打火成功,就能得到有效的气体保护。
劈刀的设计优化
设计特殊规格的劈刀,有三个方法:一是对现有劈刀的角度进行修正,将劈刀的外角度改小,或内角度改大,在脚位上焊接时,不会将铜线切断;另一种方法是将劈刀嘴的结构进行变更,劈刀嘴结构中,有几个重要之处,阳角、阴角的大小确定,劈刀头锥度的选择,劈刀材料的选择以及表面光洁度的选择。
粗铜丝引线键合劈刀的外形具体尺寸(结合图6-图8):
优化后的φ3mil粗铜丝引线键合劈刀关键尺寸:
H:孔径是102μm、CD:倒角直径是152μm、CA:内倒角孔角是120°、T:头部直径是356μm、FA:端面角是4°
优化后的φ4mi粗铜丝引线键合劈刀关键尺寸:
H:孔径是140μm、CD:倒角直径是180μm、CA:内倒角孔角是120°、T:头部直径是419μm、FA:端面角是4°
优化后的φ5mil粗铜丝引线键合劈刀关键尺寸:
H:孔径是175μm、CD:倒角直径是215μm、CA:内倒角孔角是120°、T:头部直径是465μm、FA:端面角是4°
芯片铝层厚度的检测
一准备
检查实验器具(200mL小烧杯,800mL大烧杯各一个,100℃温度计两根,小电炉一个,纯度大于85%的磷酸溶液及秒表一个)。在框架两头处分别装上2-3个同型号待测芯片,按照型号对装有待测芯片的框架进行归类、标记。
二烧水
将50mL左右磷酸倒入干净的小烧杯中。打开电炉,将装有三分之二自来水的大烧杯放在炉上加热,同时将小烧杯放到大烧杯中,注意别让大烧杯中的水流入小烧杯。
分别在两烧杯中放入温度计,同时观察温度变化,等到大烧杯中水温到60℃时关掉电炉(不必取下烧杯),轻轻摇动小烧杯,并观察小烧杯中温度计读数,到58℃以上时就可以把框架放入。
三溶解
在小烧杯中放入粘有芯片的框架,确保框架其中一头的芯片至少1颗完全沉入溶液,同时在表格上记录初始时间。观察芯片气泡产生以及小烧杯中的温度计读数,如果温度超出62℃,则可将小烧杯取出,保证其温度在58℃-62℃。当观察到芯片表面有部分颜色变暗,则表示铝层即将溶解完全,准备记录时间,待芯片白色铝层褪尽,表面无气泡产生时,溶解完成。把结束时间记录在表格上。
四实验结束并处理数据
将所有芯片的实验完成并记录完数据后,关电炉,收拾器具(用过的磷酸可妥善保存,重复利用)。把结束时间减取开始时间,再转化成秒得到溶解时间,再参看附表一,确定其厚度,若仍未能确定,则先定厚度范围,再根据该范围的平均溶解速度计算其厚度值。
铝层厚度=(溶解时间-最邻近标样溶解时间)*该范围平均溶解速度+该标样厚度实验流程图见图9所示。
附表一:
关键工艺参数的优化
先调整参数使内焊点打不粘,然后逐步增加功率,压力以及压焊时间,直到金球剪切力达到工艺要求为止。不要过分追求过大的剪切力,而追求金球剪切力的CPK值。在实际批量生产过程中遇到压焊区铝垫面积大小不等的各类芯片,压焊区面积大小不同,则解决弹坑与失铝的办法也不尽相同。
具体情况如下:
压焊区面积较大的芯片(金球直径尺寸>150μm)可用加初始压力的办法解决。在实践中经常将此数值设置到150~400g之间;初始时间的数值设置在2~5ms之间;初始功率的数值设置在10~50W之间。这样在超声能量没有加到压焊区铝垫之前,先加一个压焊压力让金球先压扁变形,在超声功率作用时就有面积较大的金球覆盖在铝垫上而不至于将铝垫撕裂或留下小洞。
对压焊区面积较小的芯片可用设置预超声能量的办法解决。因为压焊区面积小,铜球变形不能过大而超出压焊区造成短路,因为采用以上的办法来解决弹坑与失铝问题。在较大的超声功率作用前,先加一个小的超声能量将铜球在铝垫上振荡压变形,这样就可避免因较大的超声功率作用导致能量过大而将铝层撕裂或留下小的空洞。当然降低tip的高度,让压焊头以一个较低的速度平缓地运动到芯片压焊区铝垫也是解决弹坑与失铝的一个有效办法。
铝层厚度4.5μm,粗铜丝φ3mil,优化后的工艺参数:
打火电流:250mA、打火时间:800us、打火尾丝:100mil
初始功率:20/80W、初始压力:350/400克、初始时间:2/2ms
焊接功率:65/150W、焊接压力:80/200克、焊接时间:10/15ms
铝层厚度4.5μm,粗铜丝φ4mil,优化后的工艺参数:
打火电流:310mA、打火时间:1500us、打火尾丝:110mil
初始功率:50/150W、初始压力:800/600克、初始时间:2/2ms
焊接功率:85/180W、焊接压力:200/500克、焊接时间:10/20ms
铝层厚度4.5μm,粗铜丝φ5mil优化后的工艺参数:
打火电流:360mA、打火时间:1800us、打火尾丝:120mil
初始功率:50/150W、初始压力:900/800克、初始时间:2/2ms
焊接功率:90/200W、焊接压力:250/600克、焊接时间:10/20ms。
在进行芯片与粗铜丝引线键合时,即按照上述优化后的工艺参数进行操作。
Claims (8)
1.一种粗铜丝引线键合的实现方法,其特征在于包括下列操作条件:
(1)铜球结晶时保护
在粗铜丝引线键合过程中,采用N2、H2混合保护气体:在焊接设备上固定有一保护气体装置,先调节好该装置相对劈刀、打火杆的位置后,接上N2、H2混合保护气体,并调整好N2、H2混合气体的流量,其中混合保护气体纯度含氧量小于1PPM;露点小于-40℃;H2体积比比例5%~10%,流量0.5l/min;
(2)劈刀的选取
在粗铜丝引线键合作业时采用如下规格的劈刀:
H:孔径是102μm~175μm、CD:倒角直径是152μm~215μm、CA:内倒角孔角是120°~150°、T:头部直径是356μm~465μm、FA:端面角是4°;
(3)芯片表面铝层厚度检测
将芯片放置在纯度大于85%的磷酸溶液中,根据铝层的溶解时间与标准样本的比较,计算出铝层的厚度;检测出来的芯片表面铝层厚度大于4μm时,用粗铜线引线键合;
(4)关键工艺参数的优化匹配
铝层厚度4.5μm,粗铜丝φ3mil~5mil,优化后的工艺参数:
打火电流:250mA~360mA、打火时间:800μs~1800μs、打火尾丝:100mil~120mil、初始功率:20/80W~50/150W、初始压力:350/400克~900/800克、初始时间:2/2ms、焊接功率:65/150W~90/200W、焊接压力:80/200克~250/600克、焊接时间:10/15ms~10/20ms;
按照优化后的工艺参数,打着打火杆,进行芯片与粗铜丝引线键合的操作。
2.如权利要求1所述的粗铜丝引线键合的实现方法,其特征在于:所述的保护气体装置主要由依次相连的固定脚、第一中间连接段、第二中间连接段以及喷嘴组装构成,其中固定脚与设备焊头部位相固定,喷嘴口为圆形,口径为φ2mm。
3.如权利要求2所述的粗铜丝引线键合的实现方法,其特征在于:所述的装在焊接设备上的保护气体装置的喷嘴,其喷嘴口要与烧球平面平行,同时离劈刀口约2mm,打火产生的火焰线条要与喷嘴方向平行,并且在其内圈0.2毫米以内。
4.如权利要求2或3所述的粗铜丝引线键合的实现方法,其特征在于:所述的第二中间连接段用绝缘胶木做成。
5.如权利要求1所述的粗铜丝引线键合的实现方法,其特征在于:在粗铜丝引线键合作业时使用的劈刀,规格有3种,适于φ3.0mil铜丝引线键合的A劈刀,适于φ4.0mil铜丝引线键合的B劈刀,适于φ5.0mil铜丝引线键合的C劈刀,其中
A劈刀:H:孔径是102μm、CD:倒角直径是152μm、CA:内倒角孔角是120°、T:头部直径是356μm、FA:端面角是4°
B劈刀:H:孔径是140μm、CD:倒角直径是180μm、CA:内倒角孔角是120°、T:头部直径是419μm、FA:端面角是4°
C劈刀:H:孔径是175μm、CD:倒角直径是215μm、CA:内倒角孔角是135°、T:头部直径是465μm、FA:端面角是4°。
6.如权利要求1所述的粗铜线引线键合的实现方法,其特征在于:所述的芯片表面铝层厚度检测采用的标准样本为:
7.如权利要求6所述的粗铜线引线键合的实现方法,其特征在于:所述的芯片表面铝层厚度检测包括下列步骤:
(1)将50mL左右磷酸倒入干净的200mL小烧杯中,打开电炉,将装有三分之二自来水的800mL大烧杯放在炉上加热,同时将小烧杯放到大烧杯中,注意别让大烧杯中的水流入小烧杯,分别在两烧杯中放入温度计,同时观察温度变化,等到大烧杯中水温到60℃时关掉电炉,轻轻摇动小烧杯,并观察小烧杯中温度计读数,到58℃以上时就可以把粘有芯片的框架放入;
(2)在小烧杯中放入粘有芯片的框架,确保框架其中一头的芯片至少1颗完全沉入溶液,同时在表格上记录初始时间,观察芯片气泡产生以及小烧杯中的温度计读数,如果温度超出62℃,则可将小烧杯取出,保证其温度在58℃-62℃,当观察到芯片表面有部分颜色变暗,则表示铝层即将溶解完全,准备记录时间,待芯片白色铝层褪尽,表面无气泡产生时,溶解完成,把结束时间记录在表格上;
(3)把结束时间减去开始时间,再转化成秒得到溶解时间,再根据铝层厚度的标准样本对比数据,确定其厚度,若仍未能确定,则先定厚度范围,再根据该范围的平均溶解速度计算其厚度值;计算公式铝层厚度=(溶解时间-最邻近标样溶解时间)*该范围平均溶解速度+该标样厚度。
8.如权利要求1所述的粗铜线引线键合的实现方法,其特征在于:所述的优化后的工艺参数包括下列几组:
铝层厚度4.5μm,粗铜丝φ3mil,优化后的工艺参数:
打火电流:250mA、打火时间:800us、打火尾丝:100mil
初始功率:20/80W、初始压力:350/400克、初始时间:2/2ms
焊接功率:65/150W、焊接压力:80/200克、焊接时间:10/15ms
铝层厚度4.5μm,粗铜丝φ4mil,优化后的工艺参数:
打火电流:310mA、打火时间:1500us、打火尾丝:110mil
初始功率:50/150W、初始压力:800/600克、初始时间:2/2ms
焊接功率:85/180W、焊接压力:200/500克、焊接时间:10/20ms
铝层厚度4.5μm,粗铜丝φ5mil优化后的工艺参数:
打火电流:360mA、打火时间:1800us、打火尾丝:120mil
初始功率:50/150W、初始压力:900/800克、初始时间:2/2ms
焊接功率:90/200W、焊接压力:250/600克、焊接时间:10/20ms。
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