Paste Reflow Vs. Hand Reworked 焊膏再流焊与手工返工
Paste Reflowed Hand Reworked
焊膏再流焊后
手工返工后
Short Circuit Fault Site 短路发生部位
The First Uncoated Failure Site 首个没有涂敷的缺陷
First Hypothesis Refuted 第一种假设被否定
Desoldering Braid 吸锡编带
Braid Strand 编带的股线
Fault Path 实际缺陷
The Fault Path Material Is Analyzed 缺陷的材质分析
• It was not feasible to chemically analyze the intact fault path material. The assembly was too large to fit into an SEM/EDS chamber, and too expensive to subject to a destructive excision. • 用化学方法对整个缺陷材质进行分析是不可行的。组件 又很大，无法放入SEM/EDS检查腔中，而进行破坏性分 析又过于昂贵 • The fault path material was excised with a precision scalpel. • 缺陷材料用一精密刀片被取下 • The material did not stretch like solder, but instead broke like glass. • 材料并不象焊料一样能够延展，而是象玻璃一样破碎
Jeff Kukelhan, BAE Systems Electronics, Intelligence, & Support Fort Wayne, Indiana
Background
背景
• BAE Systems EI&S manufactures avionics hardware at its Fort Wayne, Indiana site • 印地安娜州的BAE Fort Wayne生产制造商业与军事航 空用电子硬件. • Many of the applications contain surface mount assemblies. • 许多产品包含表面贴装组件. • The surface mount assemblies are made with either Sn63/Pb37 or Sn62/Pb36/Ag2 solder pastes • 生产中使用的焊膏既有Sn63/Pb37也有Sn62/Pb36/Ag2. • The assemblies are stencil printed, and reflowed on a conventional belt oven • 生产工艺为模板印刷和再流焊.
SEM Micrograph Of Fault Path Material 缺陷的SEM显微照片
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Location 1: Attached Solder 位置1：附加的焊料
Elt. Line Intensity (c/s) Cu Ka 3.09 Sn La 114.39 Pb Ma 57.43 Error 2-sig 0.098 0.596 0.422 Atomic % 3.823 72.726 23.451 100.000 Conc 1.769 62.853 35.378 100.000 Units wt.% wt.% wt.% wt.%
• The fault paths were not invisible, but were difficult to see below 30X magnification. • 肉眼不可见，且在低于30X放大镜下也很难看到 • Each path consisted of one or more needle like structures that measured a few microns in diameter. • 每处都含一个或多个直径只有几微米的针形结构， • The fault paths were confined to gull wing devices with a lead pitch of 0.65 mm or less. • 缺陷只发生在节距为0.65mm及以下的鸥翼形引线 • The fault paths were always found between hand reworked solder joints. • 缺陷总是在手工返工的焊点处发现
Early Conclusions 早期结论
• The fault paths were clearly not a thin string of solder left behind by a solder iron. • 缺陷并非焊接后烙铁遗留的细丝 • Molten solder’s surface tension favors the formation of solder balls, not micron wide needles that meet at angular intersections. • 熔融焊料在表面张力的作用下是形成球形而 非成一个角度相交的微米级针形
Total
Location 2: Needle Like Material 位置2：针形物质
Cu6.7Sn5
Elt. Line Intensity (c/s) Cu Ka 50.44 Sn La 84.87 Pb Ma 4.53 Error 2-sig 0.343 0.445 0.103 Atomic % 56.408 41.917 1.675 100.000 Conc 40.241 55.863 3.896 100.000 Units wt.% wt.% wt.% wt.%
An Unusual Production Failure Mode 一种不寻常的缺陷模式
• • • • • Adjacent fine pitch component leads were short circuiting 细节距器件相邻引脚发生短路. There was no visible failure path 短路肉眼看不到. The fault condition disappeared from the assembly when the affected component was removed • 当发生问题的器件被拆下后，缺陷也随之消失. • The fault condition also disappeared from the removed component • 拆下的器件也没有发现短路缺陷.
The First Hypothesis 第一种假设
• Fine pitch solder joints are hand reworked to eliminate solder paste reflow bridges. • 手工返工是为了消除细节距焊点处的再流焊桥连 • The excess solder is drawn off with desoldering braid: a woven ribbon of fine copper wire. • 多余的焊料被吸锡编带吸走 • Hypothesis: A short circuit path forms when an individual strand of copper wire breaks away from the desoldering braid, and lodges between two fine pitch leads. • 假设：短路是由一根从吸锡编带上断掉的铜丝造成的
Copper Tin Intermetallic Crystals And Their Role In The Formation Of Microbridges Between Hand Reworked Fine Pitch Component Leads
铜锡金属化合物晶体及其在细节距器件手工返工时微桥接形成 中的作用
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Fault Path Chemistry 缺陷的化学性质
• The actual fault path material contained virtually no lead (Pb). • 实际的缺陷材料几乎不含铅 • It was composed primarily of tin (Sn) and copper (Cu). • 主要是锡和铜的化合物 • The copper and tin were present in ratios that closely matched that of Cu6Sn5, a known intermetallic compound. • 铜与锡的比例接近众所周知的金属间化合物Cu6Sn5 • EDS analysis of additional production fault sites yielded the same results. • 对于生产中缺陷位置附加的EDS分析也得出相同结论。
A Key Test Engineering Observation 测试时的一个重要发现
• The invisible short circuit fault paths could sometimes be eliminated by running a test probe between the affected fine pitch leads. • 不可见的短路有时会因有测针在引脚间划过 而消失
“Cleared” Short Circuit Fault Paths “被清除”的短路
Probe Scrape Marks 测针划痕
Broken Remnant 被破坏的短路残留
Isolating The Fault Condition 缺陷的分离