SMT缺陷及防范措施之一

Identify SMT defects

Identify the SMT defects according to IPC-A-610 (Rev: D)
根据 IPC-A-610 (Rev: D)确定缺陷．
     
This standard is a collection of visual quality acceptability requirements for electronic assemblies.

IPC-A-610是关于电子组装外观质量验收条件要求的文件：

缺陷的主要分布

urce: MPM’s A user’s guide to more Precise SMT printing

Know the common defect
       了解常见缺陷类型
 Analysize the possible cause
       分析可能的原因
 Countermeasure for the defects
         基于以上原因采取的对策

一、Tombstoning/ 立碑

hip components standing on a terminal end (tombstoning).
片式元件末端翘起（立碑）

Countermeasure/ 对策
Mechanism: Surface tension in component terminal is uneven in reflow.
原理：在回流过程中零件两末端的表面张力不均衡.
1、Component terminal heat distributed unevenly
零件两末端受热分布不均衡.
  Insufficient soak time: Reflow profile optimization.
 保温区时间太短:优化回流曲线参数.
2、PCB Pad design issue (the pads distance is too big): improve PCB pad design.
PCB焊盘设计问题(焊盘间距太大):改善PCB焊盘设计.

   
     3、Component terminal oxidization or contamination : Solderability test if necessary and RTV the defect material.
    元件末端氧化或者受污染: 根据情况做可焊性实验并且退还缺陷物料.

4、Printing misalignment: adjust printing machine parameters.
    丝印偏位: 校正丝印机的参数.
      5、Placement misalignment: Optimize the P&P machine parameters.
    贴片偏位: 优化贴片机的参数.
      6、Stencil aperture design issue: study and improve the aperture design.
     钢网开孔的设计问题: 研究并且改善开孔设计.
二、Solder bridge  桥联
 A solder connection across conductors that was joined.
 焊锡在导体间非正常连接
• Solder has bridged to adjacent non-common conductor or component.
 焊锡桥连到相邻的非导体或元件

Countermeasure/对策
　1. Screen printing issue/丝印问题：
     a) Paste height out of UCL:  Adjust the printer to control the paste height.
         锡膏高度超出控制上线: 校正丝印机, 控制锡膏高度
     b) Paste printing misalignment or bridging: Fine tune printing machine.

锡膏印刷偏位或者桥联: 优化丝印机
     c) Icicle printing: Fine tune printing machine or change to lower viscosity paste.
         丝印拉尖: 优化丝印机或者使用低粘性的锡膏 
     d) Solder paste collapse: change to higher viscosity paste.

锡膏塌陷:使用高粘性的锡膏
     e) Nonstandard stencil aperture opening : Study and improve the aperture.

不标准的钢网开孔: 研究并改善开孔

Icicle printing 丝印拉尖

Bridging 桥联

Paste collapse  锡膏塌陷

Misalignment 印刷偏位
Countermeasure/对策
    2、Pick & Placement/贴片
    a) Component placing misalignment: Fine tune the P&P machine .
        元件贴片偏位: 优化贴片机的参数.
    b)  High pressure for placement: reduce pressure to proper value .
        贴片压力过大: 减少压力到适当的参数.
    3. Wave soldering 波峰焊
    a) Low conveyor ramp: Raise the conveyor ramp per actual status.
        传送带角度过小: 根据实际情况加大传送带角度
    b) Less flux on board : Increase the flux spray volume on board before going through wave soldering.
        板上的助焊剂较少: 波峰焊接之前加大助焊剂的喷射量

三、元件损坏
Nicks, cracks, or stress fractures.
缺口，裂纹，压痕

Countermeasure/对策
1、Raw material damaged: purge the batch material
来料损坏： 清除这批来料.
2、Damaged during SMT process (Fixture or machine touch it) : Investigate and take corrective actions for nonstandard operation.
      在SMT流程中损坏(夹具和设备接触)：调查分析并且对不规范的操作作出矫正行动.
3、Fast cooling rate on reflow process : Control the cooling rate to below 4degree per second.
      在回流过程中冷却速率过快： 控制冷却速度使斜率保持在每秒4度以下.
 Typical reflow profile
回流曲线典例：

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