SMT焊接不良及图片说明8--Tombstone(墓碑)
日期:2009-12-07 21:26 点击:1517
8.Tombstone( 墓碑 )
拒收 良好情r
缺c描述 :
Chip component standing in a vertical or near vertical position with only one termination soldered to the PCB.
PROBABLE CAUSES
- Excessive heating rate
- Uneven heating resulting a differential across the component terminals
- Solderability of the two terminations of the component and PCB pads
- Uneven paste deposition on the two solder pads
- PCB contamination
- Pad size mismatch
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- Insufficient tack force of the solder paste to hold component due to temperature and humidity effect
- Excess movement during and after reflow operation
- Inadequate placement force to make intimate contact between the paste and the termination of pads
- Adhesive sparsely applied
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POSSIBLE PREVENTIONS
- Maintain the solder temperature accordingly to the manufacturers’ specifications
- Increasing the preheating temperature so as to reduce the temperature differential between the two terminations of component
- Ensuring consistent solder paste deposit heights between pads
- Selecting components and PCBs with consistently solderable leads and pads
- Avoiding inefficient tack force by avoiding extreme environmental conditions
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- Minimizing the amount of movement the assembly sees during reflow
- Increasing component placement force to ensure contact of the component terminal to the solder paste deposit
- Ensure that the board is free of contaminants
- Adhesive applied to hold the component down must just be sufficient enough to ensure proper holding strength
- Make solder pad smaller
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REWORK
For SMD :
The solder joint is desoldered with the help of a chisel-shaped soldering tip and solder wick.The solder wick should be pressed into the corner long enough to let the heat travel into the gap underneath the component so that the solder is sucked out and the damaged component is replaced by another good one as solderability is doubtful.
