SMT (Surface Mount Technology), like other SMD Soldering and PCB assembly technology is not ZERO-Defect soldering process. There will always be some or the other defect in any Electronics PCB assembly in both Thru-Hole and SMT.
Here I will discuss some of the most common faults and causes of SMT defects and possible solution and troubleshooting.
Common Faults in SMT:
Here I will discuss some of the most common faults and causes of SMT defects and possible solution and troubleshooting.
Common Faults in SMT:
- Solderballs
- Solder beading
- Bridging
- Open- Insufficient
- Tombstoning
- Unmelted paste
- Excessive filet
- Slump
- Dewetting
- Disturb joint
- Orange Skinning
Possible cause:
Possible cause:
Possible cause:
Possible cause:
- Too slow ramp up in reflow profile
Remedy: Run recommended profile and see if problem stays
Possible cause:
- Too rapid ramp up in flow profile
Remedy: Run a slower ramp up profile to give volatiles to evaporate
- Solder Paste smearing on underside of stencil.
- What is squeegee pressure?
- Is stencil underside cleaned with a solvent and is solvent still present after cleaning?
- Is stencil proper aligned with PCB?
- Check squeegee pressure
- Check for proper gasketting and alignment
- Check if cleaning solvent is completely evaporated before printing
- Was paste shipped refrigerated?
- Did paste spend a long time in a hot area?
- Was old paste returned to jar?
- Was jar put back into refrigeration after opening?
- Is alloy sensitive to oxidation?
Possible cause:
- Squeegee pressure too high
- Paste gets squeezed out between stencil and board
Possible cause:
- Drying out of paste after printing
- What is specified tack time of paste?
Possible cause:
- Too slow ramp up in reflow profile
Remedy: Run recommended profile and see if problem stays
Possible cause:
- Too rapid ramp up in flow profile
Remedy: Run a slower ramp up profile to give volatiles to evaporate
SOLDER BEADS
Possible cause:
Possible cause: Excessive amount of solder paste on component pads
- Reflow profile ramp up slow
- Capillary action draws unreflowed paste away from pad to somewhere under the component, it reflows there and form a bead of solder which comes out from under the component side.
Possible cause: Excessive amount of solder paste on component pads
- What is stencil thickness?
- Are apertures reduced?
- Dispense time for a dot?
- Reduce aperture size of stencil or use thinner stencil
- Use smaller needle and/or reduce purge time on dispenser
- What is squeegee pressure?
- Is stencil underside cleaned with a solvent and is solvent still present after cleaning?
- Is stencil properly aligned with PCB?
- Check squeegee pressure
- Check for proper gasketting and alignment
- Check if cleaning solvent is completely evaporated before printing
BRIDGING
Possible Cause:
- Cold slumping
- Is paste flowing apart after printing, height of deposit reduces and surface increases.
- Check viscosity of paste, too low viscosity may result in cold slumping
- Check print speed, too rapid print speed may result in shearing of paste and degrading its thickness
- Check temperature in printer, too high temperature brings viscosity down
- Hot slumping
- Is paste flowing apart during ramp up part of reflow profile
- Shorten duration of ramp up cycle in reflow profile
- Paste smearing on underside of stencil
- Paste can be outside pad area and form solder balls between two component leads resulting in a bridge
- Reduce squeegee and check pcb-stencil alignment and gasketting
- Excessive solder paste being deposited on the pads
- While placing a component on the pads the paste is smeared out and may form a bridge to an adjacent pad
- Reduce amount of solder paste
- Increasing print speed may
- Reduce stencil thickness
Open-insufficient
Possible cause:
Remedy: Unblock apertures and clean stencil
Possible Cause:
Remedy: Check viscosity and metal content
- Scooping during printing
Excessive squeegee pressure on a polypropylene squeegee may cause scooping
- Reduce squeegee pressure or use a harder durometer type of squeegee or use a metal squeegee
Remedy: Unblock apertures and clean stencil
Possible Cause:
- Foreign material on solder pad
- Was solder mask printed on pad?
- Use another PCB
- Too high squeegee speed
- Paste cannot get into the apertures
- Reduce squeegee speed
Remedy: Check viscosity and metal content
TOMBSTONING
Possible cause: Unequal placement of components on pads prior to Reflow results in unbalanced solder forces.
Remedy: Check if placement equipment places properly.
Possible cause: Unequal heat sink i.e. ground planes inside PCB layers may draw heat away from pad.
Remedy: Increase soak time (plateau) or reflow profile so that all components are on.
Remedy: Check if placement equipment places properly.
Possible cause: Unequal heat sink i.e. ground planes inside PCB layers may draw heat away from pad.
Remedy: Increase soak time (plateau) or reflow profile so that all components are on.
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UNMELTED PASTE
Possible cause: To cold reflow profile
- Solder paste cannot melt completely
Excessive Fillet
Possible cause:
- Too much solder paste deposited on pads
- If excess solder occurs on all components reduce overall stencil thickness or reduce dispenser purge time
- If excess solder occurs on some places only reduce stencil thickness or dispense purge time for these components only
Slump
Cold Slump
Possible cause: Viscosity of paste to low or metal content to low
Remedy: Use different type of paste with higher viscosity or higher metal content
Possible cause: Paste came in contact with a cleaning solvent or other alien product
Remedy:
Possible cause: Temperature of paste is too high while printing or dispensing
Remedy:
Possible cause: Too slow ramp up in reflow profile
Remedy: Increase ramp up temperature, make sure to have a ramp up between 2 Degree Celsius to 3 Degree Celsius per second
Possible cause: Viscosity of paste to low or metal content to low
Remedy: Use different type of paste with higher viscosity or higher metal content
Possible cause: Paste came in contact with a cleaning solvent or other alien product
Remedy:
- Make sure there are no solvents presents after cleaning the screen
- Never try to revive the paste by adding some compound
- Squeegee pressure to high
- Paste is shearing due to excessive pressure applied to it thickeners in paste are destroyed
Possible cause: Temperature of paste is too high while printing or dispensing
Remedy:
- Check temperature inside printer
- Reduce pressure on squeegee
- Reduce on pressure on syringe when dispensing
Possible cause: Too slow ramp up in reflow profile
Remedy: Increase ramp up temperature, make sure to have a ramp up between 2 Degree Celsius to 3 Degree Celsius per second
Hot Slump
Possible cause: Too slow ramp up in reflow profile
Remedy: Increase ramp up temperature, make sure to have a ramp up between 2 Degree Celsius to 3 Degree Celsius per second
Remedy: Increase ramp up temperature, make sure to have a ramp up between 2 Degree Celsius to 3 Degree Celsius per second
DEWETTING
Possible cause:
- Unwanted material on surface which prevents solder from attaching to surface, i.e. solder mask, fingerprints or oxides.
- Clean boards first
- Use different batch of boards
- Bad alloy in HAL process, i.e. too much Cu elevates melting point of HAL alloy
- Increase peak temperature in reflow
- Use different batch of boards
Disturbed Joint
Possible cause: A source of vibration that is transmitted through the pcb during the liquids state of the reflow profile
Remedy:
Remedy:
- Find and fix source of vibration
- Adjust reflow
Orange skinning
Possible cause:- Too high in peak zone
- Residue is burned or rosin was cooking
- Lower peak zone temperature
- Too long exposure to temperatures between activation temp and reflow = (depending on alloy)
- Shorten time in soak or lower soak temperatures
- Preheating too high
- Lower preheat temperatures
Very informative post. Keep up the good work. I would really look forward to your other posts
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