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SMT Troubleshooting

SMT Troubleshooting (SMT / SMD Problem and Solution) – SMT is NOT ZERO-Defect soldering process. Problems will Occur, but Solutions are there.

SMT Troubleshooting will always be there. SMT is NOT ZERO-Defect soldering process. Problems will Occur, but Solutions are there.

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.

SMT Troubleshooting

SMT Troubleshooting

Common Faults for SMT Troubleshooting

  • Solder balls
  • Solder beading
  • Bridging
  • Open- Insufficient
  • Tombstoning
  • Unmelted solder paste
  • Excessive filet
  • Slump
  • Dewetting
  • Disturb joint
  • Orange Skinning

Solder Balls – Problem and Solution

Possible causes:

Solder balls=numerous tiny solder balls trapped along the outside edge of the flux residue

Solder balls = numerous tiny solder balls trapped along the outside edge of the flux residue

 

  1. Solder Paste smearing on underside of stencil.
  2. What is squeegee pressure?
  3. Is stencil underside cleaned with a solvent and is solvent still present after cleaning?
  4. Is stencil proper aligned with PCB?

Solution to Solder Ball Problems:

  1. Check squeegee pressure
  2. Check for proper gasketting and alignment

    Solder balls=numerous tiny solder balls trapped along the outside edge of the flux residue

    Solder balls = numerous tiny solder balls trapped along the outside edge of the flux residue

  3. Check if cleaning solvent is completely evaporated before printing

Oxidised Paste -Problem and Solution

Possible cause

  1. Was paste shipped refrigerated?
  2. Did paste spend a long time in a hot area?
  3. Was old paste returned to jar?
  4. Was jar put back into refrigeration after opening?
  5. Is alloy sensitive to oxidation?

Solution:

  1. Run fresh paste from a different lot under same conditions and see if solder bars go away.

Possible cause

  1. Squeegee pressure too high
  2. Paste gets squeezed out between stencil and board

Solution: Reduce squeegee pressure

Possible cause:

  1. Drying out of paste after printing
  2. What is specified tack time of paste?

Solution: Run a PCB with fresh paste and see if problem goes away

Possible cause:

  1. Too slow ramp up in reflow profile

Solution: Run recommended profile and see if problem stays

Possible cause:

  1. Too rapid ramp up in flow profile

Solution: Run a slower ramp up profile to give volatiles to evaporate

SOLDER BEADS -Problem and Solution

Possible cause:

Solder beading : solder balls that are alongside a components

Solder beading : solder balls that are alongside a components

  1. Reflow profile ramp up slow
  2. 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.

Solution: Run a more rapid ramp up profile of 1.5 Degree Celsius to 2.5 Degree Celsius per second.

Possible cause:

  1. Excessive amount of solder paste on component pads
  2. What is stencil thickness?
  3. Are apertures reduced?
  4. Dispense time for a dot?

Solution:

  1. Reduce aperture size of stencil or use thinner stencil
  2. Use smaller needle and/or reduce purge time on dispenser

Possible cause: Paste smearing on underside of stencil

  1. What is squeegee pressure?
  2. Is stencil underside cleaned with a solvent and is solvent still present after cleaning?
  3. Is stencil properly aligned with PCB?

Solution:

  1. Check squeegee pressure
  2. Check for proper gasketting and alignment
  3. Check if cleaning solvent is completely evaporated before printing

BRIDGING -Problem and Solution

Possible Cause:

Bridging=solder running from one component contact to another resulting in a short circuit

Bridging = solder running from one component contact to another resulting in a short circuit

  1. Cold slumping
  2. Is paste flowing apart after printing, height of deposit reduces and surface increases.

Solution:

  1. Check viscosity of paste, too low viscosity may result in cold slumping
  2. Check print speed, too rapid print speed may result in shearing of paste and degrading its thickness
  3. Check temperature in screen printer, too high temperature brings viscosity down

    Bridging=solder running from one component contact to another resulting in a short circuit

    Bridging = solder running from one component contact to another resulting in a short circuit

Possible Cause:

  1. Hot slumping
  2. Is paste flowing apart during ramp up part of reflow profile

Solution: Shorten duration of ramp up cycle in reflow profile

Possible Cause:

  1. Paste smearing on underside of stencil
  2. Paste can be outside pad area and form solder balls between two component leads resulting in a bridge

Solution: Reduce squeegee and check PCB-stencil alignment and gasketting

Possible Cause:

  1. Excessive solder paste being deposited on the pads
  2. While placing a component on the pads the paste is smeared out and may form a bridge to an adjacent pad

Remedy:

  1. Reduce amount of solder paste
  2. Increasing print speed
  3. Reduce stencil thickness

Open-insufficient – Problem and Solution

Possible cause:

Opens and insufficient=insufficient or no solder to make a complete bond between the lead and the pad

Opens and insufficient = insufficient or no solder to make a complete bond between the lead and the pad

  1. Scooping during printing
  2. Excessive squeegee pressure on a polypropylene squeegee may cause scooping

Remedy: Reduce squeegee pressure or use a harder durometer type of squeegee or use a metal squeegee

Possible Cause: Blocking of stencil aperture with dried up paste

Remedy: Unblock apertures and clean stencil

Possible Cause:

Opens and insufficient=insufficient or no solder to make a complete bond between the lead and the pad

Opens and insufficient = insufficient or no solder to make a complete bond between the lead and the pad

  1. Foreign material on solder pad
  2. Was solder mask printed on pad?

Remedy: Use another PCB

Possible Cause:

  1. Too high squeegee speed
  2. Paste cannot get into the apertures

Remedy: Reduce squeegee speed

Possible cause: Solder paste viscosity and/or metal content too low

Remedy: Check viscosity and metal content

TOMBSTONING – Problem and Solution

Tombstoning=chip type components standing up on one end after reflow caused by unequal forces on the components end

Tombstoning = chip type components standing up on one end after reflow caused by unequal forces on the components end

Possible cause: Unequal placement of SMD Electronic Components on pads prior to Reflow results in unbalanced solder forces.

Solution: Check if placement equipment places properly.

Possible cause: Unequal heat sink i.e. ground planes inside PCB layers may draw heat away from pad.

Solution: Increase soak time (plateau) or reflow profile so that all components are on.

Unmelted Solder Paste – Problem and Solution

Possible cause:

Unmelted paste=paste shows characteristics of powder after reflow, joints are dull not shiny. May be on some components only

Unmelted paste = paste shows characteristics of powder after reflow, joints are dull not shiny. May be on some components only

  1. To cold reflow profile
  2. Solder paste cannot melt completely

Solution: Check reflow profile, make sure peak temperature and time above liquids (183C) are high enough and soak (plateau) is long enough.

Excessive Fillet – Problem and Solution

Possible cause: Too much solder paste deposited on pads

Solution:

Unmelted paste=paste shows characteristics of powder after reflow, joints are dull not shiny. May be on some components only

Unmelted paste = paste shows characteristics of powder after reflow, joints are dull not shiny. May be on some components only

  1. If excess solder occurs on all components reduce overall stencil thickness or reduce dispenser purge time
  2. If excess solder occurs on some places only reduce stencil thickness or dispense purge time for these components only

Slump – Problem and Solution

Cold Slump

Possible cause: Viscosity of paste to low or metal content to low

Solution: Use different type of paste with higher viscosity or higher metal content. Alpha Cookson Solder Paste is the Best.

Excessive fillet=bulbous appearance of joint where outlines of the leads are obscured by the quantity of solder on them

Excessive fillet = bulbous appearance of joint where outlines of the leads are obscured by the quantity of solder on them

Possible cause: Paste came in contact with a cleaning solvent or other alien product

Solution:

  1. Make sure there are no solvents presents after cleaning the screen
  2. Never try to revive the paste by adding some compound

Possible cause:

  1. Squeegee pressure to high

    Slump=deformation of paste deposit after printing or dispensing deposit height will reduce while surface expands

    Slump = deformation of paste deposit after printing or dispensing deposit height will reduce while surface expands

  2. Paste is shearing due to excessive pressure applied to it thickeners in paste are destroyed

Solution: Use new paste and reduce squeegee pressure

Possible cause: Temperature of paste is too high while printing or dispensing

Solution:

  1. Check temperature inside printer
  2. Reduce pressure on squeegee

    Slump=deformation of paste deposit after printing or dispensing deposit height will reduce while surface expands

    Slump = deformation of paste deposit after printing or dispensing deposit height will reduce while surface expands

  3. Reduce on pressure on syringe when dispensing

Hot Slump

Possible cause: Too slow ramp up in reflow profile

Solution: Increase ramp up temperature, make sure to have a ramp up between 2 Degree Celsius to 3 Degree Celsius per second

DEWETTING – Problem and Remedy

Dewetting=bad attachment of molten solder to surface

Dewetting = bad attachment of molten solder to surface

Possible cause:

  1. Unwanted material on surface which prevents solder from attaching to surface, i.e. solder mask, fingerprints or oxides.

Remedy:

  1. Clean boards first
  2. Use different batch of boards

Possible cause:

Dewetting=bad attachment of molten solder to surface

Dewetting = bad attachment of molten solder to surface

  1. Bad alloy in HAL process, i.e. too much Cu elevates melting point of HAL alloy

Remedy:

  1. Increase peak temperature in reflow
  2. Use different batch of boards

Disturbed Joint – Problem and Solution

Possible cause: A source of vibration that is transmitted through

Disturbed Joint=dull, rough appearance of solder in an alloy that is normally bright and shiny

Disturbed Joint = dull, rough appearance of solder in an alloy that is normally bright and shiny

the pcb during the liquids state of the reflow profile

Remedy:

  1. Find and fix source of vibration
  2. Adjust reflow

Orange skinning – Problem and Solution

Possible cause:

Orange skinning=dull, rough appearance of solder, joint texture is orange skin like

Orange skinning = dull, rough appearance of solder, joint texture is orange skin like

  1. Too high in peak zone
  2. Residue is burned or rosin was cooking

Remedy:

  1. Lower peak zone temperature

Possible cause:

  1. Too long exposure to temperatures between activation temp and reflow = (depending on alloy)

Remedy:

Orange skinning=dull, rough appearance of solder, joint texture is orange skin like

Orange skinning = dull, rough appearance of solder, joint texture is orange skin like

  1. Shorten time in soak or lower soak temperatures

Possible cause:

  1. Preheating too high

Remedy:

  1. Lower preheat temperatures

Final Words on SMT Troubleshooting

I hope you found this guide and tutorial very useful. Please share this article with other so that other can also benefit and do SMT Troubleshooting.

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Santosh Das

Santosh, founder of this Electronics Tutorial Website, is an Electronics Geek, Blogger and Young Entrepreneur. He possesses vast experience in the field of electronics, electronic components, PCB, Soldering, SMT, Telecommunication, ESD Safety, and PCB Assembly Tools, Equipment and Consumables. Keep visiting for daily dose of Tips and Tutorials.

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11 Responses

  1. There are a variety of faults which present in SMT. In this blog I found exclusive solutions for defected SMT. This blog really helps me to overcome the SMT problems.

  2. MFG engineer says:

    I have a problem with soldering and can’t seem to be able to find any information in the web. I am using a alpha 0m5100 sn pb ag solder paste, the paste is applied to panel by a stencil printer then components placed by a pick and place and it enters the reflow oven, the solder looks ok until you remove forcefully the soldered component after the reflow procces in which underneath the solder looks dull, dusty, mate and overall like it didnt melt inside, only the surface has the shiny finish. According to my thermal profile the oven recipe meets the solder paste specifications. Ive also tried cleaning the panel prior applying solder paste, change solder paste tube to a new one, adjusted force in which paste is applied by squeege. Do you have any recomedations that could help with this issue?

    • Santosh Das says:

      Are you bothered about the Dull Looking Solder Joint or Poor Solder Joint and Electrical Performance. Lead-Free Solder give Dull Looking Solder Joints but you should not be bothered about the Dull Look. Performance will be excellent. If you use Sn/Pb Paste (Removing Sn), you will get shiny Solder Joints.

  3. MFG engineer says:

    I was worried because the exterior of the joint looks shiny and the interior look like it is unmelted, we don’t have any problem the electrical performance, but I wasn’t sure if this appearence was normal or not since the solder paste is not lead-free, and if this could represent a future problem.

  4. Sanjaye says:

    hi guys
    having open solder on 1210 resistor networks after reflow.
    any thought.

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