SMT Defect List

SMT Defect List and SMT Troubleshooting (SMT / SMD Problem and Solution)

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:

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

Solder Balls – Possible cause:

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
   

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

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

Oxidised paste – 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?
   

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

4. Was jar put back into refrigeration after opening?
5. Is alloy sensitive to oxidation?

Solution to Oxidised Solder Paste Problems:

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

Oxidised paste – 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 – Possible cause:

1. Reflow profile ramp up slow
   

   Solder beading : solder balls that are alongside a components

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 – Possible Cause:

1. Cold slumping
   

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

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 printer, too high temperature brings viscosity down
   

   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 may
3. Reduce stencil thickness

Open-insufficient – Possible cause:

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:

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

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.

UNMELTED PASTE – Possible cause:

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

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

Excessive Fillet

Possible cause: Too much solder paste deposited on pads

Remedy:

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 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:

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

Remedy: Use new paste and reduce squeegee pressure

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

Remedy:

1. Check temperature inside printer
2. Reduce pressure on squeegee
3. Reduce on pressure on syringe when dispensing

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

DEWETTING – 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:

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 Possible cause:

A source of vibration that is transmitted through the pcb during the liquids state of the reflow profile

Remedy:

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

Orange skinning – Possible cause:

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:

1. Shorten time in soak or lower soak temperatures

Possible cause:

1. Preheating too high

Remedy:

1. Lower preheat temperatures

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