twitterfacebookgoogle pluslinkedinrss feedemail

Thursday 9 February 2012

SMT Troubleshooting (SMT Problem and Solution) 78

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:
  • Solderballs
  • Solder beading
  • Bridging
  • Open- Insufficient
  • Tombstoning
  • Unmelted paste
  • Excessive filet
  • Slump
  • Dewetting
  • Disturb joint
  • Orange Skinning
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
Possible cause:
  • 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?
Remedy:
  • Check squeegee pressure
  • Check for proper gasketting and alignment
  • Check if cleaning solvent is completely evaporated before printing
Possible cause: Oxidised paste
  • 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?
Remedy: Run fresh paste from a different lot under same conditions and see if solder bars go away.
Possible cause:
  • Squeegee pressure too high
  • Paste gets squeezed out between stencil and board
Remedy: Reduce squeegee pressure
Possible cause:
  • Drying out of paste after printing
  • What is specified tack time of paste?
Remedy: Run a PCB with fresh paste and see if problem goes away
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 beading : solder balls that are alongside a components

SOLDER BEADS

Possible cause:
  • 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.
Remedy: Run a more rapid ramp up profile of 1.5 Degree Celsius to 2.5 Degree Celsius per second.
Possible cause: Excessive amount of solder paste on component pads
  • What is stencil thickness?
  • Are apertures reduced?
  • Dispense time for a dot?
Remedy:
  • Reduce aperture size of stencil or use thinner stencil
  • Use smaller needle and/or reduce purge time on dispenser
Possible cause: 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 properly aligned with PCB?
Remedy:
  • Check squeegee pressure
  • Check for proper gasketting and alignment
  • Check if cleaning solvent is completely evaporated before printing
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

BRIDGING

Possible Cause:
  • Cold slumping
  • Is paste flowing apart after printing, height of deposit reduces and surface increases.
Remedy:
  • 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
Possible Cause:
  • Hot slumping
  • Is paste flowing apart during ramp up part of reflow profile
Remedy:
  • Shorten duration of ramp up cycle in reflow profile
Possible Cause:
  • Paste smearing on underside of stencil
  • Paste can be outside pad area and form solder balls between two component leads resulting in a bridge
Remedy:
  • Reduce squeegee and check pcb-stencil alignment and gasketting
Possible Cause:
  • 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
Remedy:
  • Reduce amount of solder paste
  • Increasing print speed may
  • Reduce stencil thickness
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

Open-insufficient

Possible cause:
  • Scooping during printing

  • 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:

  • Foreign material on solder pad
  • Was solder mask printed on pad?
Remedy:
  • Use another PCB
Possible Cause:
  • Too high squeegee speed
  • 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 = chip type components standing up on one end after reflow caused by unequal forces on the components end

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.
Click thumbnail to view full-size
Unmelted paste = paste shows characteristics of powder after reflow, joints are dull not shiny. May be on some components onlyUnmelted 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
Unmelted paste = paste shows characteristics of powder after reflow, joints are dull not shiny. May be on some components only

UNMELTED PASTE

Possible cause: To cold reflow profile
  • 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 = bulbous appearance of joint where outlines of the leads are obscured by the quantity of solder on them

Excessive Fillet

Possible cause:
  • Too much solder paste deposited on pads
Remedy:
  • 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 = 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

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:
  • Make sure there are no solvents presents after cleaning the screen
  • Never try to revive the paste by adding some compound
Possible cause:
  • Squeegee pressure to high
  • 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:

  • Check temperature inside printer
  • Reduce pressure on squeegee
  • 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

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 = bad attachment of molten solder to surface
Dewetting = bad attachment of molten solder to surface

DEWETTING

Possible cause:
  • Unwanted material on surface which prevents solder from attaching to surface, i.e. solder mask, fingerprints or oxides.
Remedy:
  • Clean boards first
  • Use different batch of boards
Possible cause:
  • Bad alloy in HAL process, i.e. too much Cu elevates melting point of HAL alloy
remedy:
  • Increase peak temperature in reflow
  • Use different batch of boards
Disturbed Joint = dull, rough appearance of solder in an alloy that is normally bright and shiny

Disturbed Joint

Possible cause: A source of vibration that is transmitted through the pcb during the liquids state of the reflow profile
Remedy:
  • Find and fix source of vibration
  • Adjust reflow
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

Orange skinning

Possible cause:
  • Too high in peak zone
  • Residue is burned or rosin was cooking
Remedy:
  • Lower peak zone temperature
Possible cause:
  • Too long exposure to temperatures between activation temp and reflow = (depending on alloy)
Remedy:
  • Shorten time in soak or lower soak temperatures
Possible cause:
  • Preheating too high
Remedy:
  • Lower preheat temperatures

1 comments: