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

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 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

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

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

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

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

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

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

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

Possible cause:

- unwanted material on surface which prevents solder from attaching to surface, i.e. soldermask, 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

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

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