SMT Trouble shooting guide to improve process.pptx

use this guide to improve in SMR

1. Solder misconnecting between two or more than two adjacent pads that come into contact to form a conductive path. Possible cause: PCB SMT pads will contribute to coplanarity issue resulting in poor gasketting during printer set-up. Possible cause: Stencil NG stencil with solder paste beneath it will lead to contamination in bear boards generating solder bridging phenomenon. 2. Stencil tension 3. Aperture Design Suggestion: Verify zero print gas set-up Ensure minimum print pressure. Increase cleaning frequency if required. Ensure good cleaning agent. 2. Ensure stencil tension is tight as per SOP. 3. For fine-pitch components opening should be slightly smaller than landing pad size for better gasketting in PCB. Suggestion: 1. Highly recommended to remove solder mask between adjacent pads especially for fine-pitch components

2. Possible cause: Screen Printer Poor gasketting – Paste oozes out beneath stencil during printing leading to potential solder bridging issue. Misaligned print will lead to bridging. Poor paste print with dog ears majorly on fine-pitch components. Dented squeegee blades will result ultimately solder bridging issue. Suggestion: There should be no gap between stencil and PCB, check if the tension is OK, check if the stencil is clean. Print must be aligned as per pad. Separation speed must be adjusted to avoid dog ears. Check squeegee blades condition. Possible cause: Component Placement. Placement inaccuracy will eventually result in solder bridging defect. Excess pressure of the part will lead to solder spattering resulting in solder bridging. Suggestion: Verify component placement pressure, verify BGA placement by X-ray, verify QFP placement by magnifier. Verify component height against program set in mounter. Component placement height should be one third of paste height.

3. Possible cause: Reflow profile 1. Increase of temperature during soak will lead to more heat into the paste resulting in hot slump phenomenon indicating solder bridging defect. Suggestion: Adopt a ramp up to spike profile without soak zone if possible. Possible cause: Solder Pate 1. Dry paste will lead to many printing error like, uneven solder printing, irregular shape, and volume. 2. Paste oozes out from the pad may stick to adjacent pad causing solder bridging defect. Suggestion: 1 . P aste expiry Operating temperature within supplier’s recommendations. Check temperature inside printer . Normal requirement around 25°C, 50%RH Do not mix using new and old paste 2. Operating temperature within supplier’s recommendations. Verify with another batch of paste to confirm problem is batch-related . Perform cold and hot slump test result using IPC-TM-650 Method 2.4.35

4. Solder paste volume/amount printed on PCB is lesser than stencil opening. Possible cause: Stencil Paste scooping effect especially on larger pad . Suggestion: 1. Segment the larger opening into smaller pads. Possible cause: Printer Paste scooping effect especially on larger pad . Suggestion: 1. Segment the larger opening into smaller pads.

5. Solder paste volume/amount printed on PCB is lesser than stencil opening that will lead to form a fillet at component leads. Possible cause: Stencil Paste sticks to the aperture walls. Suggestion: Aspect ratio > 1.5 Area ratio > .65 There should be no burr on stencil aperture edge. Possible cause: Printer 1. Print program. Suggestion: Verify print set-up and program. Reduce print speed.

6. Solder paste volume/amount printed on PCB is lesser than stencil opening that will lead to form a fillet at component leads. Possible cause: Reflow Profile 1 . Mismatch in CTE between component and PCB can cause solder wicking effect which may look like insufficient solder on pads. Suggestion: • Attach thermocouple on component and PCB. • Apply soak profile to minimize delta T before reflow zone. • Set bottom zones to be higher temperature if possible, to keep PCB hotter than component leads Possible cause: Solder paste 1. Solder paste viscosity. Suggestion: Check paste condition.

7. After reflow various solder particles form away from the main solder pool. Possible cause: Stencil 1. Paste adheres beneath the stencil might cause solder balls formation Suggestion: • Zero gap between stencil and PCB. Stencil must be cleaned. Stencil tension must be OK. Print pressure must be OK. Possible cause: Reflow Profile 1. Fast ramp-up rate, preheat rate will not allow sufficient time for the solvent to vaporize eventually. Suggestion: Slow preheat rate is recommended, typically < 1.5°C/sec from room temperature to 150°C.

8. After reflow various solder particles form away from the main solder pool. Possible cause: PCB moisture 1. Moisture trapped in PCB may result in vaporization during reflow eventually forming solder balls. Suggestion: 1. • Especially for lower grade PCBs such as FR2, CEM1, tends to absorb moisture. Bake 120°C for 4 hours if necessary. Possible cause: Solder Paste Especially for water soluble paste which is hygroscopic, it tends to have limited stencil life cause of moisture absorption. Suggestion: 1. • Minimize exposure time • Printer temperature and humidity to be within recommendation • Try new lot of solder paste to verify paste integrity. • Use coarser powder size if possible as fine powder size has more oxides and tends to slump more readily.

9. Possible cause: PCB Paste dissociation. Paste not adheres on the solder mask. Suggestion: Zero gap between stencil and PCB. Stencil must be cleaned. Stencil tension must be OK. Print pressure must be OK. Possible cause: Reflow Profile 1. Fast ramp-up rate, preheat rate will not allow sufficient time for the solvent to vaporize eventually. Suggestion: Slow preheat rate is recommended, typically < 1.5°C/sec from room temperature to 150°C. After Reflow large solder balls are formed on the side of the chip components between the terminations and away from the pads.

10. Possible cause: Stencil Design. 1 . Excess paste squeezes underneath component body tends to dissociate with the main body of solder during reflow Suggestion: Home plate or U-shape design may help to reduce the amount of paste potentially squeezed under the component body, onto the mask. Possible cause: Screen Printer 1. Paste smearing on solder mask Suggestion: Printer set up for zero print gap, verified by paste height consistency without dog ears Print alignment accuracy

11. Possible cause: Component Placement Excess mounter pressure will spatter the solder from the pad. Suggestion: Verify actual component height against data entered in the machine. Component placement height should be ±1/3 of paste height Possible cause: Screen Printer 1. Increase of temp in soak will result in extra ‘ heat to the paste resulting paste slump phenomenon. Suggestion: Adopt a ramp up to spike profile without soak zone if possible.

12. Possible cause: PCB Pad Design Bad Pad design (smaller than body part) generally leads to tombstone in small parts. Unequal pad size will lead to tombstoning generally especially when one of the pad is grounded. Suggestion: If component termination are not covering >50%, there are higher chances that part will get lifted from one side during reflow. (Feedback to supplier) Uneven cause results in different solder paste volumes that will cause tombstoning during reflow, If due to design issue use a gradual soak ramp rate. Tombstoning is the phenomenon when one end of the part is partially or completely lifted off the pad.

13. Possible cause: Placement accuracy and pressure 1. Misaligned placement will generally result in tombstoning due to uneven wetting force. Suggestion: Re-teach fiducials if all component shifted, else edit that specific location manually. Tombstoning is the phenomenon when one end of the part is partially or completely lifted off the pad. Possible cause: Reflow Profile 1. Extend soak zone can aid in balancing the wetting force on both pads before paste reaching to molten state Suggestion: Focus at 30°C before alloy liquids temperature, e.g., for SAC305, liquids @ 220°C, ensure soak at 190~220°C for minimum of 30 seconds

14. Possible cause: PCB Handling of boards. Bare boards contamination. Suggestion: 1. • Do not mix clean and washed boards. • Open fresh PCBs from package when ready to run. • Ensure working area is cleaned thoroughly and not contaminated with solder paste remains . 2. Inspect bare PCBs to capture and filter solder found on bare PCB before printing station. Solder spattering phenomenon generates like solder balls and sticks to the golden pads of the PCBs. Possible cause: Screen Printer Dirty Stencil. Not Dry Stencil Suggestion: Stencil must be cleaned with good cleaning agent preferably SC10. Frequently and effective cleaning of stencil. Possible cause: Reflow profile 1. Control the flux outgassing rate to minimize explosive solder scatter on Golden pads Suggestion: For SAC 305, set slow ramp rate of 0.3-0.4°C/sec from 217-221°C

15. Voiding is the phenomenon when empty spaces are formed between the pad and component other than solder paste joint. Tow main contributors of voiding are Flux resided in solder paste outgassing and resulting in voiding. Excess oxidation. Foreign particle between pads and part. Possible cause: PCB Micro via hole in PCBs containing extra flux and moisture. Suggestion: Typically via holes>6 mils, will be more difficult to vaporize the flux, moisture, or air trapped. Plug the blinded via hole before printing. Double print the via hole locations. Use finer powder size. Avoid printing paste on via holes instead aperture opening designed around it.

16. Possible cause: Stencil For bigger pads like LGA larger pads will contain extra solder paste that will be containing extra flux, during reflow extra flux will be vaporizing leading to void in PCBs Suggestion: Reduce amount of solder paste. With solder mask in between break the larger pad into smaller pads. Without solder mask cut a large round opening in the middle. Possible cause: Reflow Profile Flux entrapped without sufficient time to outgas. Oxidation rate predominates. Suggestion: 1. Establish soak zone from 170~220°C for 60-90 sec. Also make sure profile set between 130~220°C for 180 sec . 2. A dopt short profile concept to preserve flux activity, no soak zone. Use nitrogen if possible

17. Head-on-pillow is an assembly defect in which the spheres from a BGA or CSP don’t coalesce with the solder paste on the PCB pad. It is important to differentiate head-on-pillow from a defect caused simply by insufficient reflow temperature, which is characterized by distinct solder spheres from the paste that have not been properly melted on the pad and BGA solder sphere. With head-on-pillow the soldering temperature is sufficient to fully melt the solder sphere and paste deposit, but an impediment to the formation of a proper solder joint exist Possible cause: Screen Printer 1. Irregular print definition across the pads may hinder some solder bump locations to be in contact with solder paste. Suggestion: Verify printing parameters and measure print height consistency. Possible cause: PCB/Component Increase paste deposition volume to better compensate for substrate warpage. BGA coplanarity issue Oxidized BGA solder balls Suggestion: Increase print volume by using square aperture vs. round opening, or enlarge overall deposition volume without jeopardizing bridging. Increase solder volume . Use higher activity paste . Use nitrogen reflow

18. Possible cause: Reflow Board warpage especially for double reflow boards or thin PCBs ( Variance in CTE between PCB and BGA Paste hot-slump effect will aggravate BGA open joints if there are coplanarity issues. Long soak profile may exhaust the flux capacity before reflow Suggestion: Critical to minimize time above Tg , (typically 130°C for FR4 boards) with BGAs mounted. Target to maintain < 2 min if possible. • For second reflow cycle, try to adopt lower preheat to reduce warpage occurrence. Ensure minimum delta temperature difference between the BGA component and the rest of the components on the board. Apply short soak if necessary. Minimize time from 150°C to liquids temperature. If a long soak is mandatory for complex board, use nitrogen cushion the flux capacity in overcoming oxidation rate.

19. Possible cause: Reflow Insufficient heat absorbed by the solder. Excessive heat imposed Cooling rate is too slow Suggestion: Ensure a TC is properly attached to this component. Verify peak temperature is at least 15°C above alloy liquids and time above liquids (TAL) > 45 sec. Adopt a ramp-to-spike profile with soak zone to minimize oxidation and flux exhaustion. If soaking is mandatory, use nitrogen reflow whenever possible. Ensure alloy cooling rate from molten solder is 3-8°C/sec. Fast cooling rate will result in fine-grain structure appearance and looks shiny. Sometimes called “Cold Solder,” it is recognized by dark, non-reflective, rough surfaces from an alloy that is normally bright and shiny.