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3.5 Defect Mechanisms & Fixes

EveryReflow soldering defects are the consequence of failed controls upstream: paste, stencil, placement, or profile. Correct troubleshooting requires disciplined analysis of the defect tellsmechanism ato storyidentify aboutthe imbalanceroot cause in the process.process Tombstones hint at uneven pad wetting, voids expose trapped volatiles, and head-in-pillow reveals warpage or weak flux. While the symptoms show up after reflow, their roots often lie upstream in printing, stencil design, or subtle profile details like soak length and TAL.chain. The fastest pathfailure to stability is rarely wholesale change, but instead the smallest targeted fix that restores balance—whether that’s adjusting apertures, cleaning the stencil, tweaking blower speed, or softeningdistinguish a thermalprint-related ramp.defect When(volume/alignment) eachfrom mechanisma isthermal-related understood,defect defects(wetting/balance) stopguarantees beingcontinuous, mysteriescostly and start becoming predictable, solvable patterns.rework.

3.5.1 HowSolder to debugBridging (the 10-minute rhythm)Shorts)

  1. ConfirmMechanism: Excessive solder paste volume connects neighboring pads, usually on fine-pitch components (QFP, BGA outer rows). The molten solder's surface tension is overcome by the print.excessive Checkvolume, SPIallowing oncoalescence thebetween parts that fail: volume/height/area stable? If not, fix pads.Chapter 7 items first.

  2. Sort by family. Are fails clustered on chips, QFN thermals, BGAs, or power pads? Each has a usual culprit.
  3. Read the oven story. Pull the last profile: ramp, soak, TAL, peak, ΔT. If it disobeys the paste or parts, correct that before anything else.
  4. Change one thing. Re-run a small lot and verify with AOI/AXI. Don’t shotgun.



3.5.2 Quick map (symptom → root → first fix)

SymptomRoot (what you see)Cause

LikelyFix rootand cause(s)Control Point

FirstProcess fix (smallest move that helps most)Check

TombstonesExcessive /Paste skew (chips)Volume

PadReduce &stencil copperaperture width (5–10%) or thickness.

SPI (Chapter 1.6) must show Area/Volume below the "Bridge" limit.

Misalignment/Smear

Printer alignment drift or poor gasket seal (solder smears under stencil).

Check thermalPrinter imbalanceSetup; (Chapter 1.5) for high squeegee pressure or slow separation speed.

Excessive Slump

Paste is too warm, past expiration, or solvent loss (rheology failure).

Check pastePaste imbalanceHandling; aggressive(Chapter 1.2) for proper thaw/open time protocol.

Too Long TAL

Excessive time above liquidus allows solder to flow too far.

Shorten rampTAL with faster belt speed (Chapter 3.2).

3.5.2 Tombstoning (Manhattan Effect)

Mechanism: Unequal wetting forces on a component's opposite pads cause it to stand on one end, creating an open circuit. This occurs when the solder on one pad melts and pulls the component upright while the solder on the opposite pad is still solid or lagging. This is most common on small, light passives (0402, 0201).

Root Cause

Fix and Control Point

Process Check

Uneven Heating (∆T)

One pad is connected to a thermal mass (plane/via), the other is isolated.

Thermal Profile (Chapter 3.2) must achieve minimal ∆T across the board (Soak profile preferred).

Unequal Paste Volume

One pad has significantly more paste (higher surface tension).

SPI must confirm volume symmetry (±5%) on both pads. Use Home-Plate Apertures (Chapter 1.4) to balance forces.

Fast Wetting

The transition to liquidus is too quick, maximizing the surface tension torque.

Slow down Ramp Rate (1–2˚C/sec) and ensure adequate preheat time.

Placement Skew

Component placed with a significant offset, causing one end to contact the paste poorly.

Verify Placement Accuracy and Coplanarity during First Article (Chapter 2.5).

3.5.3 Head-in-Pillow (HIP)

Mechanism: The solder ball on the BGA component and the solder paste on the PCB pad melt but fail to fully coalesce, forming a weak mechanical interface (a latent defect that often passes electrical test but fails in the field under stress). This is almost exclusively due to warpage and oxidation.

Root Cause

Fix and Control Point

Process Check

Package/PCB Warpage

Component or board lifts during the high-heat zones, allowing the ball to oxidize.

Implement PCB/BGA flatness screening at incoming. Use lower peak temperature (if possible) to minimize warpage.

Oxide Layer

Oxide forms on the lifted BGA ball, and the paste's flux is too weak/spent to clean it upon recontact.

Use home-plate/invertedHigh-Activity Flux shapes;paste. biasEnsure pasteTAL towardis coldsufficient pad;to smoothmaximize rampflux (≤~2activation °C/s)and joint collapse.

BridgingInsufficient (fine pitch)Volume/Contact

Over-print;Starved clogged/dirtydeposit stencil;at fastthe BGA corners (where warpage is worst).separation

Narrow aperture 5–10%; add/shortenUse cleaning;Aperture slow peelReduction offwith thevolume boardbiasing on outer rows/corners (Chapter 1.4) to ensure contact.

QFN/LFPAK voidsAtmosphere

SolidExcessive slugoxygen ofcontent pastein can’t vent; too hot/shortthe reflow zone accelerates oxidation.

Window the center padSwitch to 50–65%Nitrogen (N2) atmosphere with(Chapter small3.3) tiles;to usesuppress aoxide gentle soak; try N₂ before adding peakformation.

3.5.4 Voiding and Cold Joints


Mechanism: Voids are gas bubbles trapped within the solidifying solder, compromising the joint's thermal and electrical conductivity. Cold joints are those where the solder paste did not reach the minimum liquidus temperature (Incomplete Reflow).

BGA head-in-pillow (HIP)Defect

Package/boardRoot warpage at melt; oxide; too short/uneven TALCause

SupportFix theand board;Control lengthen/smoothPoint

Process TAL; add N₂; verify VIPPOs are filled + cap-platedCheck

GrapingVoiding (grainyThermal beads on pads)Pads)

StarvedVolatiles paste(from onflux/solvents) tinyare apertures;trapped longby soak;the oxidationlarge molten solder mass.

Stencil Design:Fresh paste; slightlyUse higherWindow-Pane peakApertures with Chimneys or(Chapter 1.4) to create vent paths.

N₂AXI; improve release (nano-foil,X-ray cleanInspection) more,must checkconfirm areavoiding ratio)is below the acceptable limit (e.g., ≤25% total area).

Solder balls/splatterBalls

TooRapid muchheating paste;(too long/hotfast soak;ramp) tiredor fluxinsufficient preheat time vaporizes solvents, causing solder to splatter.

ShorterExtend soakPreheat/Soak; trimtime peak(Chapter slightly;3.2) reduceto aperture;allow keepsolvents beadto fresh/cleanevaporate gradually before reflow.

Reflow Profile must show a controlled, gradual ramp rate (≤3˚C/sec).

DullCold jointsJoint/Poor / poor wettingWetting

LowInsufficient peak;TAL oxidizedor finish;peak weaktemperature fluxfor atthe endalloy, ofor lifepoor pad solderability.

+5Increase °CBelt peakSpeed (to extend TAL) or a touch longer TALZone Temp; consider(to N₂;raise confirm paste age/finish choice

Opens on LGA/QFN edges

Paste starvation; poor release; fast beltpeak).

Profile PlotEnlarge ormust de-reduceconfirm edgeminimum apertures;TAL/Peak improvetargets releasewere (nano-coat/cleaning); slow belt a notchhit.


Final Checklist: Troubleshooting Reflow Defects


3.5.3 Tombstones & skew (chip passives)

WhyDefect it happens: one pad wets first (hotter copper or more paste), pulling the part upright or sideways.
 Fix order:Observed

Primary

  1. Root Cause Zone

StencilKey first: moveAction to Take

home-plateBridging

Printing (trimExcess toe)Volume)

Reduce orstencil aperture area; verify invertedSPI volume Cpk.

Tombstoning

Profile (trimThermal heel)Imbalance)

Tune shapes;profile micro-windowfor 01005–0402.longer soak; check

  • Balance copper:SPI iffor onevolume pad ties into a plane, symmetry.bias paste down on that side.
  • Profile: smooth the ramp; avoid spiky preheat that “starts the race” early.
  • Placement: confirm centered sit-down—nozzles and board support matter.



    • 3.5.4 QFN/LFPAK voids & float/tilt

    Why it happens: big center pads trap volatiles; too much paste floats the part.
     Fix order:HIP

    1. Window-pane the thermal pad to 50–65% coverage; keep tiles 1.0–1.5 mm with 0.3–0.5 mm webs. Add a chimney slot to an edge if the pad is huge.
    2. Profile: longer, gentler soak; consider slightly lower peak so volatiles leave before freeze.
    3. Atmosphere: try N₂—often worth 5–10% fewer voids.
    4. Paste: if still stubborn, evaluate a paste tuned for low voiding on slugs.



    3.5.5 BGA/CSP head-in-pillow (HIP)

    Why it happens: at liquidus the ball and paste separate (warpage or oxide) and never merge.
     Fix order:

    1. Mechanics: solid board support in printer and PnP; keep panels flat through the oven (edge rails/center fingers).
    2. Profile: extend and steady TALMaterials/Profile (don’t just spike peak); aim for even heat so both sides melt together.Warpage/Oxide)
    3. Atmosphere:

     addUse N₂ ifatmosphere; wettingconfirm isadequate marginal.

  • Upstream: ensure VIPPOs are filled/cap-plated; respect MSL/bake for the package so balls aren’t oxidized or popcorn-prone.



    • 3.5.6 Graping (grainy “bunches” of solder)

    Why it happens: small paste deposits partially dry or oxidize; at peak they bead into “grapes” instead of flowing.
     Fix order:

    1. Printer hygiene: shorten cleaning interval, keep a smaller bead, and refresh paste more often.
    2. Release help: nano-coated or electroformed foils; check area/aspect ratios on the worst pads.
    3. Reflow: a touch higher peak or N₂ to kick wetting over the edge; avoid long soaks that dry flux.
    4. Materials: verify paste age/storage; consider finer powder only if area ratios demand it (then tighten handling).



    3.5.7 Bridges (fine-pitch gull-wing, QFN edges)

    Why it happens: too much paste or paste smeared during peel; dirty stencil.
     Fix order:

    1. Apertures: narrow 5–10% on the crowded side; add tiny relief nicks at inner corners; consider stencil step-down nearby.
    2. Printer: slow separation, ensure “wipe-clean” pressure, add wet+vac cycles when SPI shows area creep.
    3. Reflow: if you must, shave peak a touch; but most bridges die in printing.



    3.5.8 Solder balls & splatter

    Why it happens: volatile flux cooked in the soak, dirty stencil lips, or brute-force pressure that squeezes paste onto mask.
     Fix order:

    1. Shorten soak or reduce mid-zone temps; keep ramp smooth.
    2. Printer: ensure “just-enough” blade pressure and regular understencil cleaning.
    3. Stencil: remove tiny paste traps (burrs, damaged apertures); nano-coat helps the edge stay clean.
    4. Paste: refresh; don’t exceed open time.



    3.5.9 Dull joints / poor wetting

    Why it happens: peaks too low; oxide (finish or paste); exhausted activators.
     Fix order: +5 °C peak or +5–10 sBGA TAL; trycheck BGA N₂flatness; confirm finish compatibility (e.g., aged OSP needs honest profiles), and paste shelf life..



    3.5.10 Opens on LGA / starved edges on QFN

    Why it happens:Voiding

    Design/Paste release(Gas issuesEntrapment)

    Revise starve the edges; belt too fast to finish wetting.
     Fix order: slightly larger or less-reduced edge apertures; improve release (foil/coating/cleaning); slow beltstencil to add avent few seconds of TAL.



    3.5.11 Make fixes stick (how to prove you won)

    • Before/after plotspaths:; reflowmonitor profile screenshotsresults with TAL/peak notes.
    • SPI heat-mapsAXI:.

    Cold the bad geometry should drop off the Pareto.

  • Joint

    • AXI/AOI snapshots: BGA collapse and QFN void distributions, pre- vs post-change.

  • Recipe & drawing updates: lock the new profile name and the revised aperture notes into the Golden RecipeProfile and(Insufficient stencilHeat)

    • Increase spec, not just an email.



    3.5.12 Pocket checklist (pin this near the oven & printer)

    • SPI is stable on the failing parts (else fix 6.xTAL/Peak first)until Profile Plot confirms targets are met.
    • One change at a time (profile or stencil or atmosphere)
    • Small DOE if torn between two fixes (3–6 boards is enough)
    • AOI/AXI prove improvement; limits unchanged (no goal-post moving)
    • Golden Recipe / stencil drawing updated; notes explain why



    By treating defects as process feedback rather than random failures, manufacturers can resolve issues with minimal disruption. The result is a reflow line that runs consistently, with fewer surprises, and a Pareto chart that steadily empties of recurring headaches.


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