Jump to content

Surface-Mount Rework

From RetroTechCollection
Desoldering with hot air
A hot-air rework nozzle surrounding a large quad flat pack, with a vacuum pick-up cup resting on top of the package to lift it once the solder melts.

Surface-mount rework is the removal and refitting of parts that are soldered to pads on the surface of a board: small-outline ICs (SOIC) with two rows of gull-wing leads, quad flat packs (QFP) with leads on all four sides, and chip resistors and capacitors. The Amiga 600 and 1200 and the Nintendo 64 are among the machines that use them; see the worked examples below. The usual tools are a hot-air station, a soldering iron with plenty of flux, a low-melt removal alloy and hot tweezers.

Surface-mount lands are fragile. NASA's surface-mount workmanship standard warns that excess movement of desoldering braid during heating will in most cases loosen or remove the land.[1] Surface-mount electrolytic capacitors are covered in Capacitor Replacement Procedure, and through-hole parts in Through-Hole Desoldering.

When to use it

[edit | edit source]
  • A surface-mount chip has been shown to be faulty. Confirm the fault first with Logic Chip Testing, Continuity Testing and Signal Tracing or Finding Short Circuits: Texas Instruments recommends removing and replacing its surface-mount parts only once.[2]
  • Joints on a fine-pitch chip are cracked or poorly wetted and need touching up with fresh flux; drag soldering is one way to rework a whole row.[3]
  • Pins are bridged with solder. SparkFun uses hot air with a flux pen to clear bridges between IC pins,[4] and Adafruit draws the hot iron tip between the joints, or uses braid where there is too much solder.[5]
  • Small chip resistors or capacitors have been damaged, as on the Atari ST Book display board, where leaked electrolyte had damaged two resistors (see the worked examples).

Safety

[edit | edit source]
  • Work on a heat-resistant surface such as a silicone mat or a piece of MDF. The air from the nozzle gets very hot very quickly, so aim it with care.[4]
  • Some stations keep blowing after they are switched off; SparkFun's 303D runs until the air is below 100 °C. Leave it to finish.[4]
  • Hot air melts the plastic of nearby headers and connectors and can discolour switches.[4] Cover the neighbouring parts with heat-resistant tape, as Texas Instruments recommends.[2]
  • Smoke, warping or black residue from the board means it is overheating: take the heat away and turn the temperature down.[4]
  • Flux fume irritates the eyes, throat and lungs and can cause asthma; use fume extraction. Wash your hands with soap and water after handling solder.[6]
  • NASA required electrostatic discharge controls for all surface-mount work.[1] Use an earthed wrist strap; see IC Removal and Socketing.

Tools and materials

[edit | edit source]
  • A hot-air rework station with a choice of nozzles. A small nozzle concentrates the air on a small part and a larger one spreads it over a bigger area.[4]
  • A board preheater or underboard heater, for large boards.[2]
  • A temperature-controlled soldering iron with a fine tip, a chisel tip and, for drag soldering, a concave "hoof" or "spoon" tip.[7]
  • Flux: a no-clean flux pen or gel flux for most jobs, and plenty of liquid flux for drag soldering.[4][7]
  • Fine desoldering braid; see Through-Hole Desoldering.
  • A low-melt removal alloy kit, where there is no hot-air station. Chip Quik's lead-free kit contains the alloy, a no-clean tack flux and alcohol pads.[8]
  • ESD-safe straight and curved tweezers,[4] and a vacuum pick-up pen or dental pick.[9]
  • Hot tweezers (soldering tweezers) for chip parts.[10]
  • Kapton or other heat-resistant tape.[2]
  • An orangewood stick or other soft tool for holding parts and straightening leads.[7][1]
  • Magnification and bright, shadowless light. NASA called for 10× to 25× on lands wider than 0.65 mm, 10× to 40× below that and 25× to 45× on lands narrower than 0.39 mm, with at least 1076 lm/m² (100 foot-candles) on the work.[1]
  • Isopropyl alcohol, a stiff brush and lint-free wipes for cleaning off flux.[11]

Choosing a method

[edit | edit source]
Surface-mount rework methods
Method Used for Notes from the sources
Hot air Removing SOIC, QFP and chip parts; reflowing paste NASA allowed hot air only where it could be shown not to reflow the neighbouring joints.[1] Air that dwells on one spot lifts the copper and the solder mask.[4]
Low-melt alloy Removing multi-pin parts with an ordinary iron The alloy blends with the old solder and stays liquid long enough to lift the part.[12] Chip Quik's leaded alloy melts at 58 °C, and the site must be cleaned thoroughly afterwards.[9]
Drag soldering Fitting or reflowing rows of gull-wing leads Needs a temperature-controlled iron, a hoof or spoon tip and plenty of liquid flux.[7]
Point to point Fitting a chip pin by pin; touching up a few leads TestEquity suggests it where only a few leads need rework.[3] Wettermann uses a chisel tip 60 to 70 % as wide as the pad.[7]
Hot tweezers Removing chip resistors and capacitors The tips grip and heat both ends of the part. IPC-7711 lists tweezer, bifurcated-tip and hot-air methods for removing chip parts.[10]
Braid Clearing pads after removal NASA allowed it for most parts but not for chip resistors, ceramic capacitors or leadless chip carriers, and set the heat at 315 ± 20 °C.[1]

Procedure

[edit | edit source]

Prepare the board

[edit | edit source]
  1. Take the board out of the machine and lay it on a heat-resistant surface.[4]
  2. Note the orientation of every part to be removed. Surface-mount ICs usually mark pin 1 with a dot, and the pin numbers increase anticlockwise from it.[13]
  3. If the chip is to be kept, bake the board first at a temperature that everything on it can stand. Texas Instruments recommends 8 to 24 hours at 105 to 125 °C for its packages, because absorbed moisture turns to steam during desoldering and can crack the package.[2]
  4. Preheat the board. Texas Instruments suggests 90 °C for 15 to 20 minutes, which prevents measling and white spots in the laminate.[2]
  5. Cover the neighbouring parts with heat-resistant tape.[2]

Remove a chip with hot air

[edit | edit source]

The sources set the air temperature in different ways. SparkFun starts with the temperature and air knobs at mid-scale, slightly above the melting point of the solder, and tests on a small part first.[4] Texas Instruments' example profile for a rework machine, with the nozzle 1.27 mm above the part, ends with 65 seconds at 370 °C for lead-free solder.[2] On the Amiga 1200, modrobert's joints improved when he raised the air to 380 °C. Tin-lead solder melts at 183 °C and SAC305 lead-free solder at 217 to 220 °C.[14]

  1. Fit a nozzle to suit the part.[4]
  2. Set a low air flow; too much air blows small parts off the board.[4]
  3. Apply flux to the pins.[4]
  4. Hold the nozzle just above the chip and move it steadily round all the pins. Do not hold it still on one spot.[4]
  5. Watch the solder. When it is shiny and molten on every pin, nudge the chip gently with tweezers.[4]
  6. Lift the chip with tweezers or a vacuum pen once it moves freely. Pushing or pulling harder pulls the pads off.[4]
  7. Let the board cool.[2]
  8. Treat the removed chip as suspect, since Texas Instruments recommends only one removal and replacement for its parts.[2]

Remove a chip with low-melt alloy

[edit | edit source]

A low-melt removal alloy lets an ordinary iron take off a multi-pin chip. The alloy blends with the solder on the joints and stays liquid long enough to lift or slide the chip off.[12] Chip Quik's leaded alloy melts at 58 °C,[9] and its lead-free kit works below 300 °F (149 °C).[8]

  1. Flux all the leads.[9]
  2. Melt the alloy evenly onto every pin.[9]
  3. Keep the alloy molten until every joint has reflowed.[9]
  4. Lift the chip with a dental pick or vacuum pen while the alloy is still liquid.[9]
  5. While the alloy is molten, move the excess away from the pads with a cotton swab and flux.[9]
  6. Heat each pad and polish it with a swab and flux until it is clean.[9]
  7. Take up what remains with flux and braid.[12]
  8. Once the board is at room temperature, wipe the area with an alcohol pad.[9]

Prepare the pads

[edit | edit source]

The old solder has to come off so that a fine-pitch part sits level on every pad.[15]

  1. Flux the pads.[15]
  2. Lay braid slightly narrower than the pad on the first pad, and press the iron on the braid.[15]
  3. Lift the iron and braid straight up together.[15]
  4. Move to a clean part of the braid and repeat on the next pad. Do not wipe the braid along a row of pads, which lifts pads and damages the solder mask.[15]
  5. Keep the heat controlled; NASA set it at 315 ± 20 °C for wicking surface-mount lands.[1]
  6. Clean the area thoroughly before fitting the new part.[1]
  7. Check every pad and track under magnification. A lifted pad needs repair before the new part goes on; see PCB Pad Repair.[5]

The sources differ on braid width: Chemtronics recommends braid as wide as or slightly wider than the pad,[16] and Wettermann uses braid slightly narrower than the pad.[15]

Fit a chip by drag soldering

[edit | edit source]
Drag soldering a fine-pitch QFP (26 seconds). Flux goes on along the pins from a syringe, a hoof tip carrying solder is drawn along the row, and the flux is scrubbed off with a brush and wipe.
  1. Check that the leads are straight and level. Straighten a bent lead with an orangewood stick; NASA did not allow metal tweezers for this.[1][7]
  2. Place the chip on its pads the right way round and align the leads, holding it with tweezers or an orangewood stick.[7]
  3. Flux two opposite corners.[7]
  4. Tack one corner lead and check that the chip is still square on its pads. If it is not, reheat and reposition it.[17]
  5. Tack the opposite corner.[7][17]
  6. Flux all the rows of leads.[7]
  7. Load the hoof tip with solder until about a third of the hollow is covered; use less for finer pitch.[7]
  8. Start on a row with no tacked leads. Angle the tip so the edge of the solder touches the lands with the face of the tip tilted away, and hold the shaft along the row at up to 30°.[7]
  9. Draw the tip slowly along the row with very light pressure, more slowly on parts with more thermal mass.[7] The solder flows only onto surfaces that are above its melting point.[3]
  10. Repeat on each row.[7]
  11. Clear any bridge by drawing the hot tip between the joints, or with braid if there is too much solder.[5]
  12. Clean off the flux and inspect the joints.[7]
  13. Tin the tip before putting the iron back in its stand.[7]

Fit a chip point to point

[edit | edit source]
SMD Soldering Technique
A soldering iron tip on the pins of a QFP (U28).
  1. Fit a chisel tip 60 to 70 % as wide as the pads.[7]
  2. Tin the pads lightly, with very little solder.[17]
  3. Align the chip and tack two opposite corners, as for drag soldering.[17][7]
  4. Flux the rows.[7]
  5. Put the tip on the pad beside the first lead, so that most of the heat goes into the pad.[5]
  6. Feed solder in from the side opposite the iron and keep the heat on until it flows between lead and pad.[5]
  7. Lift the iron and let the joint cool undisturbed.[5]
  8. Repeat for each lead in turn.[17]

Fit a chip with paste and hot air

[edit | edit source]

IPC-7711/7721 lists solder paste with hot air among its installation methods for gull-wing and chip parts.[10]

  1. Apply fresh solder paste to the cleaned pads with a syringe dispenser.[1]
  2. Place the part the right way round and align it with a hand tool.[1]
  3. Heat the part with hot air, keeping the nozzle moving, until the solder is shiny and molten on every joint.[4]
  4. Keep the air off the neighbouring joints; NASA allowed hot air only where it did not reflow them.[1]
  5. Wash off the flux. SparkFun warns that water-soluble flux left on the board causes corrosion.[4]

Replace a chip resistor or capacitor

[edit | edit source]
Hot tweezers removing a chip capacitor (Aisart)
Hot tweezers gripping both ends of a chip capacitor beside a QFP. The photographer describes it as desoldering a size 1210 capacitor without flux.
  1. Grip the part across both ends with hot tweezers.[10]
  2. Lift it away as soon as both joints melt.[4]
  3. Without hot tweezers, heat both ends with hot air and lift the part with ordinary tweezers.[4]
  4. Clear the pads with braid and clean them, as above.[1]
  5. Tin one pad.[17]
  6. Hold the new part in place with tweezers and reheat the tinned pad to fix that end.[17]
  7. Solder the other end, heating the pad first.[17][5]
  8. Check that the part sits flat; NASA rejected a chip tilted by more than 25 % of its thickness.[1]

For the polarity of surface-mount electrolytic and tantalum capacitors, see Capacitor Replacement Procedure.

Checking the repair

[edit | edit source]
NASA-STD-8739.2 Figure B-6, gull wing lead solder fillet
NASA-STD-8739.2 Figure B-6: minimum and maximum solder on a gull-wing lead. A heel fillet is required in both.
NASA-STD-8739.2 Figure B-3, chip part solder fillet
NASA-STD-8739.2 Figure B-3: the fillet on a chip part must reach at least half the thickness of the part.
  1. Inspect every joint under magnification and good light.[1]
  2. On gull-wing leads, look for a concave fillet that wets lead and land, a heel fillet at the back of the foot, and the outline of the lead still visible under the solder.[1]
  3. Reject a lead that overhangs its land at the side or toe by more than a quarter of its width, a foot that stands more than 0.26 mm above its pad, or a fillet that stops short of the edges of the land.[1]
  4. On chip parts, look for a concave fillet on each end face that reaches at least half the thickness of the part, with solder flowed under the ends.[1]
  5. Look along each row for bridges between pins.[5]
  6. Test continuity from each reworked lead's track to the next point on its net, and between neighbouring pins; see Continuity Testing and Signal Tracing. Probe the track or a via: a probe pressing on a marginal joint can make it read good.[5]
  7. Clean off the flux residue.[4]

Common mistakes

[edit | edit source]
  • Lifting a chip before every pin is molten, which pulls pads off. Nudge it first.[4]
  • Holding hot air on one spot. The adhesive under the copper lets go as black goo and the solder mask comes away.[4]
  • Air flow set too high, which blows small parts off the board.[4]
  • Leaving plastic parts nearby unprotected. Headers melt and switches discolour.[4]
  • Wiping braid along a row of pads.[15]
  • Wicking solder from a chip resistor or ceramic capacitor that is still fitted; NASA excluded those parts from the braid method.[1]
  • Straightening leads with metal tweezers.[1]
  • Heating the lead and not the pad, so that the solder never flows onto the pad.[5]
  • Too much solder, the usual cause of bridges.[5]
  • Leaving low-melt alloy on the pads. Chip Quik's instructions are to clean the site thoroughly before installing the new chip.[9]
  • Fitting a removed chip elsewhere as a known-good part, when Texas Instruments recommends only one removal and replacement.[2]

Worked examples on this wiki

[edit | edit source]
[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 NASA, NASA-STD-8739.2 Workmanship Standard for Surface Mount Technology, 31 August 1999, PDF: sections 6.3 (at least 1076 lm/m² on the work), 6.4 (shadowless inspection light), 6.5 (electrostatic discharge control to NASA-STD-8739.7), 12.2 (magnification for soldered connections: 10× to 25× for lands wider than 0.65 mm, 10× to 40× below 0.65 mm, 25× to 45× below 0.39 mm), 12.9 (chip parts: concave fillet on the end faces, good wetting, rejection for tilt over 25 % of part thickness, fillet under 50 % of part thickness, negative wetting angle, no solder flow under the ends, side overhang over 25 % of part width; gull-wing leads: concave fillet with a mandatory heel fillet, lead discernible, rejection for side or toe overhang over 25 % of lead width, foot more than 0.26 mm above the pad, fillet short of the land edges, no heel fillet), 13.2 (metal tools such as tweezers not to be used to rework leaded parts; orangewood stick or soft tool), 13.3 (manual realignment with a hand tool; new paste from a syringe dispenser), 13.4 (hot air permissible if it does not reflow adjacent joints; braid permissible except for leadless chip carriers, ceramic capacitors and resistors, with a caution that lands are fragile, heat should be 315 ± 20 °C and excess braid movement will in most cases loosen or remove the land; clean before new paste) and Appendix B, Figures B-3 and B-6. Retrieved 2026-10-25.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Colin Martin, Surface Mount Package Removal Application Note, Texas Instruments SLVA439A, September 2010, revised December 2010, PDF: cover adjacent components with heat tape; underboard heating to prevent warping; only one removal and replacement recommended for TI surface-mount components; preheat to 90 °C for 15 to 20 minutes to prevent measling and white spots; bake boards and parts 8 to 24 hours at 105 to 125 °C (J-STD-033) at a temperature the parts and board can withstand, because absorbed moisture expands and can delaminate or crack the package; example lead-free removal profile with the nozzle 1.27 mm above the part (soak 225 °C for 90 s, ramp 335 °C for 30 s, reflow 370 °C for 65 s, then a cool-down stage); do not reuse a removed part. Retrieved 2026-10-25.
  3. ↑ 3.0 3.1 3.2 TestEquity, "Drag Soldering: Your Answer to Rework and Touchup", September 2020: drag soldering solders or touches up a whole row of fine-pitch leads at once; a concave tip holds the molten solder; solder flows only onto surfaces above its melting point (217 °C for SAC305); point-to-point soldering may suit a job where only a few leads need rework. Retrieved 2026-10-25.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 4.24 4.25 SparkFun Electronics, "How to Use a Hot Air Rework Station", learn.sparkfun.com: tweezers, flux pen and heat-resistant surfaces (MDF or silicone mat); the 303D blows until the air is below 100 °C after switch-off; small and large nozzles; hot air with a flux pen to remove solder jumpers; temperature slightly above the melting point of the solder and modest air flow, starting from mid-scale and testing on a small part; move the nozzle back and forth until the solder is shiny and molten, then nudge or grab the part with tweezers; keep the nozzle moving; headers melt and buttons discolour; smoke, warping or black goo means too much heat; wash off water-soluble flux, which causes corrosion; high air flow blows parts away; dwelling on one spot releases the copper adhesive as black goo and lifts the solder mask; nudge gently, since pushing too hard pulls the pad. Retrieved 2026-10-25.
  5. ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 Bill Earl, Adafruit Guide To Excellent Soldering, Adafruit Learning System: pages "Surface Mount Components" (most of the heat on the pad, solder from the side opposite the iron, heat kept on until the solder flows, joint cooled undisturbed), "Common Soldering Problems" (surface-mount pins not wetted because the pin was heated instead of the pad; solder bridges cleared by drawing a hot tip between the joints or with a sucker or wick, and caused by too much solder; lifted pads) and "Soldering FAQ" (a probe pressing on a marginal joint can make it read good). Retrieved 2026-10-25.
  6. ↑ University of Sussex Health and Safety, Soldering Safety Guidance, document HS G077, version 1.0, September 2024, PDF, sections 4.3 and 4.4: washing hands with soap and water after soldering; rosin flux fume causes eye, throat and lung irritation, sensitisation and asthma; fume extraction. Retrieved 2026-10-25.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 Bob Wettermann (BEST Inc.), "Knocking Down the Bone Pile: Soldering for QFPs and Other Gull Wing Leaded Parts", SMT007 Magazine, August 2022: IPC-7711/7721 section 5.5 methods; point to point with a chisel tip 60 to 70 % of the pad width; inspect the leads for coplanarity; align the part and hold it with tweezers or an orangewood stick; flux and tack two opposite corners; flux the rows; drag soldering with a concave hoof or spoon tip loaded to about a third with solder (less for finer pitch), starting on an untacked row, the edge of the solder on the land and the tip face tilted away, the shaft along the row at up to 30°, very light pressure and a speed set by the thermal mass; clean and inspect; tin the tip before holstering; needs a temperature-controlled station and plenty of liquid flux. Retrieved 2026-10-25.
  8. ↑ 8.0 8.1 Electronic Design staff, "Kit Offers Lead-Free Way Of Removing Chips From Boards", Electronic Design: the Chip Quik lead-free SMD removal kit contains removal alloy, no-clean tack flux and alcohol pads; flux all pins, melt the alloy onto the joints with a low-temperature iron, and lift the part with a vacuum pen; removal below 300 °F. Retrieved 2026-10-25.
  9. ↑ 9.00 9.01 9.02 9.03 9.04 9.05 9.06 9.07 9.08 9.09 9.10 Chip Quik Inc., Chip Quik Removal Alloy (SMD4.5), datasheet revision 1.0, 2015, PDF copy at Chip Dip: melting point 58 °C (136 °F); apply flux to all leads; melt the alloy uniformly on all pins and keep it molten long enough for complete reflow; lift the chip with a dental pick or vacuum pen; while molten, move the excess to an unused part of the board with a swab and flux; polish each pad with swab and flux while heating until thoroughly clean; at room temperature clean the residue with an alcohol pad. Retrieved 2026-10-25.
  10. ↑ 10.0 10.1 10.2 10.3 IPC, IPC-7711C/7721C Rework, Modification and Repair of Electronic Assemblies, January 2017, table of contents in the ANSI webstore preview: section 2.5 baking and preheating; 3.3 chip component removal (bifurcated tip, tweezer and hot air methods); 3.6 and 3.7 removal of two- and four-sided gull-wing parts; 4.1 land preparation; 5.3 chip component installation (paste and hot air, point to point); 5.5 gull-wing installation (multi-lead, point to point, paste and hot air, hook tip and blade tip methods). The standard itself is paywalled. Retrieved 2026-10-25.
  11. ↑ NASA, NASA-STD-8739.3 Soldered Electrical Connections, with Change 4, copy at EverySpec, section 6.13: isopropyl alcohol among the approved solvents for removing flux. Cancelled in 2011, when NASA adopted IPC J-STD-001ES. Retrieved 2026-10-25.
  12. ↑ 12.0 12.1 12.2 Little Bird Electronics, "Chip Quik SMD Removal Kit with Lead-Free Alloy (SMD1NL)", product page (discontinued item): the alloy blends with the existing solder and stays liquid long enough to lift or slide the part off; steps of flux, alloy, lift, then clean the pads with flux, solder wick and an alcohol wipe. Retrieved 2026-10-25.
  13. ↑ SparkFun Electronics, "Polarity": surface-mount ICs such as SOIC, SSOP and QFN usually have a dot near pin 1; for all IC packages pin numbers increase anticlockwise from pin 1. Retrieved 2026-10-25.
  14. ↑ Kester, "Alloy Temperature Chart": Sn63Pb37 183 °C; Sn96.5Ag3Cu0.5 (SAC305) 217 to 220 °C. Retrieved 2026-10-25.
  15. ↑ 15.0 15.1 15.2 15.3 15.4 15.5 15.6 Bob Wettermann (BEST Inc.), "Knocking Down the Bone Pile: Solder Excavation and Rework", SMT007 Magazine, May 2021: residual solder must be removed so that fine-pitch parts are coplanar; IPC-7711 section 4.1.3 braid method with fluxed pads and braid slightly smaller than the pad, the tip moved up and down with the braid between, and the braid advanced to a clean section; do not "swab the deck" across a row of pads, which lifts pads and damages the solder mask. Retrieved 2026-10-25.
  16. ↑ Chemtronics, Soder-Wick Desoldering Braid technical data sheet TDS #SWick, rev. E (02/05), PDF copy: choose braid equal to or slightly larger than the pad. Retrieved 2026-10-25.
  17. ↑ 17.0 17.1 17.2 17.3 17.4 17.5 17.6 17.7 Anne Barela, "LCARS-inspired Circuit Board Panel: Soldering", Adafruit Learning System, 18 October 2023: lightly tin the IC pads, place the chip, heat a corner lead to anchor it, check that it is square and reposition if needed, tack the opposite lead, then solder the leads one by one; for chip resistors, tin one pad, place the part with tweezers, heat to fix it, then solder the other end. Retrieved 2026-10-25.
Retrieved from "https://retrotechwiki.com/index.php?title=Surface-Mount_Rework&oldid=30982"