Soldering, Rework and Board Assembly

• Solder wets, it does not glueA good joint forms when both the pad and the lead reach solder's wetting temperature and flux has removed the oxide. Heat the joint, not the solder. A joint made by melting solder onto a cold pad is a cold joint: it looks dull, it is mechanically weak, and it will fail intermittently later.Heat both surfaces first, then feed solder into the joint. Total contact time on a small pad: 2-3 seconds.
• Flux is the active ingredientFlux chemically strips oxide so the solder can wet. Cored solder has some; it burns off in seconds. Extra flux from a pen or syringe is what makes drag soldering, fine-pitch work and rework possible. No-clean flux residue is generally safe to leave; rosin and water-soluble types must be cleaned.
• Alloys and temperatureSn63/Pb37 is eutectic: it goes from solid to liquid at one temperature (183 C), which makes joints easy and forgiving. Lead-free (SAC305) melts around 217-220 C, needs a hotter iron, and produces duller joints that are harder to inspect. Know which one you are using and set the iron accordingly.Typical tip temperature: 320-350 C for leaded, 350-380 C for lead-free. Hotter and faster beats cooler and longer.
• Thermal mass and tip choiceA ground-plane-connected pad sinks heat away fast; a fine tip cannot deliver enough power and you end up dwelling, damaging the pad. Use the largest tip that fits the joint, and expect to preheat or use a hot plate for large planes. Thermal reliefs on pads exist precisely for this reason.
• SMD techniques: tack, drag, and hot airTwo-terminal passives: tin one pad, place the part, tack it, then solder the other end. Fine-pitch ICs: tack two opposite corners, flood with flux, drag-solder the row, then wick away the bridges. QFNs and BGAs need hot air or a hot plate with paste, since the joints are under the part.
• Inspection criteriaIPC-A-610 is the industry standard for what an acceptable joint looks like. The essentials: a smooth concave fillet wetting both pad and lead, no cracks, no voids visible, no bridges, no lifted pads, and for through-hole, solder visible on the top side of the barrel where required.
• Desoldering and reworkBraid plus flux for pads and bridges; a solder sucker or desoldering iron for through-hole. Adding fresh solder before removing it is counter-intuitive but correct — it improves thermal contact and mixes flux in. Lifted pads are usually caused by dwelling too long with too little heat.
• Safety and hygieneSolder fumes are flux fumes and they irritate airways; ventilate or use a fume extractor. Lead is a contact hazard, not an inhalation one at soldering temperatures — wash your hands, do not eat at the bench. Observe ESD precautions for sensitive parts.

ProblemYou inspect a board and see four suspect joints. Identify each fault, its cause, and the fix.

1. 1
   Joint A: dull, grainy, ball-shaped, solder sitting on the pad without spreading. This is a cold joint — the pad never reached wetting temperature. Cause: tip too small, tip dirty and not transferring heat, or the pad tied to a plane with no thermal relief. Fix: add flux, reheat with a bigger tip until the solder visibly collapses and wets, hold 2 seconds, remove.
2. 2
   Joint B: shiny but a huge blob covering the pad and the neighbouring pad. This is a bridge. Cause: too much solder, or drag-soldering without enough flux. Fix: flux the area, lay clean braid over the bridge, press with the iron until the braid draws the excess away, then inspect for the fillet underneath.
3. 3
   Joint C: through-hole part, solder only on the bottom side, barrel not filled. The joint may pass a continuity test today and crack in six months. Cause: insufficient heat or insufficient dwell for the barrel to wick. Fix: heat the lead and the pad from the bottom until solder wicks up and appears at the top of the barrel.
4. 4
   Joint D: pad detached from the board, copper trace lifted. This is unrecoverable by reheating. Cause: excessive dwell time, excessive mechanical force while hot, or repeated rework. Fix: scrape back the trace to expose fresh copper, solder a fine jumper wire from the part lead to the trace, and secure it mechanically with adhesive. Document it — a repaired board is not a production board.
5. 5
   Root-cause pattern: three of the four are heat-delivery problems, not skill problems. Before blaming technique, check tip size, tip cleanliness, tip temperature, and whether the pad is thermally connected to a plane.

AnswerA = cold joint (reflow with more heat and flux). B = bridge (wick with flux and braid). C = unfilled barrel (heat from below until solder wicks through). D = lifted pad (jumper repair, document as rework).

Tool: A practice board and a USB microscope or phone macro lens

• Get a cheap SMD practice kit (widely available for a few units of currency) with 0805, 0603, SOIC, TQFP and through-hole footprints. Or use any scrap board with parts to remove and replace.
• Set your iron to the correct temperature for your alloy and clean the tip until it is bright and tinned.
• Solder a row of 0805 resistors using the tack-one-end method. Photograph each joint at high magnification.
• Solder a TQFP by drag soldering: tack two opposite corners, flood the row with flux, and draw the iron along the leads at a steady pace. Photograph before and after wicking.
• Deliberately create each of the four faults from the worked example: make a cold joint, a bridge, an unfilled through-hole barrel. Photograph them.
• Now repair each deliberate fault and photograph the result. This before/after pair is your evidence of rework capability.
• Grade every photograph against the IPC-A-610 acceptability criteria: is there a concave fillet? does solder wet both surfaces? is the lead's outline visible through the solder?
• Do a continuity and short check with the multimeter across every joint you made, including between adjacent pins.

• Adafruit - Guide to Excellent Solderingfreereading~45 minThe best free illustrated guide to through-hole technique and joint appearance. Read the 'Common problems' page twice.
• iFixit - How to Solder and Desolder Connectionsopen licencereading~30 minCC BY-NC-SA. Strong on desoldering and rework, which most guides skip.
• NASA-STD-8739.3 - Soldered Electrical Connectionsfreereference~60 minPublicly available, extremely detailed photographic standards for acceptable and defective joints. Use the photo galleries as your grading rubric.
• EEVblog - SMD soldering tutorialsfreevideo~90 minSearch for 'SMD soldering tutorial' and 'drag soldering'. Watching hands at real speed is worth more than any written description here.

Assemble a small board from scratch containing at least: six 0805 or 0603 passives, one SOIC-8, one TQFP-32 or SSOP, four through-hole parts including one connector, and one part you then remove and replace. Then produce an inspection report.

Deliverables

• The physically assembled board, powered up if it is a functional design (a practice kit is acceptable if you do not have a real board yet).
• A photographic inspection report: at least 12 magnified photographs of individual joints, each graded accept/reject against a stated criterion, with the criterion quoted.
• A rework record: photographs of one part before removal, the bare pads after removal, and the replacement part fitted, with a note on the technique and tip/air temperature used.
• A defect log: for every joint you had to redo, what the defect was, what caused it, and what you changed.
• Electrical verification: a continuity table proving every intended net is connected and a short-check table proving no adjacent pins are bridged, taken with a multimeter.
• A statement of your process parameters: alloy, tip temperature, tip geometry, flux type, and whether flux residue was cleaned and how.

Acceptance criteria — how you know it is good enough

• Every joint in the photographs shows a wetted, concave fillet, or is explicitly graded as a reject with a stated reason.
• No bridges remain between adjacent fine-pitch pins, proven by measurement not by eye.
• The removed-and-replaced part sits flat with no lifted pads.
• The defect log shows root causes, not just descriptions.
• The continuity and short checks cover every pin of the fine-pitch part, not a sample.

If you want to push further

• Reflow a QFN with hot air or a hot plate using solder paste and a stencil, then verify the hidden joints by X-ray-free means: continuity, current draw, and functional test.
• Measure the resistance of a deliberately cold joint versus a good one at the milliohm level using a four-wire measurement, and report the difference.

• I can produce a joint with a smooth concave fillet on 0603 passives and fine-pitch ICs.
• I can identify a cold joint, a bridge and an unfilled barrel from appearance alone.
• I use flux deliberately and can drag-solder a fine-pitch package.
• I can remove and replace a surface-mount part without lifting pads.
• I inspect my own work against a written standard rather than 'it looks fine'.
• I verify assembly electrically, checking both continuity and shorts.