Hand-soldering defects on a lead-free 0805 and SOIC batch
A small Pune contract-assembly unit hand-solders 200 mixed boards a week. Each board carries forty 0805 resistors and capacitors, two SOIC-8 ICs and a 4-pin header, on an FR-4 board with a lead-free SAC305 HASL finish. The operator uses a 350 °C iron with a 1.2 mm conical tip, 0.8 mm SAC305 wire and a no-clean flux pen. Incoming inspection pulls 20 boards and finds three boards with 0805 capacitors standing on one end, two boards with dull, grainy, balled-up joints on the SOIC leads, one solder bridge between two SOIC pins, and one board where the copper pad lifted off the laminate during a capacitor rework. Rework rate is 30 per cent. The customer drawing calls for IPC-A-610 Class 2.
1. Diagnose the cause of the dull, balled SOIC joints and the tombstoned capacitors, and specify the changes you would make to tip, temperature, wire diameter and technique.
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Dull, grainy, balled joints. These are cold joints: the joint froze before the solder and the pad reached a common temperature. SAC305 melts at about 217–220 °C, roughly 35 °C higher than Sn63Pb37, and it does not spread or brighten the way leaded solder does. Three things in the setup cause it:
- a 1.2 mm conical tip has a tiny contact area, so heat transfer into a SOIC lead and pad is slow;
- 350 °C measured at the tip may be only 300 °C at the joint once the iron is loaded, especially on a HASL board with any copper area;
- the flux in no-clean pen form is quickly consumed by a long dwell, so the oxide is not removed and the solder balls up instead of wetting.
Changes. Use a 1.6–2.4 mm chisel tip (large flat face = better heat transfer), set the station to 350–370 °C for lead-free work, verify with a tip thermometer rather than trusting the dial, use 0.5 mm SAC305 wire for 0805 and 0.8 mm for the SOIC, and add flux so dwell can be short. Touch the pad and the lead simultaneously for 1–2 s, feed solder into the joint (not onto the iron), remove the wire, then the iron. Keep dwell under about 3 s. If the board has thermal mass, preheat to 100–120 °C, particularly for ground-connected pads.
Tombstoning. One end of an 0805 lifts because the two joints did not melt or cool together: the second joint's heat, a placement offset, uneven pad sizes or dirty pads pull the part upright on surface tension. Tack one end, check alignment, then solder the other end while holding the part with tweezers.
2. For the bridged SOIC pins and the lifted pad, describe the correct repair sequence and decide what happens to the lifted-pad board under IPC-A-610 Class 2.
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Bridged SOIC pins. Do not drag a dry tip across the pins or you will scratch the solder mask. Flood the bridge with flux, lay a clean desoldering braid over it, press with a chisel tip and lift the braid as it wicks; repeat with a fresh piece of braid, then inspect at 10× magnification and clean the residue. If the bridge is between two leads with a wide gap, drag soldering with plenty of flux from the inside out is faster. Re-inspect for solder removal from the heel and for any lifted lead.
Lifted pad. The board is not automatically scrap, but it is now a repair, not a build.
- Assess how much copper remains: if only the pad has lifted and the track stub is intact, an adhesive-backed copper foil repair bonded and over-coated is an accepted IPC-7711 technique. If the track has gone with it, a fine engineering wire (30 AWG) can replace the connection, staked with epoxy and routed clear of other pads.
- Check the laminate under the pad for charring, blistering or delamination. If the base material is damaged, the assembly must be scrapped – no repair brings back the dielectric.
- A Class 2 assembly may accept this repair only if the customer drawing or contract permits repair and the repair is recorded: rework log, photo, operator ID, and customer approval before shipping.
Also cap the rework cycles at two or three per board and mark that board for 100% inspection.
3. The customer's inspector rejects the whole batch. Outline your containment and corrective action, and the process controls that prevent a repeat.
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Containment (same day)
- Quarantine all 200 boards plus stock in the customer's hands; issue a formal hold notice.
- 100% visual inspection at 10× of every board against IPC-A-610 Class 2 criteria, plus a functional test on the SOIC boards.
- Sort into reworkable and scrap; rework only after the process change is in place, not before, or the defect simply repeats.
Root-cause analysis – a simple 8D or 5-why. The data already point to two causes: iron tip geometry and dwell (cold joints, bridges) and unequal pad heating plus placement (tombstones). Ask why the operator used a conical tip and 350 °C – usually no work instruction, no tip thermometer and no training record.
Corrective action
- Write and post a work instruction with tip type, temperature range, wire diameter, flux and dwell for each package family.
- Buy a tip thermometer and a supply of chisel tips; add 0.5 mm wire.
- Train and certify both operators; re-qualify with a sample board each quarter.
- Add first-article inspection and an in-process check every 25 boards, with the results on a simple defect Pareto.
- Record and investigate the lifted pad as a separate machine/laminate issue.
Reply to the customer – a formal 8D within the agreed response time, with the objective evidence: inspection sheets, the updated instruction and the rework log. Rework rate must fall below 5% before the batch is re-submitted.