Drag Soldering SOIC and QFP Packages

Section Overview

You know what drag soldering is and why it works (7.1); this section is the actual procedure on the two packages you'll drag most — the SOIC and the quad flat package (QFP). The routine is the same for any gull-wing IC. First, prep and tack: find pin 1 (6.4), flux the pads heavily (Chapter 3), place the part square so every foot sits on its pad, check coplanarity (6.4), and tack two diagonal corners to lock it (6.5). Then load the tip's reservoir with a solder bead (7.1) and draw it steadily along a row of leads: as the tip passes, solder flows onto each foot and forms a heel fillet (6.7), while surface tension carries the excess along. A SOIC has two long rows — drag one side, then the other. A quad flat package has four sides of leads — you rotate the board and drag each side in turn, with more pins and finer pitch demanding more flux and care. Between passes, a back-drag — a second, cleaner sweep back over the row — picks up excess solder and clears many bridges before you even reach for braid. Then finish the same way every time: wick any remaining bridges with fluxed braid (5.5; 6.6; full bridging is 7.3) and inspect every heel fillet and gap (6.7). Two faults dominate: the bridge, which is expected and wicked away, and the solder skip — a pin the drag left without solder, from too fast or too dry a pass — which you simply touch up by hand. Pin 1, heavy flux, tack two corners, load a bead, drag each side, back-drag, then wick and inspect — that is drag soldering an IC in practice.

Why This Matters

This is where the concept of drag soldering becomes a real, repeatable skill you can use on a live repair. This matters because a procedure you can follow step by step is what turns "I understand drag soldering" into "I can solder this 44-pin QFP": the ordered routine — prep, tack, load, drag, back-drag, wick, inspect — is what makes the result consistent instead of hit-or-miss. It matters because the tack step is what everyone skips and regrets: dragging an untacked part shoves it out of alignment, so tacking two diagonal corners first (6.5) is what lets you drag confidently without the chip walking. It matters because the QFP is the part that scares people, and the four-sides-by-rotation method makes it routine: a quad flat package is just a SOIC with two more rows, done the same way one side at a time. It matters because the back-drag is the pro move that separates a clean result from a bridge-covered mess: a second light pass carries off the excess and clears bridges before cleanup, dramatically cutting the wicking you have to do. It matters because knowing the two faults — bridge and skip — and their fixes removes the fear: bridges wick away and skips get a touch of the iron, so neither is a disaster. And it matters because this exact procedure scales: the finer-pitch QFPs and the bridging work (7.3) all build on this stroke, and a technician who can drag a SOIC and a QFP can hand-solder almost any leaded IC fast. Learn the procedure here and fine-pitch ICs stop being slow, dreaded jobs.

Required Prerequisites

What Is Drag Soldering? — Section 7.1 explained the concept, the drag tip and its reservoir, and why flux and surface tension make it work; this section is the hands-on procedure. You should also be comfortable tacking two diagonal corners and reading a heel fillet (6.5; 6.7), finding pin 1 and checking coplanarity (6.4), and wicking a bridge (5.5), with a drag tip and plenty of liquid flux ready (Chapters 4 and 3).

Recommended Consumables

Liquid or gel flux — a lot of it (Chapter 3) — the non-negotiable enabler; keep the row wet with flux
Thin flux-cored solder — to load the tip and feed the bead
Scrap boards with SOIC and QFP parts (7.1; 6.5) — to practice both packages
Desoldering braid (5.5; 6.6) — to wick the bridges a drag leaves
Isopropyl alcohol and a brush — to clean the heavy flux residue and inspect (6.7)
Eye protection and strong ventilation — drag soldering moves molten solder and uses heavy flux (Chapter 3)

Recommended Practice Hardware

A drag tip — hoof or mini-wave (Chapter 4; 7.1) — the tip that carries the bead down the row
A temperature-controlled iron (Chapter 4) — for steady, adequate heat
A magnifier — loupe, visor, or microscope (Volume 2, Chapter 9) — to place, drag, and inspect fine pitch
A board holder or vise that rotates — to steady the board and turn it for a QFP's four sides
Good light, a parts tray, and an ESD-safe surface

Real-World Applications

Drag-soldering a SOIC and a QFP is a routine repair task, and the procedure is always the same. A technician replacing a 16-pin SOIC fluxes the pads, tacks two corners, and drags each side in one pass — then wicks the one bridge it left and inspects the heels — done in a minute. A repairer installing a 48-pin QFP tacks diagonal corners, then rotates the board and drags each of the four sides in turn, back-dragging each to shed excess, and finishes with a bridge sweep and heel check (6.7). Someone whose first drags were bridge-covered learns the back-drag: one clean return pass, and most of the bridges are gone before the braid comes out. A repairer who kept missing a pin recognizes the solder skip — a lead the drag ran past too fast — and touches it with the iron and a dab of solder. And a beginner who tried to drag without tacking watches the chip slide askew — and learns to lock two corners first. The failures this procedure prevents: the part dragged out of alignment, the bridge-covered row from no back-drag, the skipped pin left open, and the cold joints from a timid, stop-start drag. Every fast fine-pitch IC install a pro does is this exact sequence — tack, drag, back-drag, wick, inspect.

Common Challenges

The chip slides while I drag. You didn't tack it, or the tack was weak — tack two diagonal corners solidly first (6.5) so the part can't move under the tip.
Every pass leaves a row of bridges. Not enough flux, too big a bead, or no back-drag — flux heavily, carry a modest bead, and add a clean back-drag; wick the rest (5.5).
A QFP's four sides are overwhelming. Do one side at a time — rotate the board so the side you're dragging is oriented the same comfortable way each time, and treat it as four SOIC-style drags.

Safety Notes

Risk Level: Low. Drag soldering is hot work that deliberately moves a bead of molten solder along the board and uses heavy liquid flux, so eye protection and strong ventilation matter.

A moving molten bead, heavy spitting flux, and hot rows

Drag soldering is hot work with the flux and molten-solder hazards turned up. The iron tip and molten solder burn instantly — 300 to 400°C (Chapter 4): keep the iron in its stand between passes, and never touch a just-dragged row or the board — both are hot. You are moving a bead of molten solder along the board — it can flick, drip, or roll off the tip, especially at the end of a stroke — so wear eye protection and keep your face out of the line of the drag. Heavy liquid flux spits and spatters when it hits the hot tip and releases a lot of fume — far more than cored solder alone, and rosin and no-clean fluxes are respiratory sensitizers (Chapter 3) — so use strong extraction or ventilation and keep back from the plume. Handle ICs with ESD care — they are static-sensitive — and by the edges (electrostatic-safety material). The parts are tiny and fine-pitch and the board gets hot — work over a tray, use magnification, and take breaks to avoid eye strain. Wash your hands after handling leaded solder and flux (lead hygiene, Chapter 2). If the board is from a device, disconnect and discharge it first (powered or mains work follows the earlier electrical-safety sections). The live hazards are the moving molten-solder bead, the spitting flux and its heavy fume, and the hot rows — eye protection and strong ventilation cover them.

Professional Tips Before Starting

Tack two corners before you drag anything. An untacked part slides under the tip — lock two diagonal corners (6.5), confirm pin 1 and coplanarity (6.4), then drag.
Keep the flux wet and re-flux between passes. Drag soldering lives on flux (Chapter 3) — a dry row bridges and skips; re-apply liquid flux before each side and before a back-drag.
Back-drag before you reach for braid. A second, light, clean pass down the row picks up excess and clears many bridges on its own — so you wick far less afterward.

Drag-Soldering a SOIC and a QFP — Step by Step

Prep and Tack — Flux and Diagonal Corners

A good drag starts before any solder moves, with a well-prepped and firmly-tacked part. Find pin 1 first — match the pin-1 indicator to the board's silkscreen (6.4) — because a multi-pin IC dragged in the wrong orientation is wrong on every pin. Then flux the pads heavily: apply a generous film of liquid or gel flux over every row you'll drag (Chapter 3), because the flux is what lets the solder wet and flow and is the single biggest factor in a clean drag. Place the part square with tweezers so every lead foot sits on its pad, and check coplanarity — that no leg is lifted (6.4). Now tack two diagonal corners (6.5): solder one corner lead, verify the part is still square and pin 1 is right, then tack the diagonally opposite corner to lock both position and rotation. This is the step people skip and regret — an untacked part slides out from under the drag tip the moment it moves. With pin 1 confirmed, the pads well fluxed, every foot down, and two corners locked, the part is ready to drag — and most of the outcome is already decided by how well you prepped.

Loading the Tip and the Drag Stroke

The stroke itself is simple once the tip is loaded and the flux is wet. Load the drag tip's reservoir with a solder bead (7.1): touch solder to the tip so its well holds a small, controlled amount of molten solder — enough to wet a run of pins, not a big blob. Make sure the row is freshly fluxed. Then bring the loaded tip to the start of the row and draw it steadily along the leads at a consistent, moderate angle and speed, keeping light contact so the reservoir feeds solder onto each foot as it passes. As the tip moves, solder flows onto each lead and its pad and forms a heel fillet (6.7), and surface tension carries the excess along with the tip rather than leaving it bridged. Drag off the end of the row — running the tip past the last pin — so the leftover solder releases cleanly rather than pooling on the final lead. Keep the motion continuous: a smooth, steady drag lays an even fillet on every pin, while a hesitant, stop-start drag leaves cold joints and skips. Reload the bead and re-flux as needed for a long row. Loaded tip, wet flux, steady continuous stroke, drag off the end — that is one clean pass.

Dragging a SOIC's Two Rows

A SOIC is the simplest drag: two long rows of gull-wing leads down its sides, done one at a time. With the part tacked (two diagonal corners) and the pads fluxed, load the tip and drag the first long side — start at one end, draw the loaded tip steadily down the row of feet forming a fillet on each, and drag off the far end. Re-flux and reload as needed. Then rotate or reposition so the second side is comfortable, and drag it the same way. Two passes, one per side, solder the whole chip. Because a SOIC's pitch is relatively coarse (6.5), it is the friendliest package to learn dragging on — the leads are far enough apart that bridges are fewer and the stroke is forgiving. After both rows, do a quick back-drag on any row that looks heavy, then wick any bridges and inspect the heels (6.7). Master the SOIC's two-row drag first: it is the same stroke you'll use on every other gull-wing package, just with fewer, more forgiving pins.

Dragging a QFP's Four Sides

A quad flat package is a gull-wing IC with leads on all four sides, and you drag it exactly like a SOIC — four times. A quad flat package (QFP) — common in fine pitch and high pin counts — has a row of gull-wing leads down each of its four edges. After tacking two diagonal corners (6.5), you drag one side, then rotate the board ninety degrees so the next side faces you the same comfortable way, and drag it — working around all four sides in turn. Rotating the board (rather than twisting your hand) keeps every drag the same easy motion. A QFP usually has more pins and a finer pitch than a SOIC, so it needs more flux, a steadier hand, and more care — and it rewards a back-drag on each side. Pay attention to the corner pins where two sides meet: they can get an extra or a missed touch, so check them specifically. Otherwise it is four SOIC-style drags: flux a side, drag it, back-drag, rotate, repeat. Then wick the bridges from all four sides and inspect every heel fillet around the part (6.7). A 44- or 100-pin QFP is not four times harder than a SOIC — it is the same drag done four times, one comfortable side at a time.

Speed, Angle, and the Back-Drag

Three controls turn a rough drag into a clean one: your speed, your tip angle, and a second pass called a back-drag. Speed: a steady, moderate, continuous pace lets each pin get enough solder and heat; too fast leaves skips and cold joints, too slow dumps excess and bridges. Angle and contact: hold the tip at a consistent shallow angle with light contact so the reservoir feeds evenly onto the feet — pressing hard or changing angle mid-row makes the delivery uneven. And the back-drag: after the first pass, draw the tip back over the same row in a second, cleaner sweep — with little or no added solder — to pick up the excess and lift many of the bridges the first pass left. A back-drag is the single most effective way to reduce the cleanup: it uses the tip and surface tension to carry off surplus solder before you ever touch braid to the board. You can back-drag toward fresh flux or off the end to shed the collected solder. Steady speed, a consistent shallow angle, light contact, and a back-drag to finish each row — these are what make a drag come out clean rather than covered in bridges.

Cleanup — Wick, Inspect, and Common Faults

The drag is not finished until you have cleared the bridges, touched up any skips, and inspected every joint. Two faults dominate drag soldering, and both are routine to fix. The bridge — solder joining two adjacent pins (5.4) — is expected even from a good drag; after back-dragging, wick any remaining bridges away with fluxed desoldering braid (5.5; 6.6), and the full diagnosis and prevention of bridging is Section 7.3. The solder skip — a pin the drag left without a joint, from too fast or too dry a pass — is fixed by simply touching that lead and pad with the iron and a dab of solder to form its fillet. Watch also for excess (too big a bead) and for the tip losing its solder mid-row (reload and re-flux). Then inspect every joint under magnification (6.7): look for a good heel fillet on each pin, sweep every gap for a bridge, and confirm no lead is skipped or lifted. Reflow any dull, cold, or skipped joint. A drag plus a back-drag plus a wick-and-inspect is one complete, fast operation — the bridges and skips are designed out at the cleanup step, not agonized over during the stroke. Clear the bridges, touch up the skips, and read every heel — then the IC is done.

Common Mistakes

Dragging without tacking. The part slides out of alignment under the tip — tack two diagonal corners first (6.5), pin 1 and coplanarity confirmed (6.4).
Skimping on flux. A dry row bridges and skips — flux heavily and re-flux between passes (Chapter 3).
Skipping the back-drag. You leave far more bridges to wick — a second clean pass carries off most of the excess before braid.
A stop-start or too-fast drag. It leaves cold joints and solder skips — keep a steady, moderate, continuous stroke and drag off the end.
Not inspecting every pin. A skipped or bridged pin hides in a long row (6.7) — sweep every gap and read every heel fillet.

Troubleshooting Guidance

Drag problems trace to tack, flux, bead, speed, or cleanup. If the part slides while dragging: it isn't tacked, or weakly — tack two diagonal corners solidly (6.5). If a row is covered in bridges: too little flux, too big a bead, or no back-drag — flux heavily, carry less solder, and add a clean back-drag; wick the rest (5.5). If pins are skipped (no solder): the drag was too fast or the flux/tip too dry — slow down, keep the flux wet, and touch up each skip by hand. If joints are cold or dull: the stroke was stop-start or the tip too cool — use adequate heat and a steady, continuous drag. If the tip stops feeding solder mid-row: the reservoir emptied — reload the bead and re-flux (7.1). If a QFP feels unmanageable: do one side at a time, rotating the board — it is four SOIC drags. If bridges won't wick: not enough flux on the braid — add flux and use fresh braid (5.5). If a corner pin is bad: the meeting of two sides gets missed — check and touch up corner pins specifically. The throughline: tack first, flux heavily, carry a modest bead, drag steadily and back-drag, then wick bridges and touch up skips — and a dragged IC comes out clean.

Verification & Testing Methods

Use this as a drag-procedure check:

[ ] I find pin 1, flux the pads heavily, place the part square, check coplanarity, and tack two diagonal corners before dragging (6.4; 6.5).
[ ] I load the tip with a modest bead, keep the flux wet, and drag each row steadily at a consistent angle, dragging off the end.
[ ] I drag a SOIC's two rows one side at a time, and a quad flat package's four sides by rotating the board.
[ ] I use a back-drag — a clean second pass — to pick up excess and clear bridges before wicking.
[ ] I wick any remaining bridges with fluxed braid (5.5) and touch up any solder skip by hand.
[ ] I inspect every heel fillet and sweep every gap under magnification before calling the part done (6.7).

Then try the practice exercises below — drag-procedure practice; scenarios differ from the quiz.

Practice Exercises

Drag a SOIC (10 minutes, applied). On scrap, prep and tack a SOIC (pin 1, flux, two diagonal corners), then drag both rows, back-drag, wick, and inspect the heels (6.7).
Drag a QFP (12 minutes, applied). Tack a QFP's diagonal corners, then rotate the board and drag all four sides, back-dragging each. Wick the bridges and check the corner pins.
Practice the back-drag (6 minutes, applied). Drag one row without a back-drag and another with one. Compare how many bridges each leaves before wicking.
Find and fix the faults (6 minutes, applied/reasoning). After a drag, identify any bridges and solder skips, and describe how you fix each (wick versus touch-up).

These core ideas — prep and tack, the drag stroke, SOIC two-rows and QFP four-sides, the back-drag, and cleaning up bridges and skips — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

The drag procedure is the same for any gull-wing IC: find pin 1, flux the pads heavily, place square and check coplanarity, tack two diagonal corners (6.5), load the tip with a bead, and drag each row steadily to form a heel fillet on every pin (6.7).
A SOIC has two long rows (drag one side, then the other); a quad flat package (QFP) has four sides — you rotate the board and drag each in turn, with more flux and care for its finer pitch and higher pin count.
A back-drag — a second, cleaner pass over a row — picks up excess solder and clears many bridges before you ever reach for braid, and is the biggest single reducer of cleanup.
Keep the stroke steady and continuous at a consistent shallow angle: too fast or stop-start leaves cold joints and a solder skip (a pin left without solder), which you touch up by hand.
Finish every drag the same way: wick any bridges with fluxed braid (5.5; full bridging in 7.3) and inspect every heel fillet and gap (6.7) — bridges and skips are cleared at cleanup, not feared during the stroke.

Skills Learned

You can now prep and tack a gull-wing IC ready for dragging.
You can now load the tip and drag a row into clean heel fillets.
You can now drag-solder a SOIC's two rows and a QFP's four sides.
You can now use a back-drag and control drag speed and angle.
You can now wick bridges and touch up a solder skip after a drag.

Glossary Additions

quad flat package — a surface-mount integrated-circuit package, abbreviated QFP, with rows of gull-wing leads on all four sides of a square or rectangular body, made in a wide range of pin counts and often at fine pitch; because its leads are gull-wing and reachable, a QFP is hand-solderable and is a natural fit for drag soldering, done one side at a time by rotating the board. Thin and low-profile variants (such as the TQFP) are common, and the finer the pitch and higher the pin count, the more flux and care the drag needs.
back-drag — a second, cleaner drag pass made back over a row of leads just soldered, usually with little or no added solder, to pick up excess solder and lift many of the bridges the first pass left; the back-drag uses the tip and the solder's surface tension to carry off surplus before any desoldering braid is used, and it is the most effective single way to reduce the cleanup after a drag. A light, steady back-drag toward fresh flux or off the end of the row leaves cleaner, more even joints.
solder skip — a lead or pin left without solder, or with too little to form a joint, after a drag-soldering pass, typically caused by dragging too fast, running the tip dry of solder or flux, or losing contact with a foot; a solder skip is an open or marginal joint and is fixed by touching that lead and pad with the iron and a small amount of solder to form its fillet. Skips are found by inspecting every heel fillet along a dragged row.

Suggested Next Sections

Must read next:

Bridging — Causes and Correction — drag soldering leaves bridges by design, so the next section is the full treatment: why bridges form, how to minimize them at the drag, and every way to remove them cleanly — from the back-drag and braid to flux and technique — so a bridged row is never a problem.

Recommended:

What Is Drag Soldering? — the concept, the drag tip and reservoir, and the flux-and-surface-tension physics this procedure applies.
Soldering SOIC and Wide-Body ICs — the two-corner tack and flood-and-wick that the drag procedure builds on.