---
# System prepended metadata

title: How to solder microcontroller pins?
tags: [Microcontroller]

---

You’re basically asking: “How do I put this tiny centipede on the board without ruining it?” 
Let’s go through it step by step for both through-hole and SMD [microcontrollers](https://www.ampheo.com/c/microcontrollers).

![10](https://hackmd.io/_uploads/SkukIkj-be.jpg)

**1. First: what kind of package do you have?**

* Through-hole (DIP) – big legs that go through holes in the PCB/[breadboard](https://www.onzuu.com/category/solderless-breadboards).
* SMD TQFP/QFP – flat square package, pins around the edges, 0.5–0.8 mm pitch.
* SMD QFN – no visible legs, just pads underneath and around the edge.

I’ll cover all three, in increasing difficulty.

**2. Tools you should have**

* Temperature-controlled [soldering iron](https://www.onzuu.com/category/soldering-irons-tweezers-handles) with a fine tip (chisel or conical, ~1–2 mm)
* Solder (0.5–0.7 mm) with flux core (lead-free is fine; leaded is easier if allowed)
* Flux pen or liquid/gel flux (this is huge for SMD)
* [Tweezers](https://www.onzuu.com/category/tweezers) for SMD chips
* Solder wick (desoldering braid)
* Isopropyl alcohol + small brush / cotton swabs (cleaning)
* Optional but very helpful:
  * Magnifier or cheap USB microscope
  * Helping hands or PCB vise
  * Hot air rework (for QFN or rework)

Safety: work in a ventilated area, don’t breathe the smoke, eye protection is smart.

**3. Through-hole microcontroller (DIP) – easiest**
**Steps**

**1. Insert the chip**

* Line up the notch/dot (pin 1) with the board marking.
* Gently bend the legs inward if needed so they fit the holes.
* Push it into the PCB; flip the board and slightly bend a couple of pins outwards so it doesn’t fall out.

**2. Tack opposite corners**

* Solder just one pin on each corner first.
* Check alignment: chip sitting flat, pins centered in pads/holes.
* If it’s skewed, reheat one corner pin and nudge.

**3. Solder the rest**

* Touch the iron to the pad and pin together, then feed a tiny bit of solder.
* When solder flows around the pin and makes a shiny cone, remove solder, then iron.
* Repeat for each pin; don’t linger too long on one pad.

**4. Inspect and clean**

* Look for dull joints, cold joints, or bridges (two pins shorted by solder).
* Clean off flux residue with isopropyl alcohol if sticky.

That’s it — through-hole is very forgiving.

**4. SMD TQFP/QFP (fine-pitch legs) – “drag soldering” trick**

This looks scary but is actually easier than soldering each pin individually.

**4.1 Align and tack the chip**

**1. Flux the pads**

Apply flux on all the PCB pads where the MCU will sit.

**2. Place the chip**

* Use tweezers; line up the pins with the pads. Check that pin 1 (notch/dot) matches the PCB marking.
* Take your time; alignment is the most important part.

**3. Tack two corners**

* Add a tiny amount of solder on one pad at a corner of the footprint.
* Place the chip, press gently with tweezers, reheat that pad so the corner leg sinks into the solder.
* Do the opposite corner next.
* Verify alignment on all sides. If it’s off, reheat a tacked corner and nudge.

**4.2 Drag solder a side**

**1. Load the tip**

* Add a small “bead” of solder on the iron tip.
* Add more flux to that side of the pins.

**2. Drag technique**

* Place the flat part of the tip at one end of the row of pins.
* Gently drag the tip along the row of pins, letting the solder “wipe” over them.
* Surface tension + flux will pull solder onto each pin/pad pair.

**3. Deal with bridges**

* If you get solder bridges (two pins shorted), don’t panic.
* Add more flux; then:
  * Clean the tip, drag lightly again (often it will “pick up” excess), or
  * Press solder wick onto the bridged pins and briefly heat to suck up extra solder.

**4. Repeat for all sides**

Reflux each side, drag solder, clean bridges.

**4.3 Inspect**

Under magnification, check:

* No bridges.
* No pins floating (solder only on top, not actually wetting pad).

If a pin’s not soldered:

* Add a dot of flux.
* Touch that pin/pad with a clean iron tip; solder should reflow.

**5. QFN (no visible legs) – short version**

These are trickier and usually easiest with hot air and solder paste:

1. Apply a thin layer of solder paste to the PCB pads (stencil is ideal).
2. Put paste also on the thermal pad in the center if the package has one (not too much).
3. Carefully place the QFN on top, aligned to the pad outline.
4. Heat evenly with a hot air station until the paste reflows; the chip often “snaps” into place from surface tension.
5. Let it cool, then inspect the edges for solder fillets; verify connections by continuity test.

If you don’t have hot air, there are hacky techniques (preheating, reflow on a hot plate) but that’s a bigger topic.

**6. Common problems & quick fixes**

**Bridges between pins**

* Add flux.
* Use clean iron tip to drag away, or use solder wick on the bridge.

**Dull, grainy joints**

Likely too cold or oxidized; apply flux and reheat until shiny.

**Lifted pad**

* Don’t pull or scrape while hot.
* If pad lifts, you might need to patch with a tiny wire to the trace/via.

**7. Beginner tips to make life easier**

* Start practicing on junk boards or cheap breakout boards before touching your actual [microcontroller](https://www.ampheoelec.de/c/microcontrollers).
* Use more flux than you think – it makes solder flow and prevents bridges.
* Keep the tip clean and tinned (wipe on sponge/brass wool, add a little solder).
* Don’t crank the temperature crazy high; ~320–360 °C is enough for most leaded solder, a bit higher for lead-free.
* Take breaks; fine-pitch work is much easier when you’re not rushing.