Soldering the boards

This is the long lab, the chapter the whole book has been climbing toward. Everything you practiced in Chapter 7 gets used for real, in six stages, in the order Chapter 9 justified: diodes, sockets, TRRS and reset, OLED jumpers and headers, controllers, screens. Each stage below tells you what the part does, how it must be oriented, the steps, and an inspection gate. Do not skip the gates; they are cheap now and expensive later.

Two standing rules for the whole chapter:

  • Before every stage, read the tape. Every component in this chapter goes on a face labeled "LEFT BACK" or "RIGHT BACK". Soldering to the unlabeled side is the classic Lily58 error, and it is committed by smart, careful people who were sure they did not need to check. Check.
  • Work one half at a time through each stage (all left diodes, then all right diodes), so a mistake teaches you before you repeat it 58 times.

Total bench time, first build: roughly 4 to 6 hours for this chapter alone. The per-stage estimates below assume a beginner going carefully.

Stage 1: diodes (58 of them, about 60 to 90 minutes)

What they do: each key has one diode, a one-way valve for electricity that lets the controller tell 58 keys apart using only a handful of pins, and lets it see several keys held at once without ghost presses. Chapter 3 explained the matrix they make possible.

Orientation: diodes are the only tiny part in this build that has a direction, and every single one must point the same way. The SMD diode body carries a painted line across one end (the cathode, the "out" end of the valve). The PCB silkscreen marks the matching direction at each diode position. Line up the diode's painted line with the silkscreen's marker, every time, all 58 times. On the Lily58 all the diodes on a half point the same way, which makes checking easy: any diode that looks rotated compared to its neighbours is wrong.

Don't be confused. The painted line on the diode and the marking on the silkscreen are two different things that must agree. The line on the tiny black body is part of the component, put there by the diode factory, and it always marks the cathode. The marker printed in white on the PCB is part of the board, and it shows which way the designer needs the cathode to face at that spot. Your job is to make the component's line sit on the board's line side. Take a photo of your first correctly placed diode and compare every later one against it.

Most current Lily58 kits ship SMD (surface-mount) diodes, type 1N4148W: black grains of rice with a metal tab at each end, sitting on top of pads rather than through holes. The tack-and-reheat technique:

  1. Confirm the tape label. Melt a small blob of solder onto one pad of the first diode position (this is "tinning" the pad), then lift the iron.
  2. Pick up a diode with tweezers. Check the painted line against the silkscreen marker. Hold it in place on its pads.
  3. Touch the iron to the tinned pad so the solder re-melts, slide the diode's end into the molten blob, remove the iron, and hold the diode still for a second while the joint sets. The diode is now tacked down.
  4. Look at it side-on: both ends flat on their pads, not tombstoned (one end lifted in the air). Nudge and reheat if needed.
  5. Solder the other end normally: iron to pad and tab, feed a little solder, remove.
  6. Return to the first end and give it a proper joint: touch the iron, add a whisker of solder if the tack was thin.
  7. After every row of about ten, stop and audit: all lines pointing the same way, no tombstones, joints shiny and concave.

If your kit has through-hole diodes instead (small glass or black cylinders with long wire legs, type 1N4148): the direction mark is a band printed around the cathode end of the body. Bend both legs down at 90 degrees using a lead-bending jig or the edge of a ruler so they match the hole spacing, insert with the band toward the silkscreen's cathode marking (often a square pad or the bar of the printed diode symbol; your board's silkscreen is the authority), solder both legs from the back, and snip the excess with flush cutters, safety glasses on. Save two or three of the clipped legs in your spares bag; they make perfect bodge wires (little repair jumpers) if a trace ever needs fixing during troubleshooting.

You are done when all 58 diodes are mounted on taped faces, every painted line agrees with its silkscreen marker, nothing is tombstoned, and a fingertip dragged gently across each row snags on nothing loose.

Stand up. Stretch. This was the fiddliest stage, and it is behind you.

Stage 2: hotswap sockets (58, about 60 to 90 minutes)

What they do: a hotswap socket is a little plastic-and-metal receptacle that grips a switch's pins, so switches push in and pull out without any soldering (Chapter 3). You solder the socket once; the switch is forever replaceable.

Orientation: the socket drops into its silkscreen outline on the back of the PCB, and it only matches the outline one way. Its two metal contact leaves land on the two big pads. If a socket does not sit into the outline naturally, it is rotated; never force it.

The critical quality bar in this stage is flatness. The socket's plastic body must sit flush against the PCB, because the switch pins that arrive from the other side in Chapter 11 are only so long. A socket soldered at a tilt, propped up on a blob of solder, will not grip its switch.

  1. Confirm the tape label. Tin one pad of the first socket position, generously; socket pads are big and drink more solder than diode pads.
  2. Place the socket into its outline. Press down on its plastic body with tweezers (the body gets hot; tweezers, not fingertip) and reheat the tinned pad until the socket sinks flat against the board. Hold it down while the solder sets.
  3. Check side-on: the body flush to the PCB all around, no rocking.
  4. Solder the second pad: iron heats pad and contact leaf together, feed solder until it wets both and forms a smooth fillet.
  5. Revisit the first pad and make it a full joint too.
  6. Tug test every socket: grip the plastic body with tweezers and pull firmly away from the board. A good socket does not move at all. These joints take real mechanical force every time you insert a switch, so a merely-decorative joint will fail in service.

You are done when all 58 sockets sit flush, every one passes the tug test, and both pads of each show a proper wetted fillet, not a ball resting on top.

This is the longest stretch of repetitive work in the build. If your joints are getting worse instead of better, that is fatigue, not failure; break here and come back.

Stage 3: TRRS jack and reset switch, one of each per half (about 20 minutes)

What they do: the TRRS jack is the headphone-style socket where the cable that links the two halves plugs in; the reset switch is the button you press to put the controller into bootloader mode for flashing (Chapter 3, and it earns its keep in Chapter 12).

Both are through-hole parts and neither has a tricky orientation: their pin patterns are asymmetric, so they only fit their holes one way. The enemy here is seating, especially for the TRRS jack. A jack soldered at a slight tilt looks fine on the bench and then fights the cable forever.

  1. Confirm the tape label. Insert the TRRS jack from the back so its pins poke through toward the front. Hold it flush with a strip of masking tape over its body.
  2. Flip the board, solder one pin only, flip back, and inspect: body flat against the PCB, connector opening square with the board edge? If not, reheat that one pin while pressing the jack home.
  3. Happy with the seating, solder the remaining pins.
  4. Repeat the same tape, one pin, check, finish sequence for the reset switch. Its little legs sometimes need a gentle inward squeeze to enter the holes; that is normal.
  5. Do the other half.

You are done when both jacks and both reset buttons sit flush and straight, and the reset button gives a crisp click when pressed.

Stage 4: OLED jumpers and headers (about 20 minutes)

What this is: the Lily58 routes the OLED screen's signals through solder jumpers, pairs of tiny bare pads sitting almost touching, which you connect by bridging them with solder. Until they are bridged, the OLED connector is electrically dead; forgetting the jumpers is the classic "my screens never worked" mistake, filed in Chapter 13.

Where exactly the jumper pads sit, how many there are, and which side of the board they are on varies with PCB revision, so this is one of the moments to open your kit's build guide and match its photo against your board. Typically they are four pairs near the controller area. Bridge only the jumpers your guide shows; other bare pads on the board are for options you are not using.

The bridging technique, which deliberately breaks the "solder is not glue, use the minimum" rule from Chapter 6:

  1. Confirm you are looking at the pads your guide identifies. Add flux from the flux pen across the pad pair.
  2. Load a slightly fat blob of solder onto the iron tip and press it across both pads at once. Drag gently. Surface tension pulls the blob into a bridge joining the pair.
  3. If the solder balls up on one pad and refuses to span, add more flux and try again; flux, not more solder, is usually the fix.
  4. Bridge all the pairs your guide shows, then solder the 4-pin header into the OLED connector holes: short pin side into the board from the back, plastic collar flush, tack one pin, check it is standing straight (a leaning header makes a leaning screen), then finish the other three.

You are done when every jumper pair your kit's guide shows is bridged with a smooth connected blob, no bridge strays onto any third pad, and the 4-pin headers stand perpendicular on both halves.

Stage 5: controllers (the decision stage, 30 to 60 minutes)

What it does: the controller is the small computer that scans the switch matrix and speaks USB (Chapter 3). It connects to the Lily58 through two rows of 12 pins, and you have a genuine decision about how: solder it in permanently, or socket it.

Don't be confused. Ordinary header pins and Mill-Max pins solve the same problem in different ways and are not interchangeable. Header pins are square-ish posts on a plastic strip, soldered to both the board and the controller: permanent. Mill-Max parts are a matched pair, machined round socket strips that solder to the PCB and slim round pins that solder to the controller, and the controller then plugs in and pulls out like a cartridge. Header pins do not fit Mill-Max sockets properly; buy and use the pair together.

Path A: direct solder with header pins (cheaper, ships with most kits, permanent). The soldering is easy; the danger is entirely in orientation, because this is the least reversible step of the build. Get it backwards or upside down and you face 24 joints of the hardest desoldering in the hobby. On the Lily58 most guides mount the controller with its component side down, facing the PCB, so you see its bare back on the finished board. But do not take that, or anything, on trust here:

  1. Find the controller footprint on the back of the PCB. The silkscreen marks how the controller sits: which end faces which way, and where the labeled pads go. Match the pin labels on your controller against the markings on the board, and check the whole arrangement against your kit's build guide photos. Only when board, controller, and guide all agree do you pick up the iron.
  2. Insert the two 12-pin header strips into the PCB from the back, long pins up. A trick for perfect alignment: seat the controller loosely on top of the pins before soldering anything, so the headers cannot lean.
  3. Solder one pin of each strip to the PCB, re-check everything is flush and the controller still fits, then solder the rest of the strip pins to the PCB.
  4. With the controller seated in its final orientation, solder all 24 pins to the controller itself. Snip any excess pin length, glasses on.

Path B: Mill-Max sockets and pins (recommended; roughly 10 to 20 dollars extra per keyboard). Controllers are the most likely part to die or be upgraded, and a socketed one swaps out in ten seconds. The whole game is keeping the pins perfectly straight, and the trick is to use the parts as jigs for each other:

  1. Solder the two Mill-Max socket strips into the PCB exactly as you would headers: insert from the back, tack one pin, verify flush and upright, finish.
  2. Now the pins. Never solder a pin that is sitting crooked; it will be crooked forever. Either (a) push the pins into the socket strips first, rest the controller on top so every pin enters its hole, then solder the pins to the controller while the sockets hold them perfectly aligned, or (b) stand the pins in a spare strip used purely as a jig and do the same. Some pins ship on a plastic carrier bar that serves as the jig; snap it off after soldering.
  3. Solder all 24 pins to the controller, let everything cool completely, then pull the controller straight up and out. Straight up: levering at an angle bends pins.
      Mill-Max stack, side view (controller shown lifted):

      [ controller PCB ]
         |  |  |  |      <- pins, soldered to the controller
         v  v  v  v
        [socket strip]   <- soldered to the keyboard PCB
      ===================
        keyboard PCB

A note on the USB port: on ATmega32u4 boards with a micro-USB connector, the port is the board's famous weak point; it is held on by small surface pads and snaps off under cable leverage. Handle the cable by pushing and pulling straight, never by yanking sideways, and consider a dab of epoxy along the connector's sides (not inside it) as reinforcement. RP2040 boards with USB-C are much sturdier here.

You are done when both controllers are mounted in the orientation your board's silkscreen and kit guide agree on, all 24 pins per controller are soldered at both levels (board and controller), and, if socketed, each controller can be removed and reinserted smoothly.

Stage 6: the OLEDs (10 minutes)

The screens ride on the 4-pin headers from stage 4. If you want them removable (worth it: they are fragile and occasionally worth replacing), solder female header sockets to the PCB in stage 4 instead and give each OLED male pins, so it plugs in like a tiny controller. Otherwise solder each OLED directly onto its header pins: seat it parallel to the main board, tack one pin, adjust until it sits level, finish the other three. The OLED's 4 pads are labeled and only line up with the header one way; match the labels on the screen module to the labels by the header on the PCB.

You are done when both screens sit level and firm on their headers.

The final inspection ritual (20 minutes, not optional)

Before anything gets assembled or plugged in, put both halves through this sequence. Every minute here saves ten in Chapter 13.

  1. Raking light pass. Hold each board under a lamp at a shallow angle and sweep it slowly. Raking light throws shadows off solder bridges, lifted diode ends, and unsoldered pins that look fine face-on.
  2. Phone-camera macro pass. Photograph each region close up and zoom in. The camera magnifies better than your eyes and the photos double as before-assembly records.
  3. Diode direction audit. One more full sweep: 58 painted lines, all agreeing with their silkscreen markers.
  4. Continuity spot checks. Multimeter to diode mode (the symbol that looks like an arrow against a bar). In this mode the meter passes a tiny current and reads out if it flows. Probe across a few diodes: with the probes one way you should get a reading (a few hundred millivolts) or a beep, and with the probes swapped, nothing. The direction that conducts is the direction the diode points. Also touch one probe to a hotswap socket contact and hunt for its connected diode pad; a beep says the joint and trace are sound. Spot-check a dozen positions across both boards, not all 58.
  5. Flux cleanup. Scrub the brown flux residue off with isopropyl alcohol on a lint-free wipe or an old toothbrush, and let the boards dry. Clean boards make every later inspection easier, and some flux residues turn conductive with humidity.

You are done when both boards are clean, photographed, and have passed all five passes with anything suspicious reworked using Chapter 8.

Takeaways

  • Read the tape label before every stage; wrong-side soldering is the error to fear, and it is entirely preventable.
  • Diodes: painted line to silkscreen marker, all 58 pointing the same way, audited every ten.
  • Sockets must sit flush and survive a tug test; TRRS jacks get taped, one-pin tacked, and checked before finishing.
  • Bridge the OLED jumpers your kit's guide shows, on the side it shows.
  • The controller stage is the least reversible: board silkscreen plus kit guide decide orientation, and Mill-Max sockets buy you a replaceable controller for a few dollars.
  • Finish with raking light, macro photos, diode-mode spot checks, and a flux scrub. Rework now, while everything is reachable.

👉 Two clean, fully soldered boards, inspected and photographed. Now they become a physical object you can rest your hands on: Assembly: plates, switches, and keycaps.