Full Stack Electronics · Stage Four · Experiment 20

Experiment 20: Assembly Day

The seventh step from the breadboard to a printed circuit board. Sort the 45 surface-mount parts by their markings, solder them by hand block by block, add the eight pins, and inspect each joint before the first power-up.

Experiment 19 ended with the order. The boards of this course came back from JLCPCB six days later: five black boards with lead-free HASL, and no parts on them. This experiment puts the parts on one board. There are 46: 23 transistors, 21 resistors and C1, which are surface-mount parts, and the eight pins of J1. The parts are small. An 0805 is 2.0 mm long, and a SOT-23 is 2.9 mm long. The NPN and the PNP transistors have the same package and the same footprint, and only a code on the top tells them apart. A PNP in an NPN position gives a joint that looks correct and a board that does not work. So this experiment sorts the parts first, solders them block by block, and inspects each joint under magnification. The result is a board that is ready for its first power-up in Experiment 21.

Two of the bare boards, black, on a grey surface. The top board shows its front: white silkscreen boxes with the block names TRIGGER COMP, DIVIDER, THRESHOLD COMP, OUTPUT, DISCHARGE, RESET and SR LATCH, the silver pads of each part, the eight round pads at the edges and the notch. The bottom board shows its back: one black area with the vias as small dots, the eight round pads at the edges and the white text 555 v3 · @dnixty.
Figure 20-1: Two of the five boards as they came from the factory, front and back. The back carries the version of the design, “555 v3”. Click to enlarge.

Resources

You will need

The parts of the board

Figure 20-1 shows the bare board. The table gives the parts that go on it, with the code that each part carries and its designators on the silkscreen.

QuantityPartCode on the partDesignators
16NPN transistor, MMBT3904, SOT-231AMQ1 to Q4, Q12 to Q23
7PNP transistor, MMBT3906, SOT-232AQ5 to Q11
84.7 kΩ resistor, 0805often 4701R1 to R7, R21
1010 kΩ resistor, 0805often 1002R8 to R11, R14, R15, R17 to R20
21 kΩ resistor, 0805often 1001R12, R13
1100 kΩ resistor, 0805often 1003R16
10.1 µF capacitor, 0805NoneC1
11×8 pin header, cut into eight single pinsNoneJ1

Many 1 % resistors carry a code of four digits: three digits of value and the number of zeros after them. So 4701 is 470 with one zero, 4.7 kΩ, and 1002 is 100 with two zeros, 10 kΩ. Some resistors carry no code at all. Keep each value in its own place from its packet to the board, because you cannot read the value of a resistor without a code. Ceramic capacitors of this size usually carry no code. The five test pads, TP1 to TP5, get no part. They stay bare copper for the probe of Experiment 21.

Sort the transistors

Of all the parts in the table, the transistors need the most care. Figure 20-2 shows why: the NPN and the PNP are the same package, with the same three legs in the same places.

Two SOT-23 transistors, top view, to scale with a 1 millimetre scale bar. Each is a dark rectangle with two legs on the left, B for pin 1 and E for pin 2, and one leg on the right, C for pin 3. Left: NPN, MMBT3904, with the code 1AM on its body, 16 parts, Q1 to Q4 and Q12 to Q23. Right: PNP, MMBT3906, with the code 2A on its body, 7 parts, Q5 to Q11. Apart from the code the two drawings are the same.
Figure 20-2: The two transistors of the board, to scale. Only the code on the top tells the NPN from the PNP.

Once a transistor is soldered, flux often covers its code, and a wrong transistor looks the same as a correct one. A wrong transistor gives no short and no bad joint. It gives a board that does not work, with no visible fault. So sort the transistors before you switch on the iron.

  1. Draw a sort sheet. On a sheet of paper, write a name for each kind of part: NPN, PNP, 4K7, 10K, 1K, 100K and 0.1 µF.
  2. Read each transistor. Under the microscope, read the code of each transistor and put it on its name: 16 with 1AM on NPN, 7 with 2A on PNP.
  3. Put the other parts on their names. Put each resistor on its value from its packet, and C1 on its name. Cut the header into eight single pins.
  4. Count. Count each group against the table. When each count is correct, the sort is complete.
A white sheet of paper with names written in pencil and the parts laid on them: 16 small black transistors over NPN, 7 over PNP, 8 resistors over 4K7, 10 over 10K, one over 100K, two over 1K, one capacitor over 0.1uF, and eight single header pins, each with its black plastic spacer.
Figure 20-3: The sort sheet of the board of this course: each part on its name, and the header cut into eight single pins. Click to enlarge.

Prepare the bench

The parts are sorted. Before the first joint, set up the bench for small parts.

Solder the parts

With the bench ready, solder one block at a time, in the boxes of the silkscreen. Start with the divider, then the two comparators and the latch, then the output and discharge transistors, and the reset block last. In each block, solder the resistors first and then the transistors. A board that you stop on halfway then shows which block comes next. The board of this course was soldered by hand with an iron, each joint in turn. Figure 20-4 shows the four steps for an 0805.

Four panels, side view of an 0805 resistor on its two copper pads on the board. Panel 1, put a bead on one pad: the iron, labelled, melts a small dome of solder on the left pad. Panel 2, slide the part into the bead: a tweezer jaw, labelled, holds the part flat on both pads, a green arrow points left, and the iron keeps the bead at the left end molten around the end cap. Panel 3, solder the other end: the left end has only a small tack of solder; the iron and the solder wire, labelled, make a concave fillet at the right end. Panel 4, add solder to the first end: the iron and the wire at the left end, and both ends now have a concave fillet from the pad up the end cap.
Figure 20-4: An 0805 soldered by hand, side view. The numbers match the steps for an 0805. Click to enlarge.
  1. Apply flux. Before you start a block, put flux from the pen on its pads.
  2. An 0805, step 1. Put a small bead of solder on one of its two pads. Leave the other pad bare.
  3. An 0805, step 2. Hold the part with the tweezers. Melt the bead with the iron and push the end of the part into it. Hold the part flat on both pads until the solder is solid.
  4. An 0805, steps 3 and 4. Solder the other end with the iron and the solder wire. Then melt the first end again, with a little more solder, so that it also has a full fillet.
  5. A SOT-23. Read its code again against the designator on the silkscreen: 1AM for Q1 to Q4 and Q12 to Q23, 2A for Q5 to Q11. Put a bead on the pad of pin 3, the leg that is alone on its side, and solder that leg as in step 2. Check that the part lies flat on all three pads. Then solder pins 1 and 2.
  6. Check the block. When a block is complete, look at each joint under the microscope before you start the next block.

A SOT-23 has two legs on one side and one on the other, so it fits its footprint in one way only. The kind of transistor is the only thing that can be wrong, and the code check at each transistor finds it.

Microscope photo of the top of the soldered board: the trigger comparator on the left, the divider R1 to R3 in the middle and the threshold comparator on the right. Each transistor and resistor sits flat on its pads, with shiny solder at each leg and each end. The codes on the transistors read 1AM on the NPN transistors and 2A on the PNP transistors. A glossy layer of flux covers the board around the joints.
Figure 20-5: The two comparators and the divider of the board of this course after soldering, under the microscope. The codes read 1AM on the NPN transistors and 2A on the PNP transistors. The gloss is flux. Click to enlarge.

The eight pins

The surface-mount parts are on the board. The pins of J1 go on last. They are the only through-hole joints, and they need more heat than the small parts. Until they are on, the board lies flat under the microscope, and nothing tall can catch a sleeve.

  1. Put a pin in pad 1. Push the short end of a single pin through pad 1 from the front, with the plastic spacer on the front.
  2. Hold it upright. Hold the pin at 90° to the board, with a little adhesive putty or with the tweezers.
  3. Solder it on the back. Heat the pad and the pin together, and feed solder until it fills the hole and forms a cone around the pin.
  4. Do the same for pins 2 to 8. The pads stand four on each side, so each pin is soldered alone.

The pins point up from the front, so the socket of a jumper lead goes onto each pin from above. In Experiment 21, eight jumper leads connect each pin to the row of the breadboard where the same pin of the NE555 sat in Experiment 1.

Inspect each joint

All the parts are now on the board. Before any voltage, inspect the board under the microscope, one joint at a time. A fault that you find now costs a minute with the iron. In Experiment 21, the same fault can stop the timer or trip the current limit of the supply. Then you must find it on a board with voltage on it. Figure 20-6 shows the four things to look for at each joint.

Four panels, each a view of 0805 parts on their copper pads from above and in front, as in a stereo microscope. Panel 1, good: a shiny, concave fillet of solder at each end, from the outer edge of the pad up the metal end cap. Panel 2, cold joint: the left end has a good fillet; at the right end a dull, grainy lump of solder sits on the end cap and the copper of the pad shows around it, with a green arrow on the lump. Panel 3, bridge: two 0805s side by side, one behind the other; one mass of solder covers both right pads and the board between them, with a green arrow on the solder between the pads. Panel 4, tombstone: the part stands up on its right end, soldered to the right pad, and a green arrow points at the bare left pad.
Figure 20-6: The four things to look for at each joint of an 0805. Click to enlarge.
FaultWhat you seeCorrection
Cold jointA dull lump of solder that does not flow onto the padApply flux and melt the joint again until the solder flows into a fillet
BridgeSolder that joins two pads or two legsPut solder wick on the bridge and press it with the iron. The wick takes up the solder.
TombstoneAn 0805 that stands on one end, with the other end upMelt the end that is down, push the part flat, and solder the other end
Wrong transistor2A where 1AM must be, or the oppositeRead each code one last time against the table. Remove a wrong part with the iron and solder the correct part.

Under the microscope, the board looks glossy, because the no-clean flux stays on it. Figure 20-7 shows this. The residue does no electrical harm. Isopropyl alcohol and a soft brush remove it, after the first power-up. Do not let the gloss hide a joint: if you cannot see a joint clearly, melt it again.

Microscope photo of the whole soldered board, turned on its side. Every transistor and resistor sits flat on its pads, in the boxes of the silkscreen. The codes 1AM and 2A are readable on the transistors. The five round test pads in the middle are bare and shiny. Flux gloss covers much of the board.
Figure 20-7: The board of this course after the inspection, under the microscope. All 45 surface-mount parts are flat on their pads, with no bridge and no tombstone, and each code matches its designator. The test pads in the middle are bare. Click to enlarge.

Heat and fumes

The tip of the iron is hot enough to burn skin and to start a fire in paper. Put the iron in its stand after each joint. Keep isopropyl alcohol, paper and batteries away from the iron. Use the fume extractor, and do not breathe the smoke of the flux.

The other method: solder paste and a hot plate

A hot plate can solder all the surface-mount parts at once. You put a small amount of solder paste on each pad, from a syringe or through a stencil, and put each part on its pads. Then the board goes on a hot plate at the temperature that the paste needs. When the paste melts, the liquid solder pulls each part to the middle of its pads. Read each code before you put the transistor on the paste, as for the iron. The pins of J1 still go on last, with the iron. The board of this course was soldered with the iron only.

Quick check

Three questions on this experiment. They ask how to tell the transistors apart, why the pins go on last, and what a good joint looks like.

1. What tells an MMBT3904 from an MMBT3906 on this board?

The two have the same SOT-23 package and the same footprint. Only the code on the top is different, so sort them by the code before you solder, and read the code again at each position.

2. Why do the eight pins go on after the surface-mount parts?

A through-hole pin needs more heat than an 0805. A board with no tall parts lies flat under the microscope, and nothing on it can catch a sleeve.

3. How does a good joint of an 0805 look under the microscope?

Solder that wets flows onto the pad and the end cap, and makes a concave fillet. A dull lump that does not reach the pad is a cold joint. Solder across two pads is a bridge.

Before moving on

The board has its 45 surface-mount parts and its eight pins. Each joint is a fillet, with no bridge, no tombstone and no wrong transistor. That is the seventh step from the breadboard to the board. The board has had no voltage yet. Experiment 21: First Power-Up applies the first voltage, with a current limit, and puts the board in place of the NE555.

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