Regulator Assembly, Soldering, and Inspection

Assembly-critical warnings, soldering profiles, inspection points and a pre-power checklist for the Board B linear regulator stage - including the negative-rail capacitor polarity trap.

Assembly procedure for the Board B linear regulator stage: U6 (L7812CD2T-TR, C13456), U7 (L7805ABD2T-TR, C86206) and U8 (CJ7912, C94173), plus their surrounding capacitors.

Two of the traps below have destroyed hardware in other people's projects and will destroy it here. Read them before touching a soldering iron.

Trap 1: CJ7912 pin 1 is GND, not INPUT

Prevention:

• Label the pins explicitly on the schematic: write "Pin 1 = GND", not just "Pin 1".

• Keep a reference-designator-to-pin table next to the symbol.

• Use a distinct schematic symbol for negative regulators so the eye does not auto-complete the 78xx layout.

Note that the tab is the pin 2 position, so the U8 tab is at -13.5V, not at ground and not at the -12V output. See Linear Regulator PCB Layout and Thermal Design.

Trap 2: negative-rail electrolytic polarity is reversed

Correct polarity for the negative rail:

C24 (input, 470µF 35V):
         ┌─────────┐
  GND ───┤+       -├─── -13.5V
         └─────────┘

C25 (output, 470µF 35V):
         ┌─────────┐
  GND ───┤+       -├─── -12V
         └─────────┘

Installing one of these backwards causes immediate capacitor failure, possibly a burst electrolytic, regulator damage, and board contamination.

Prevention:

1. Put clear polarity markings on the silkscreen, and make the negative-rail parts visually distinct from the positive-rail parts.

2. Carry an assembly note in the BOM for C24 and C25.

3. Check polarity with a DMM before applying power.

The ceramics on the same nets - C16 (470 nF, input) and C19 (100 nF, output) - are non-polarized, so they carry no orientation risk. The distances still matter: keep the ceramic within 5 mm of the pin it decouples and the electrolytic within 10 mm.

Trap 3: TVS3 orientation on the -12V rail

TVS3 (SMAJ15A, unidirectional) sits on the -12V output. Its orientation is locked by dec-tvs3-orientation-lock in scripts/schgen/decisions.json:

• Cathode to GND (0V)

• Anode to the -12V rail

Fit it the other way round and the part is forward-biased at -12V - a dead short through one diode drop across the rail.

Soldering

Reflow (recommended for production)

• Peak temperature: 260 °C maximum

• Time above 220 °C: 60-90 seconds

• Solder paste: SAC305 or a comparable lead-free alloy

• Stencil thickness: 0.125 mm (5 mil)

Hand soldering

• Iron temperature: 350 °C maximum

• U8 (CJ7912): solder pin 1 (GND) first as a mechanical reference, then pin 2 (INPUT), then the tab.

• U6, U7 (D2PAK): solder the two leads first, then the tab.

• Apply solder to the tab from the component side.

• Get real thermal contact between the tab and its pad - the tab is the entire heat path on these packages, and a partly-wetted tab turns a working thermal budget into a failing one.

Inspection

After assembly, check:

1. Electrolytic capacitor polarity - critical on the negative rail; verify the positive terminal is at GND for C24 and C25.

2. Pin solder joints - smooth fillet, no bridges.

3. Tab solder joint - full coverage, no visible voids.

4. Component alignment - centred on the pads.

5. No cold joints - a good joint is shiny and smooth.

6. Thermal via filling - solder should wick into the vias.

Pre-power checklist

• C24 polarity: positive terminal to GND, negative terminal to -13.5V

• C25 polarity: positive terminal to GND, negative terminal to -12V

• C20, C21 polarity: positive terminal to the positive rail (the normal case)

• U8 orientation: pin 1 to GND, pin 2 (tab) to -13.5V, pin 3 to the -12V output

• TVS3 orientation: cathode to GND, anode to -12V

• Visual inspection complete

• Continuity test: GND to U8 pin 1

• Resistance test: no short between -12V and GND

• Resistance test: no short between -13.5V and GND

• Resistance test: no short between +12V and GND, or +5V and GND

Common mistakes

Reversed electrolytic capacitors

Problem: installing a negative-rail electrolytic (C24 or C25) with the polarity that would be correct on a positive rail. Consequence: capacitor failure or rupture, regulator damage. Prevention: large silkscreen warnings, DMM check before power-up, and the checklist above.

Confusing positive and negative regulator pinouts

Problem: assuming pin 1 is the INPUT because that is how the 78xx positives are laid out. Consequence: wrong connections, no output. Prevention: see Trap 1.

Insufficient dropout voltage

Problem: feeding the regulator an input that is not far enough from the output - for example -12V in for a -12V out. Consequence: no regulation; the output simply follows the input. Prevention: the magnitude of VIN must exceed the magnitude of VOUT by the dropout voltage plus margin. fact-cj7912-vi-guarantee-band records that the CJ7912 output band is only guaranteed for an input between -14.5V and -27V, and the design runs it at -13.5V - already outside the guaranteed band, which is why fact-cj7912-rail-headroom carries a NEEDS BENCH verdict. Confirm the DC-DC setpoint on the bench and treat a marginal -12V as a headroom problem before suspecting the regulator.

Shared copper pour between rails

Problem: letting the -12V copper touch the +12V copper. Consequence: a direct short between rails, catastrophic. Prevention: a separate pour per rail, at least 2 mm of clearance between pours at different potentials, and a DRC run to prove it. The U8 tab island at -13.5V is the easiest one to get wrong, because it looks like a ground pour and is not.

Related

• Linear Regulator PCB Layout and Thermal Design

• Power Rail Bench Test Procedure

• Board B - synth power conversion