1. First: Why Rigid-Flex Soldering Is Tricky

• Rigid FR-4: Handles heat well (can take 260°C+), but it’s stiff—components can’t shift, so alignment matters before soldering.
• Flexible PI: Melts or warps at 250°C if heated too long. It also bends easily, so SMDs can slide out of place while you solder.
• The transition zone: Where FR-4 meets PI is extra sensitive. Heat here can weaken the bond between layers, leading to delamination later.

2. Prep First: Get the Board & Tools Ready (5-Minute Setup)

• Secure the board: Tape the flex section to a flat surface (use heat-resistant Kapton tape, not regular tape—it melts). This stops it from bending while you solder. For the rigid section, clamp it gently to your workbench (avoid clamping components or traces).
• Pick the right solder & flux: Use “no-clean” solder with 63/37 tin-lead (melts at 183°C, lower than PI’s danger zone). Add a tiny dot of rosin flux to each pad—flux helps solder flow fast, so you spend less time heating.

• Set your iron correctly: Use a 30W soldering iron with a small chisel tip (1mm wide). Set the temperature to 280°C—hot enough to melt solder, but cool enough to avoid burning PI. Test the iron on a scrap PI piece first: if it turns brown, turn it down 10°C.
  

3. Method 1: Soldering SMDs on the Rigid Section (Easy Start)

• Tack one pin first: Place the SMD on the pads (use tweezers to line it up). Touch the iron to one corner pin and a pad for 2 seconds, then add a tiny bit of solder. This “tacks” the component in place so it won’t shift.
• Solder the rest quickly: Go around the other pins one by one. Hold the iron on the pin and pad for 1–2 seconds, then feed solder. Stop as soon as the solder flows into a smooth “hill” on the pad.
• Fix bridges (if they happen): If solder connects two pins (a “bridge”), touch the iron to the bridge while blowing gently—excess solder will melt and pull away.

4. Method 2: Soldering SMDs on the Flex Section (Trickier, But Doable)

• Use “drag soldering” for small parts: For tiny SMDs (like 0402 resistors), add flux to the pads, place the component, then drag the iron tip across both pins quickly (1 second total). The flux will pull the solder into place without extra heat.
• Avoid holding the iron too long: Never leave the iron on a flex pad for more than 2 seconds. If the solder doesn’t flow, lift the iron, add a little more flux, and try again (waiting 5 seconds between tries to let the board cool).
• Cool fast: After soldering, blow gently on the pad for 3 seconds to cool it down—this sets the solder before the flex layer can warp.

5. Method 3: The Transition Zone (Most Critical Spot)

• Heat the rigid side first: If a component spans both layers (e.g., a connector with one end on FR-4, one on PI), solder the FR-4 side first—it can handle more heat.
• Use a heat sink for the flex side: Clip a small metal heat sink (or even a paperclip) to the flex pad’s edge. It draws away excess heat so the PI doesn’t warp.
• Solder in short bursts: Touch the iron for 1 second, lift, wait 3 seconds, and repeat—until the solder flows. Patience beats rushing here.

6. Check for Stability (Make Sure It Holds)

• Tug gently: Use tweezers to pull the SMD slightly. It shouldn’t move—if it does, re-solder with more flux.
• Inspect with a magnifier: Solder joints should be shiny and cover half the pad. Dull or lumpy joints mean cold solder (weak and prone to breaking).
• Bend the flex layer: Fold the flex section back and forth 10 times. If a joint cracks, it’s either too cold or you heated the PI too much—re-solder with faster touches.

Wrapping Up