FPC Assembly: SMT Process Requirements on Flexible Boards

Placing surface mount devices on a flexible circuit is not the same operation as placing them on a rigid board, and treating them as equivalent is how a flexible assembly acquires defects that only appear after bending. The differences are mechanical before they are electrical: the substrate moves, it is not flat, and it cannot be clamped the way a rigid panel can. FPC assembly therefore depends on how the part is held as much as on how it is soldered.

Conventional SMT on Flexible Circuits

The simpler class of work is undemanding. Component counts are low, the range of devices is dominated by resistors and capacitors with perhaps the occasional odd-form part, and the placement tolerance required is modest.

The sequence is familiar, with flexible-specific twists at each stage.

Paste is printed with the FPC located by its outline on a dedicated printing pallet. A small semi-automatic printer is typical; manual printing is possible but produces visibly worse quality, because the operator cannot apply the same pressure and speed consistently across a moving substrate.

Placement is often done by hand, with manual pick-and-place equipment used for the few components whose positions demand more accuracy.

Soldering is normally reflow. In specific cases spot soldering is used instead, but reflow through a controlled profile remains the default.

<img src="https://www.gopcba.com/wp-content/uploads/2026/05/SPI.jpg" alt="FPC assembly on a carrier pallet during solder paste printing” />

High-Density Assembly

When the design is dense, the process requirements change sharply.

The FPC must carry fiducial marks for machine vision, and the circuit itself must be flat — a requirement that is not automatic on a flexible substrate and that has to be designed and handled for. Holding the part in position is difficult, and consistency between units is hard to maintain in volume production, which places high demands on the equipment and on process control. Both paste printing and placement become harder to control than the equivalent operations on a rigid board.

The reason is that the flexible circuit and its fixture never form a perfectly rigid unit. Small gaps between the FPC and the pallet are unavoidable, and they are the fundamental difference from a rigid substrate. Those gaps change the effective printing height and the support under a component during placement, so equipment parameters have a direct and significant effect on print quality, placement accuracy and the resulting solder joints.

Fixturing: The Deciding Factor

The circuit is fixed to a carrier pallet from printing through placement and reflow, and never released in between. The pallet has to be dimensionally stable under heat, so a material with a low coefficient of thermal expansion is used.

There are two established fixing methods, and the choice follows the finest lead pitch on the assembly.

Method A is used where the finest QFP lead pitch is 0.65 millimetres or greater. The pallet is fitted over a locating template, the FPC is secured to the pallet with thin high-temperature tape, and the pallet is then separated from the template for printing. The tape specification matters: adhesive strength has to be sufficient to hold the circuit, the tape must peel easily after reflow, and it must leave no adhesive residue on the flexible circuit.

Method B is used where the lead pitch is finer than 0.65 millimetres. Here the pallet is custom-made and must deform only marginally after repeated thermal excursions. Location is provided by T-shaped locating pins set slightly taller than the flexible circuit, which hold the part positively without relying on adhesive.

Paste Printing on an Uneven Surface

The pallet carrying an FPC with locating tape is not a flat, uniform surface. Component heights and tape thickness both produce steps, which is why an elastic squeegee blade is used rather than a rigid metal one: it conforms to the surface and maintains contact where a rigid blade would ride over the high points and starve the low ones.

Paste composition has a strong influence on the printed result, so the material has to be selected against the printing behaviour rather than only against the reflow requirement. Where the finer-pitch method is used, the printing stencil itself requires special treatment to achieve acceptable deposit at the smaller apertures, the same area-ratio problem that governs any fine-feature printing and that is described in this note on solder paste mask openings.

Placement Equipment

The printing machine should carry an optical alignment system. Without vision alignment, the cumulative error between the FPC, its fixture and the stencil is not corrected, and the effect on solder quality is significant.

Placement equipment faces the same issue from the other direction: the part is fixed on the pallet, but the small, variable gap between the flexible circuit and the pallet means the machine’s height reference is less reliable than it is on a rigid panel. Nozzle contact force, placement height and dwell all interact with that uncertainty, so process control is stricter on flexible assemblies than on rigid ones for the same component mix.

Before the Line: Dry the Circuit

Flexible materials absorb moisture. Assembling a circuit that has absorbed water vapour drives that moisture out rapidly during reflow, and the resulting pressure can cause blistering and delamination between the layers.

The countermeasure is a bake before assembly, which is why drying is treated as a process step rather than a precaution. The temperature and duration have to suit the material, and the drying interval has to be short enough that the circuit does not reabsorb moisture before it reaches the printer.

Once boards are built, the reflow profile that holds all of this together is monitored rather than assumed, as described in this discussion of reflow profile measurement and control. On flexible circuits the tolerance for a profile that is too aggressive is smaller, because the substrate cannot restrain the assembly the way a rigid laminate does.

The mechanical design of the flexible circuit itself sets constraints that no process adjustment can overcome. Bend radii, stiffener placement and the copper type used in the flex layers all determine whether the finished assembly survives service, as set out in this guide to flex PCB design and bend radius.

What Goes Wrong

The defects on flexible assemblies cluster around the fixture rather than the solder. A rigid squeegee bridging the low areas leaves a thin deposit, which becomes a weak fillet that passes inspection and fails in bending. A component placed over an unsupported region drifts during reflow and lands outside its pads. A circuit that was not dried blisters between layers, and the damage is invisible from the outside. Because the substrate is compliant, inspection is harder as well: the same assembly can look perfect flat and reveal a cracked joint once it is formed into its service position.

FAQ

Why is manual paste printing worse on a flexible circuit? Because the surface is uneven and compliant. Consistent speed and pressure matter more than usual, and an operator cannot reproduce them as reliably as a machine.

Why is an elastic squeegee used? The pallet, tape and components create height differences across the printing area. A blade that conforms keeps contact with the stencil; a rigid blade bridges the low areas and leaves them short of paste.

How is the fixing method chosen? By the finest lead pitch on the assembly. Coarser pitches tolerate the tape-based method, while pitches below 0.65 millimetres need a custom pallet with locating pins that holds the circuit without adhesive.