Innovating with LPM: Design Tips for Engineers

Sacha Fabien
Mar 9
4 min read

Updated: May 21

The integration of electronics into increasingly dense and miniaturized structures, subjected to extreme operational stresses, is forcing engineers to redefine traditional methods for protecting printed circuit board assemblies (PCBAs). Historically, protecting components from external hazards relied on two major technologies: thermoset resin potting and high-pressure injection molding. Traditional potting comes with significant production constraints, including curing times ranging from 24 to 72 hours and the need for tedious manual masking to protect connectors. In contrast, high-pressure molding subjects parts to injection pressures of 100 to 150 bar and melt temperatures reaching 320 °C, which damage or tear the solder joints of surface-mount components (SMDs).

Low-pressure molding (LPM) solves this problem by offering a gentle and rapid alternative. By operating at very moderate injection pressures, typically between 1.5 and 40 bar, and at process temperatures of 180 to 240°C, LPM allows fragile assemblies to be encapsulated directly without inducing harmful physical stresses.

One of the key innovations of LPM lies in the concept of "skylining." Unlike encapsulation, which requires a permanent external housing to contain the liquid resin, LPM uses the cavity of a temporary metal mold to perfectly conform to the electronic board. The injected material directly forms the device’s outer sealed shell while incorporating secondary mechanical functions such as strain relief for cables. This reduction in unnecessary physical parts lowers the BOM and makes the final product lighter.

Geometric Design Tips and Thermomechanical Tolerances

The success of a low-pressure overmolding project depends on rigorous CAD modeling that accounts for the rheology and physical shrinkage properties specific to hot-melt resins. Unlike conventional injection-molded plastics, the very low viscosity of polyamides alters the filling kinetics and heat transfer upon contact with the mold.

Wall Thicknesses and Volumetric Shrinkage Kinetics

The thickness of the overmolded shell should preferably be maintained within a range of 1.5 mm to 6.0 mm. A wall thickness of less than 1.5 mm carries a high risk of premature solidification during injection, as the hot material rapidly loses heat upon contact with the aluminum mold, which acts as a heat sink. This premature solidification prevents complete filling of the cavity, resulting in material shortages or incomplete injections. Conversely, any local section with a thickness exceeding 6.0 mm excessively slows the cooling of the thermoplastic’s core. This thermal inertia generates significant internal tensile stresses as the material contracts, promoting the formation of surface sink marks or structural micro-voids.

To achieve greater thicknesses of up to 12.0 mm without causing sink marks, the engineer must design a two-stage molding process to split the heat input.

It is also imperative to ensure symmetrical distribution of the polymer around the printed circuit board. The overmolding thickness ratio between the top and bottom surfaces of the PCBA must not exceed 2.5:1. A greater imbalance creates a differential in thermomechanical stresses during the cooling phase, causing board warping and shear stresses that can break delicate components.

Draft angles, fillet radius, and component spacing

To ensure the assembly can be removed from the mold without mechanical deformation, the engineer must incorporate systematic draft angles of 1.5 to 3 degrees on the part, allowing for easy removal. Without this draft angle, the force required to overcome friction during opening can induce local tensile stresses greater than the polymer’s adhesion strength to the circuit, leading to local delamination of the Technomelt.

The presence of sharp internal or external corners on the part must be avoided, as these areas act as geometric stress concentrators. Under the effect of operating temperature cycles, these points of thermal and mechanical concentration initiate structural cracks. Corners must be rounded with a radius between 1/4 and 1/8 of the nominal wall thickness. Similarly, abrupt changes in thickness must be replaced by gradual transitions to maintain smooth flow and uniform cooling of the polymer.

The spatial distribution of components on the printed circuit board also dictates the quality of the fill. The clearance between components taller than 3.0 mm must be at least 1.0 mm. A narrower channel restricts fluid flow, causing local pressure drops that trap air and create cold solder joints with low mechanical strength.

Integration of Preforms

Designers can integrate alignment bosses or threaded metal spacers directly onto the printed circuit board or onto a preliminary subassembly before performing the final low-pressure overmolding. These elements serve as rigid mechanical guides to perfectly secure the board in the mold, preventing it from moving or vibrating during injection.

Strategic Recommendations for Engineers

The successful implementation of low-pressure overmolding relies on a structured engineering approach prior to manufacturing:

Characterization of the final environment: First, define thermal stresses, UV exposure, and contact with chemicals or solvents to select the appropriate thermoplastic resin formulation.
Strict adherence to wall thickness guidelines: Design part walls within a thickness range of 1.5 mm to 6.0 mm to eliminate the risk of premature solidification or surface sink marks, and maintain a symmetry ratio of less than 2.5:1 to prevent warping of the circuit.
Incorporation of draft angles and fillets: Include draft angles of at least 1.5 to 3 degrees on the part to ensure safe demolding, and systematically round internal and external corners with fillets equivalent to 1/4 or 1/8 of the nominal wall thickness to eliminate concentrations of mechanical stress.