While designers place components and route circuits on multiple layers, manufacturers have to fuse the disparate layers of the board into a single, continuous product. High heat and pressure cause board laminate material to flow throughout the crevices of the design and encapsulate the board shape; this process is reversible (sometimes destructively so) under certain environmental conditions. Preventing PCB delamination is difficult as manufacturers cannot reactively adjust to delamination, especially once a board is in service. Instead, careful material and process management throughout procurement and manufacturing will limit its occurrence.
| How Manufacturers Limit PCB Delamination | |
|---|---|
| Controlled environment storage | The storage of laminates post-procurement and pre-lamination should limit exposure to extreme temperatures and moisture. |
| Laminate baking | Bake-out eliminates moisture in the laminates by heating them to an elevated temperature and holding them for an extended period. |
| Material selection | Lower resin-content laminates are less susceptible to moisture absorption since most absorption occurs within the epoxy resin. |
| Copper thieving | By removing copper from flooded pour layers, manufacturers ensure a better flow and adhesion of resin during lamination. |
How Trapped Moisture Damages Boards
PCB delamination undoes the fabrication process as heat and moisture separate the board layers. This defect mode affects the shape and form of the board (generally contributing to worse delamination as stress builds in the still-fused sections) and signal propagation as the surrounding atmosphere and dielectric have significantly different electrical properties. For most boards, delamination becomes a much smaller concern in application settings and field deployment after manufacturing (for common PCB materials, instantaneous delamination usually requires temperatures above 250℃). However, the assembly process (wave, reflow, and manual soldering all require elevated temperatures) can introduce delamination defects outright or significantly stress the material to reduce the threshold for future delamination events. Delamination is increasing, especially considering the general switch from eutectic tin-lead solder alloy to higher temperature lead-free solders for RoHS compliance.
Controlling moisture throughout manufacturing is one of the most important avenues for reducing delamination. Critically, this includes baking board materials before lamination and pressing to desiccate the laminates fully. Any remaining volatile liquids present in the laminates during lamination become superheated and gaseous; gas trapped between layers of board material can then build pressure with continued temperature elevation before erupting violently in a process known as outgassing. Outgassing voids sections of the PCB’s materials and poses an issue to board yield, but, more importantly, it can be dangerous to technicians if unaccounted for.
Better Boards? Try Baking
While temperatures and hold times will vary based on material selection, manufacturers generally need to heat and hold board materials between 100℃ to 125℃ in a process known as bake-out to ensure complete elimination of moisture. Additionally, materials and boards should be shipped and stored in controlled temperature and humidity environments to limit exposure to potential wear. Note that prevention of moisture absorption is often more critical to long-term material performance than elimination in pre-fabrication processing; excessive moisture buildup can lead to material failure, even during baking. With suspected or known moisture exposure, manufacturers can weigh the laminate(s) before and after baking against an environmentally-controlled laminate or measure the capacitance change (water acts as a relatively good dielectric at room temperature) to determine the absorption level.
The Interplay of Board Materials and PCB Delamination
Because materials exhibit different absorption rates, laminate selection can also prevent delamination if all other factors related to material selection are equal. Note that for standard fiberglass-weave/epoxy-resin materials, moisture uptake concentrates more within the resin than the glass fibers. Additionally, absorbed water is either bound to polar sites within the epoxy by Van der Waal’s force (displacing the standard hydroxyl group) or free-flowing within cavities or voids of the resin. Resin content will vary between substrates – generally, thicker cores with heavier glass weaves (like 2116, 1652, and 7628) will have a relatively lower resin content of approximately 50%, while more “standard” laminates (like 106 and 1080) will have resin contents closer to 70% – but this change in resin content will also affect signal propagation (and other) properties like the dielectric constant (Dk), dissipation factor (Df), and the material’s general anisotropy.
While designers may wish to select lower resin contents and combat moisture uptake of more resinous laminates, poor lamination due to voids formed by incomplete resin flooding during lamination can also produce delamination results. This delamination vector can also arise from imbalanced copper layers – manufacturers typically employ copper thieving on copper-flooded layers for better lamination results. More exotic materials like polytetrafluoroethylene (PTFE) also exhibit far lower moisture absorption rates than standard FR4s. Assuredly, designers are unlikely to switch from an FR4 substrate to a PTFE just for delamination considerations, as PTFE has many additional manufacturing challenges. However, it’s worth weighing all the options from a design function/performance and manufacturability standpoint.
Your Contract Manufacturer Has Your PCBA Production Covered
PCB delamination is a significant defect mode for board manufacturing, but careful environmental control of materials during storage and processing will limit the potential detriments. Much like for the performance of controlled impedance structures in the stackup, designers and manufacturers will want to discuss material selection and processing that can limit a board’s exposure to moisture. For designers interested in learning more about the environmental effects of PCB materials on service life, VSE can help. Our engineers are committed to building electronics for our customers, including informing them how to optimize material selection. We’ve been realizing life-saving and life-changing products for over forty years with our valued manufacturing partners.
