All but the simplest boards today utilize multiple layers to accommodate increasingly dense interconnections or smaller board areas. Four-layer PCBs are exceedingly popular and pose little additional processing difficulties for manufacturers; further layer counts are also easily achievable. However, the presence of multiple conductor layers means manufacturers must be aware of these layers’ linear and angular translation to ensure everything lines up correctly – even minor deviations during manufacturing can undermine entire production lots. To counter the effects of PCB misregistration, manufacturers utilize various analysis techniques and careful lamination and drilling processes to ensure a board is assembly-ready.
| How PCB Misregistration Affects In-Circuit Performance | |
|---|---|
| Shorts/Opens | Significant misregistration can cause catastrophic failure by shorting different nets or producing opens that lead to lot scrappage. |
| Clearance | Clearance to nearby vias, board edges, and other elements provides mechanical integrity. |
| EMI | Misregistration can reduce the space between susceptible traces and nearby aggressive lines, making it easier for them to couple and create issues during functional testing. |
PCB Misregistration: Causes and Corrections
PCB misregistration is the out-of-plane alignment of the board layers relative to the ideal artwork, as indicated by the maximum observed variance of pad center lines within a plated through-hole (PTH). Misregistration is not necessarily a defect (though perhaps a cause for concern to the manufacturer) when it is within acceptable variance. However, the greater tendency of high-density interconnect (HDI) layouts makes for increasingly slim registration tolerances. Even a tiny translational shift between layers can be enough to cause a trace routed in the design to pass over split reference planes (significantly increasing the electromagnetic interference of the signal) or lead to shorts/opens for vias that lead to lot scrappage. Additionally, the effects of misregistration amplify as HDI layouts move to smaller microvia structures to support the denser layout.
Since misregistration occurs during fabrication, it is relatively easy to diagnose and correct. The issue occurs toward the beginning of the manufacturing process, which limits the cost overrun of material/energy/labor inputs (assuming a low-volume production lot). Misregistration is most often attributable to the following manufacturing oversights:
- Board material dimensional instability – Considerable anisotropy in board materials can produce different stress and strain values in various board directions; under thermal expansion, the board dimensions don’t increase at the same rate.
- Registration of the artwork – The artwork doesn’t reference the same origin across layers, or the manufacturer fails to scale the artwork appropriately (boards shrink post-lamination).
- Careless punching or unaligned laminate plates – Technicians’ failure to recognize pre-lamination misregistration will produce defects no matter how sophisticated the design and manufacturing process is.
Historically, manufacturers diagnosed misregistration by optically examining board cross-sections through the middle of a plated through-hole (PTH). However, this destructive analysis method offers slow throughput, making it infeasible for higher-volume lots. X-ray inspection is a much quicker process that allows manufacturers to scale analysis to higher-volume production lots without sacrificing robustness. By comparing the difference between centerlines of PTH terminal pads, inspection technicians can rapidly quantify the amount of misregistration present during lamination.
How Lamination and Material Selection Impact Misregistration
Different manufacturing processes for lamination exist within the PCB fabrication industry, which can contribute meaningfully to board misregistration:
- Standard hydraulic lamination – Heating elements like steam, hot oil, or electrical resistance fuse the board materials and enable resin flow and encapsulation of the circuitry.
- Vacuum-assisted hydraulic lamination (VAHL) – A pre-press vacuum cycle pulls out moisture and volatiles before lamination, reducing the pressure necessary to achieve resin flow conditions while improving registration. It is the most widely used lamination method and can reduce overall misregistration by 1~2 mils board-wide.
- Autoclave lamination – Similar to VAHL, autoclave lamination uses a sealed vacuum chamber and a high-pressure heated gas. While this produces a uniform hydrostatic pressure during lamination, the process is more time-intensive during the vacuum and heating cycles, making it unsuitable for larger volume production lots.
The lamination method available will depend on the manufacturers’ processes (VAHL is the most common among the three listed), yet material selection for the stackup will also be a factor to consider. PCB laminates come in many styles and variations for various circuit applications, and the best material properties for high-speed aren’t necessarily the same for a high-power board. For example, manufacturers may recommend a higher resin-content laminate to reduce the chance of drill bit deflection and wander during drilling. However, this can affect the performance of high-speed signals, which now encounter significant impedance mismatches and energy loss over the transmission length. When designing the stackup pre-layout, it’s worth consulting with a manufacturer to determine the tradeoffs between board performance and manufacturing reliability/yield for an optimal DFM solution.
Your Contract Manufacturer Hits the Mark for PCB Quality
PCB misregistration has dire consequences for board performance and reliability: even slight deviations from the theoretical position can lead to board scrap or deviations during functional testing that can be difficult to diagnose. While most misregistration concerns will fall to the manufacturer, designers can work with an experienced CM while devising the stackup to synergize their efforts pre-layout and improve yield. At VSE, our engineers are committed to building electronics for our customers, including a commitment to manufacturing quality through our trained staff and certified facilities. We’ve been realizing life-saving and life-changing products for over forty years with our valued manufacturing partners.
