Designing a good Printed Circuit Board (PCB) takes more than selecting drills and routing traces per electrical schematics. PCB and assembly manufacturability also needs to be considered. PCB manufacturability and long-term reliability is affected by many variables – some more obvious and others, yet equally important. This post will examine ten general design rules and questions to consider when designing reliable and manufacturable PCBs and how PalPilot’s Field Application Engineers (FAE) can help.
1. Material Selection: One of the first decisions is what type of material will be needed for the PCB. Will the board need a high-Tg FR4 for lead-free assembly and dimensional stability (ex: IT-180A or S1000-2), mid loss for mid-range digital applications (Ex: FR408HR, TU-872 SLK), low-loss/ultra-low-loss for high-speed digital, optical, or RF applications (Ex: Meg6, EM-888) or halogen-free for environmentally-friendly and EU end-users (Ex: EM-370 or TU-862 HF)? PalPilot FAEs can even help select the best material for flex PCBs or a combination of materials for rigid-flex PCBs.
2. Impedance: Will the board need impedance control for timing on clock or transmission lines, USB, DDR, or other unique impedance-controlled devices? PalPilot FAEs can model impedance using a 2D field solver based on the properties of the PCB material selected.
3. Thru-Vias vs. HDI Technology: Early in the design stages, it helps to determine what kind of via structures will be needed to rout out the board. Will thru hole vias be adequate or will lasered microvias need to be used for fine-pitch devices (0.5mm & 0.4mm)? Will blind or buried via spans be needed for isolation or real estate utilization on the backside of the PCB? Consulting a PalPilot FAE to discuss what the design intent is before starting to rout blind and buried structures is critical to a manufacturable PCB with robust long-term reliability.
4. Drills and Padstacks: Drills and their associated pads are critical to both manufacturability and long-term board reliability. Vias sizes, aspect ratio (board thickness relative to min drill size), and annular ring requirements directly impact PCB manufacturability. Drill-to-copper is a critical dimension for long-term reliability that is often overlooked. Drills that are too close to copper features can cause field failures such as CAF (Conductive Anodic Filament). A good design is the best way to prevent CAF failures. For fine-pitch devices or densely-routed boards, consult an FAE for drills, padstacks, and drill-to-copper design advice.
5. Traces and Spaces: Etching of fine line traces and spaces (even isolated to only under one QFN or fine-pitch device) can quickly decrease PCB manufacturability. The tightest etch constraint on a PCB determines the etch capability of the entire design. When possible, use the largest traces and spaces available. Consult a PalPilot FAE for help with 0.65mm and smaller devices and for guidelines on running two-track between pins on 1mm devices.
6. Surface Finish Selection: PalPilot recommends working with both the PCB supplier and the assembler on the preferred finish. Usually, the end-use application and assembly method will determine the finish needed. Common finishes are HASL (lead-free or leaded), Imm Ag, ENIG, OSP, and Electrolytic Ni/Au. Edge finger connectors usually require a dual finish for the solderable pads in addition to Ni/Au finish on the edge fingers. Contact a PalPilot FAE for design options with segmented or recessed “hot swap” style edge fingers.
7. Via Treatments: Whether you call it Via-In-Pad (VIP), Via-In-Pad Plated Over (VIPPO), or Plated Over-Filled Vias (POFV), via-in-pad technology usually require the vias to be resin-filled with a copper cap Cu or microvias to be copper-filled (plated shut with copper) to prevent solder paste loss and maintain a planar surface during assembly or aid in testing after assembly. Will isolated vias need to be plugged and tented with soldermask to encapsulate and protect the hole walls for robust long-term reliability? Often an afterthought for designers, how 2D soldermask data is applied to 3D holes during PCB fabrication can have a large impact on the long-term reliability. Contact a PalPilot FAE to explore the various via treatment options available.
8. Mechanical Features: What kind of mechanical features will need to be added and dimensioned on a fab drawing? Will backdrills be needed to reduce unwanted signal antennas on high-speed signals? Will edge-milling be used to thin board edges down for mechanical card guides or will edge finger contacts need bevel for ease of insertion? PalPilot FAEs can help with the feasibility of various mechanical features as well as give recommendations on manufacturable dimensions and tolerances on fab drawings.
9. Thieving: Most designers don’t usually consider adding thieving beyond the necessary traces, pads, and planes but thieving can be one of the most important tools for increasing PCB manufacturability and reliability. Thieving on external—and plated internal—layers help to distribute the plating evenly. Boards with no external thieving can overplate isolated traces and holes while simultaneously plating less under dense BGAs or areas with closely-routed traces—all of which etch at different rates. Most PCB shops also have standard thieving rules and patterns which can be allowed at vendor’s option by fab note or readme doc on a part-by-part basis. Designers also don’t necessarily realize the impact partial planes and large areas void of copper on internal layers have on the reliability of the PCB. Areas where copper-fill is absent in the same Z-axis location on multiple layers are prone to laminate voids especially when low-resin-content prepregs and/or heavy Cu weights (2 oz or greater) are used. Thieving on internal layers can help balance localized resin fill and help ensure good PCB reliability.
10. Interactions: Are all the interactions of multiple PCB complexities being realized during the design stages? While each individual constraint may not be a manufacturing concern, combinations of complexities can quickly take a manufacturable board to something that is extremely hard to produce reliably. For example: resin-filled vias, high aspect ratio drills, fine external traces and spaces with blind vias and electrolytic Ni/Au finish are all fine individually but when used in combination, make a PCB difficult to manufacture. Probably the best service a PalPilot FAE can provide is to understand the combination of complexities and help designers navigate through or around them.
*image by Rube Goldberg
PalPilot has value-added FAEs on staff to work closely with designers in all stages to ensure the most reliable and manufacturable product is designed. Contact PalPilot for stackups, impedance modeling, fab drawing reviews, cursory Gerber reviews, and general technical PCB manufacturability inquiries. Teresa Shelton is a Senior Field Application Engineer at PalPilot with 21 years combined PCB manufacturing & engineering experience and an educational background in technical writing.