Completing a Pre-Layout Design Checklist Can Keep Your PCB Design from Crashing

Many PCB design projects stall out or incur significant rework due to problems not caught early. Issues in component selection, PCB stackup configuration, communication with manufacturing, or electrical interfaces often surface late - delaying product release.

A comprehensive checklist validated at the start of board layout can preempt such issues. This article details key pre-layout considerations to prevent scrapped work or costly respins. We will explore critical checkpoints under categories of components, stackup planning, design rules, analysis, and manufacturing handoff through example checklists tailored to both simple and complex boards.

The High Costs of Late Design Changes

While no PCB development cycle proceeds perfectly smoothly, many problems arise from oversights early on. A survey by Valor found that up to 57% of designs need at least one respin - but over half originate from preventable issues in planning or component selection. Each respin can incur 8 weeks production delay and $50,000 in engineering time. Worse yet, hardware delays might cause missing market windows in fast-paced technology sectors.

Catching issues pre-layout with comprehensive checklist reviews limits wasted efforts. Spending more time in design preparation pays dividends later through manufacturing readiness. Let’s explore critical checklist contents that prevent crashing later.

Checklist for Simple PCB Layouts

Even for basic PCBs, planning errors creep in - we will work through a simplified checklist using an IoT sensor hub PCB example:

Function: Wireless connectivity and sensor aggregation

Layer Count: 2 layer

Technology: Low speed digital and power

Quantity: 10,000 units

While straightforward, such designs still mandate checks.

Components Checks

Schematic Capture Complete

• All devices entered and wired without errors

BOM Populated

• Digikey part numbers listed for procurement

Footprints Assigned

• LCSC components have properly mapped packages

Ratings Verified

• All passives, crystals, connectors meet voltage/current needs

Skipping these ensures change orders when parts arrive or footprints misalign during layout.

PCB Stackup Definition

Despite simple 2 layer build, stackup details still matter:

Dielectric Selected

• FR4 material meets cost and performance needs

Copper Weights Chosen

• 1 oz copper offers adequate current capacity

Finishes Declared

• Immersion silver plating for good solderability

Thickness Defined

• 1.6 mm board handles mechanical rigidity

Stackup misconfigurations prompt expensive panel revisions if missed pre-layout.

Design Rules Established

Layout software needs defined constraints for success:

Net Classes

• High speed clocks separated from low speed nets

Routing Rules

• Default widths and spacing by net type

Plane Definitions

• Continuous bottom plane pour for rigidity

Lacking organized rules sets leads to scrambled layout efforts.

Analysis Expectations

Even without complex SI or thermal needs, best practice analyses help:

Design for Manufacturing (DFM)

• Ensure adequate breakout routing space

Test Point Locations

• Enable safe probing of key nets

Doing checks early allows adjustment opportunity prior to release.

Manufacturing Handoff

To enable prompt assembly quoting:

Layer Stack Documentation

• Illustrate dielectric buildup, copper weights, finishes, soldermask

Panel Size Defined

• Format accommodating 10 boards for array assembly

File Checklist Declared

• Netlist, drill file, gerbers, pick and place file, docs

Clarifying expectations avoids mismatched data delivered to CM.

While basic, following this type of shortened checklist guards against unnecessary errors even on simple 2 layer boards before layout work launches.

Advanced Checklist for Complex Layouts

High speed digital, RF, or complex board geometries introduce more opportunity for oversight:

Function: 48 channel optical transceiver

Layer Count: 16 layer

Technology: PCIe Gen 4, DDR4 Channels

Quantity: 1,000s per year

Managing intricate designs demands an expanded checklist:

Components Check

Digikey Part Numbers Assigned

• BOM & schematic rows aligned to purchase all devices

Package Footprint Reviews Complete

• Xilinx FPGA pinout & pin spacing verified against IBIS model

Backup Parts Qualified

• Alternate memory ICs tested for drop-in compatibility

Deep oversight prevents delays when primary parts lapse EOL during layout.

PCB Stackup Definition

With intricate layer planning critical, validation requires rigor:

Impedance Targets Set

• 50 Ohm differential trace across all high speed lanes

Dielectric Selected

• Megtron 6 at 5.2 dielectric constant suits datarate

Sequential Bonding Confirmed

• Ensure laminate supplier can deliver 16discrete layers

Plane Alignment Verified

• All power and ground planes checked for symmetry

Stackup missteps lead to signal integrity failures that dictate full board respins.

Design Rules Signoff

Intricate constraints require layered rule hierarchy:

High Speed Constraints

• Velocity matched trace length tuning to 110 ps edge

EMI Containment

• No stubs longer than 200 mils

Memory Channel Rules

• DDR4 routes match module pinout

Attempting complex layouts without mature rules fails unrecoverably.

Analysis Prep

Advanced modeling helps steer layouts accurately:

Channel EM Models

• Validate differential pair gap/width against IL/RL targets

Plane Cavity Resonance Sims

• Identify problematic lamination dimensions

Analysis needs definition in checklist prevents late surprises that require respins.

Manufacturing Handoff

To enable first time success, fabrication release requires all details:

Layer Stack Drawings

• Full bookset communicating laminate & prepreg choice

Panel Size

• 850 x 550 mm frames suit board outline

Coupon Types

• Include daisychains, comb patterns, and test points

Only capturing every fabrication instruction prevents error discoveries after release.

While more extensive, complex designs must safeguard far greater risk factors from the beginning through comprehensive checklist diligence before layout software ever initializes.

General Checklist Guidelines

For simplicity, the previous sections used specific product examples to illustrate checkpoint categories. However, the following guidelines apply when scoping any custom pre-layout validation checklist:

Components

• Reference designators match schematic
• Required tolerances noted
• All footprints pre-verified
• Backup parts pre-qualified

Stackup

• Dielectrics finalized
• Copper weight suitable
• Layer count adequate
• Need for advanced materials checked

Rules

• Net classes defined
• Width/spacing/via rules set
• Plane connections mapped

Analysis

• SI models available
• Thermal needs documented
• DFM checks planned

Manufacturing

• Layer stack fully illustrated
• Panel sizes chosen
• Required coupon types listed
• All documentation requirements captured

Adjusting specifics to the product, following these principles steers every PCB design correctly regardless of complexity or technology towards manufacturing readiness.

Conclusion

In summary - skipping necessary planning steps before rushing layout risks discovering nasty surprises late that prompt scrapped work and expensive respins. Leveraging checklists matched appropriately to design complexity safeguards weeks of effort and prevents delays that might cause missed market windows. Especially for advanced boards, breakfast spent validating every component selection, modeling expectation, stackup nuance, design rule, and fabrication handoff requirement pays dividends minimizing iteration cycles. While exhaustive pre-layout reviews seem tedious initially, the certainty gained prevents crashing later. Adopt these checklist guidelines as standard practice and ensure your next PCB design launches smoothly!

Frequently Asked Questions

What are the most common sources of mistakes before PCB layout?

Typical oversights include inaccurate component selection, inadequate stackup planning, lack of defined design rules, mismatched analysis expectations, and failed manufacturing handoff instructions.

What risks occur without checklists for simple boards?

Even basic boards might encounter issues like shorted nets from space violations, clearance failures from insufficient rules, bad signal performance from stackup choices, or mounting troubles from footprint errors.

What risks occur without checklists for complex boards?

Advanced boards face exponentially greater challenges around impedance discontinuities, resonance from plane alignments, noise coupling into critical circuits, manufacturability with high density parts, and reliability failures without rigorous checking before layout.

How can checklists reduce needed design respins?

By methodically validating all facets upfront, checklists prevent the 57% of respins originating from avoidable planning oversights rather than true design flaws needing rework.

Should checklist rigor scale with design complexity?

Yes, while all boards need prep verification before layout, more sophisticated technologies merit additional checks around signal and power integrity, manufacturability, component packaging restrictions, and documentation needs increasing likelihood of first time design success.