AltiumLive 2017: Effective Methods For Advanced Routing

AltiumLive 2017:Effective Methods forAdvanced RoutingCharles PfeilSenior Product ManagerDave CousineauSr. Field ApplicationsEngineer

Charles Pfeil – Senior Product ManagerOver 50 years of experience in the PCB design and software industry. Designerand owner of service bureau. Worked for Racal-Redac, ASI, Cadence,Intergraph, VeriBest, Mentor Graphics, and now Altium.Architect of products with automation that gives the designer control, qualityresults, fast performance. Holder of 11 patents.Wrote a design book titled, "BGA Breakouts and Routing". Inducted into thePCB Design Hall of Fame by the UP Media Group.2

Agenda31Routing Problems2Techniques3Breakouts – Fine Pitch BGAs4Quality5Rules6Differential Pair Routing

Routing ProblemsRouting can be difficult and time consuming Dense board area and limited layers Before increasing layer count, consider alternatives River routing, via-in-pad, stacked µvias Reduce trace widths, use thin dielectrics High-speed concerns Manage all concerns based on edge rates and data rates Timing – Use realistic tolerances, tune gaps. Driver compensation? Crosstalk – Manage clearance to aggressor relative to reference plane Impedance – Realistic management of the appropriate concerns4

Routing ProblemsAdditional routing difficulties Over-constrained rules Can make design needlessly complex – unnecessary rules don’t improve the design Compromise in the right places – identify problems and review with engineering early Fabrication requirements Know your fabricator’s capabilities – clarify discrepancies at the beginning of the design Create templates based on fabricator and via technology used. Investigate if HDI can be lower cost – HDI Handbook, Happy Holden presentationsThe art of PCB design is discovering and applying the suitable compromises that enablethe circuits to work as desired, while meeting the time and cost budgets.5

Agenda61Routing Problems2Techniques3Breakouts – Fine Pitch BGAs4Quality5Rules6Differential Pair Routing

Routing TechniquesImproved routing software enables productivity Take advantage of automation Does it allow designer control? Does it produce manual quality? River routing Reduce vias, manage impedance Gloss & Retrace Better diff pair quality, even over manual route Pad entry, change width gapInteractive routing tools continue to increase levels of automation.7

Routing TechniquesImproved routing software enables productivity Copy & paste Simplifies fanouts and makes them consistent Use a grid! Factor of pin-pitch, reset origin to pin Pin swap Can make routing much more direct, less layers Keyboard shortcuts Learn the shortcuts, use them Create a list of the ones you use the mostAutomated interactive routing tools increase productivity with user control and quality.8

Agenda91Routing Problems2Techniques3Breakouts – Fine Pitch BGAs4Quality5Rules6Differential Pair Routing

Breakouts – Fine Pitch BGAsBreakout of fine-pitch BGAs Fanout & escape tracks for .65mm pitch Difficult challenge Will get worse as pin-count increases Via-in-pad methods – Best approach Location of via can improve route density Reduce trace widths Thru-via vs µVia If pin-count high, need µVias or high layer countToday, stacked µVias is by far the best solution for very dense HDI designs.10

Breakouts – Fine Pitch BGAsPush perimeter fanouts If perimeter pins need fanouts, push awayfrom BGA Can eliminate one escape layer for large BGAs Especially useful if there are power & groundpins on the outer two rowsCreating effective fanout patterns is puzzle solving that can increase route density.11

Breakouts – Fine Pitch BGAsVia in Pad – VIP – Pattern A µVia in the center of ball pad Via hole must be filled and smoothed Eliminates trapped air voids during soldering Route density may not be maximized B-C-D patterns may be better dependingon pitch VIP pattern for .4mm pitch BGAsExperiment with different patterns and find what is most effective.12

Breakouts – Fine Pitch BGAsOffset VIP – Pattern B The µVia offset from center, still completelyinside the ball pad May not need to fill & smooth µVia Likelihood of trapped air voids is much smallerthan in the case of the A pattern. This Offset VIP is optimal for .5mm and .65mmpitch BGAsPatterns may be applied on the whole BGA or locally as needed.13

Breakouts – Fine Pitch BGAsPartial VIP – Pattern C µVia hole intersects the edge of the ball pad Location determined by route density Maximize the number of traces that can be runthrough the channel between µVias µVia hole may not need filling & smoothing Work with fabricator and assembler Find out if not filling the µVia is supported C pattern good for .65mm & .8mm pitchRoute as much as allowed on outer layers, and push outer fanouts away from BGA.14

Breakouts – Fine Pitch BGAsPartial VIP – Pattern CPartial VIP – Inner Layers Increase the potential number of tracksrunning between the vias. Useful for critical signals, power andground, diff pairsVertical channels may be blocked depending upon via and track sizes.15

Breakouts – Fine Pitch BGAsNear VIP – Pattern D µVia hole completely outside edge of ball pad Soldermask opening concern Make µVia hole completely outside the soldermaskopening for the ball pad Potential maximum density May enable the best alignment of the µVias incolumns and rows for the greatest route density D pattern good for .65mm and .8mm pitchExperiment, but make sure you discuss solutions with fabricator.16

Breakouts – Fine Pitch BGAsNear VIP – Pattern DNEAR VIP – Inner Layers Maximize the potential number of tracksrunning between the vias. Can be 25% greater space than Partial VIPTake advantage of pins that don’t need fanouts and consider them in patterns.17

Breakouts – Fine Pitch BGAs0.65 mm BGA - VIP 300 µm Ball Pad 100 µm Trace 180 µm µVia18

Breakouts – Fine Pitch BGAs0.65 mm BGA - VIP 300 µm Ball Pad 100/75 µm Trace 275 µm µVia19

Breakouts – Fine Pitch BGAs0.5 mm BGA - VIP 200 µm Ball Pad 100 µm Trace 200 µm µVia20

Breakouts – Fine Pitch BGAs0.5 mm BGAOffset VIP 250 µm Ball Pad 75 µm Trace 200 µm µVia Better than VIP21

Breakouts – Fine Pitch BGAs0.4 mm BGA –VIP 180 µm Ball Pad 75 µm Trace 180 µm µVia .35 mm BGA Same approach as .4 mm But smaller feature sizes22

Breakouts – Fine Pitch BGAs0.4 mm BGAOffset VIP 180 µm Ball Pad 75 µm Trace 180 µm µVia Worse than VIP Fewer total tracks VIP 1 per channel Offset VIP .7523

Agenda241Routing Problems2Techniques3Breakouts – Fine Pitch BGAs4Quality5Rules6Differential Pair Routing

Route QualityQuality is about routing efficiently and meeting requirements Efficiency - Reduce meandering, vias, route segments Makes it easier to edit later Signal behavior requirements Symmetry looks nice, but avoid enabling crosstalk A design that works is the #1 priority Fabrication recommendation: “Space is King” Use automation that improves quality I certainly appreciate artistic work, but An artistic appearance is a result, not the cause, of quality design.25

Route QualityQuality is about routing efficiently and meeting requirements Efficiency - Reduce meandering, vias, route segments Makes it easier to edit later Signal behavior requirements Symmetry looks nice, but avoid enabling crosstalk A design that works is the #1 priority Fabrication recommendation: “Space is King” Use automation that improves quality I certainly appreciate artistic work, but An artistic appearance is a result, not the cause, of quality design.26

Agenda271Routing Problems2Techniques3Breakouts – Fine Pitch BGAs4Quality5Rules6Differential Pair Routing

RulesTo ensure success, properly constrain the routing Use the rules for DRC and control Avoid wantonly ignoring rules Automation can make it easier Simplify the rule definitions Avoid over-constrainingAppropriately defining the rules makes routing and editing easier and faster.28

RulesOver Constraining Conservative data sheets Skew budget (length tolerances) often in unnecessarily small Why? Desire to eliminate possibility of potential problems Finding the right balance of compromise Often not possible to over-constrain everything without sacrificing one or more of these: cost, time,size, performance or reliability Design Software If the software enables you to easily address the over constrained rules, why not use it? Allows the margin (available skew) to be reserved for other effectsOver constraining a design may result in more layers and higher cost.29

RulesOver Constraining The truth Rick Hartley’s presentation “The Truth About Differential Pairs in High Speed PCBs” illustrates thatover constraining differential pairs may not be necessary Rick’s view on differential pair length tolerance based on circuit speed30

RulesThe other side of the coin If it isn’t broke, don’t fix it Organizations continue conservative design practices simply because they worked in the past Reluctance to analyze constraints to determine if they are really necessary Pushing the envelope Continually strive to make process more efficient and effective Most important question At what data rate can a particular diff pair concern become significant and how is it to be managed?Analyze constraints and solutions to reduce time to market and fab yield31

Agenda321Routing Problems2Techniques3Breakouts – Fine Pitch BGAs4Quality5Rules6Differential Pair Routing

Differential Pair RoutingAddressing concernsbased on data rates Know the problems & solutions Manage the primary concerns Timing, impedance, crosstalk Avoid over-constraining Reasonable tolerances Auto compensation Allows for greater timing margins Pad entry Converge ASAP with equal length33

Differential Pair Routing Diff pair may be shorter due to faster propagation Avoid “Creepy Lengths” when the target lengthkeeps growing if routed in positive tolerance In length matching, include via-used and pinpackage lengths Tuning compliments together results in no skewdifference, it is always balancedRick Hartley’s “Truth About Diff Pairs”Diff Pair 3xH AggressorH34Reference planeTo prevent crosstalk on diff pairCorner and pad entry skew adjustments

Differential Pair RoutingCornerKeep stitch vias equidistant to DP viasPad EntryPhase match start at driver, bump at mismatch Fiberweave routing If loose weave makes one compliment is over glass and the other over resin Search: “Jeff Loyer fiber weave effect” – Best presentation on topic35

Differential Pair Routing Impedance discontinuities can cause reflections which may add noise to signal Cannot avoid all of these discontinuities, so try to minimize the number of them It is “simply” a matter modifying the routing to keep the same impedance, with reason36

Differential Pair Routing37Remove stubs with backdrillingEliminate stubs with µVias

Differential Pair RoutingUsing arcs When does it matter? “When the data rate is 16Gb/s everythingmatters”Arcs are beautiful, but usually not necessary38

Differential Pair RoutingPad entry Converge ASAP with equal length This will help to eliminate skew at thestart and end of the routing Pad entry gloss may work well Fabrication concerns Teardrops, etch traps, soldermask Balanced pad entryVery GoodUsually WithinTolerancePad entry is a good example in which automation can make routing a lot easier39

ConclusionRouting tools and methods are improving: Tools continually enhanced for new circuit, component and fabrication technology New methods with effective automation can increase productivity and quality Using the available skew for each circuit will increase productivity Avoid over-constraining and making routing easier, yet still effectiveGet the most out of the available automation for routing.40