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IPC Irvine 2010

BGA Fanout Patterns

Charles Pfeil

Engineering Director

Systems Design Division

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Mentor Graphics

BGA Breakout Solutions IPC Irvine

2010

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Mentor Graphics

Charles Pfeil Background

1966-1987 PCB Designer

1978-1987 Founder of Computer Circuits Inc., Fairfax VA, PCB Design Service Bureau

Marketing and/or engineering management at these software vendors, specializing in automatic and interactive routing:

1983-1985 Racal Redac

1988-1991 ASI/Cadence

1991-1999 Intergraph/VeriBest

1999-2010 Mentor Graphics

Original product architect for Expedition PCB

An inventor of XtremePCB & XtremeAR


2010

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Abstract

Choosing the appropriate fanout patterns for routing BGAs can enable fewer layers and better signal integrity

When using HDI, many options are available for fanout patterns

This session demonstrates different fanout patterns in the context of HDI stackups and how they can be successfully applied on large dense BGAs

BGA Breakout Solutions

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Definition of “Breakouts”

The combination of fanouts and escape traces, having a purpose of routing out of the BGA pin array


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Why Care About Fanouts?

If the BGA device has too many pins in a dense array, the only way to minimize the number of layers is to utilize all the available space inside the component area with a pattern of fanouts and escape traces

24-36% improvement in route density using effective fanouts


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Why Care About Fanouts?

With good fanout patterns, you can effectively reduce the size of the array for routing

With HDI, 1760 pins effectively reduced 41% to 1024


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Overview


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Overview - Scope

Routing of multiple 1mm pitch BGAs >1500 pins, are the primary influence on layer count

A few large BGAs on a board can be routed without impact on layers

Emulation, network and server boards with many large BGAs are difficult to route with minimal layers

Usually an HDI approach will enable fewer signal layers

Effective fanouts will also contribute to fewer layers

Netlines Display

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Overview - Challenge

Future will bring >2000 pins, .8mm pitch

HDI and efficient fanouts will be required

Each board has many variables with different priorities

Understanding the principles for via models, fanouts and escapes will allow you to succeed on a variety of conditions

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Overview – In Theory…

Theoretical solutions have limited application

Simply calculating the number of route channels ignores the impact of signal and power integrity, design rules, and available via models

Some common methods

Reassign and align pwr/gnd & unused pins to open channels

Ignore diff pairs

Ignore netline direction

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Overview – Practical Side

Red = Difficult

Green = Easy

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Finding an effective fanout pattern within the context of the stackup and via models will have a significant impact on route-ability

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Alignment of vias increases space

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For large designs with numerous BGAs, layer-biased breakouts may be most effective

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Design Rules – 1mm Pitch BGA

Rules used for various breakout methods

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Design Rules – Notes

Diff Pairs

If there isn’t adequate room to get proper spacing between compliments in a diff pair, a compromise must be made

o Space them 0.1mm (4th) inside the BGA with a rule area and spread them once the breakout is completed

o Split the diff pairs

o Change the via pattern and maybe add more layers to allow greater spacing

Rule Areas

It is common to see smaller widths and spacing for all signals inside the BGA perimeter

o Will case an impedance discontinuity, but each engineer has to decide if it will be significant

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Fanout Patterns – Signal Integrity


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Diff Pair Via-Via Crosstalk

When diff pairs are routed on the laminated core layers the buried-vias show insignificant crosstalk

o At 5Ghz, the via-to-via crosstalk between the diff pairs (vias spaced 1mm/4th) is around -35db. This is only 15db greater than if the vias were spaced 24th apart.

o In the context of the whole circuit, thisnoise should not be significant

The via stubs affect the diff pairs less than the single-ended nets

Remove unused pads on the blind-vias to minimize noise

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Fanout Patterns - Signal Integrity


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Single-ended Via-Via Crosstalk

Micro-vias reduce potential for crosstalk – try to route single-ended nets on the buildup layers

o Buried-vias if spaced too close together will cause significant crosstalk between the single-ended nets

o Buried-vias can also become noisy via stubs

Remove unused pads on all buried-vias

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Fanout Patterns


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Fanout Patterns – Overview

The effectiveness of a fanout pattern on large BGAs contributes significantly to the route-ability of a design which impacts the layer count, which in turn affects the cost of the board fabrication

Since there are many variables involved in determining fanout patterns, such as layer stackup, via models, via spans and design rules, we will explore fanout patterns BGAs in the context of large dense boards where minimizing layer count is important


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Fanout Patterns – Goals

The goal is to use a fanout pattern that increases route density and reduce the effective size of the ball pad array.

Eliminate the BGA as the most significant contributor to layer count


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1760 pins effectively reduced

41% to 1024

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Fanout Patterns – Alignment

When aligning fanout vias, the relationship between diff pairs may be shifted and spread apart

Pin swapping (within restrictions) can help


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Fanout Patterns – Through-Vias


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When using through vias, there are not too many options due to the large 0.5mm via pad relative to the 1mm ball pitch

Either a “Quadrant Dog-Bone” or “Via-in-Pad” method is appropriate


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Quadrant Dog-Bone

Advantages (Over Via-in-Pad) Opens up additional routing channels in

the center row and column

o However, there is room for only two or three more routes

On the side of the board opposite the BGA mount, the column and row channel is a convenient place to add capacitors and pull-up resistors

Lower cost and less risk of soldering problems related to the via-in-pad


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Quadrant Dog-Bone

Disadvantages (Compared to Via-in-Pad) If you have a ground or power plane on the BGA

mount side, the fanout via pads prevent a continuous plane fill under the BGA


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Via-in-Pad

Advantages (Over Quadrant Dog-Bone)


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If you do not use the mount layer for a plane, then you have an additional routing layer for the BGA - albeit a surface layer which is not recommended for high-speed nets

If you have a ground or power plane on the BGA mount side, the via-in-pads allow a continuous plane fill under the BGA

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Via-in-Pad

Disadvantages (Compared to Quadrant Dog-Bone)


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No additional route channels in the center column and row

Less room for capacitors and resistors on the opposite side under the BGA since the fanout via array is full

If you have unused pins and you do not add fanout vias for them; you will have some room in those locations for components

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Via-in-Pad

Disadvantages (Compared to Quadrant Dog-Bone)


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There will be a slightly higher cost for filling the vias and ensuring a smooth surface for the soldering of the ball pads

There is some risk of BGA soldering problems (de-lamination or pop-corning) with via-in-pad while using lead-free solder

An experienced assembly company should be able to manage this risk and make it a non-issue

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Combine methods using both via-in-pad and quadrant dog-bone


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Power, Ground and Unused Pins

One method proposed to increase route channels on inner layers is to not use fanout vias when possible for power, ground and unused pins

When using through-vias there is very little benefit

The power and ground pins will be assigned to the center of the device and distributed among the other pins; and it is highly unlikely they will be distributed in nice columns and rows

Unused pins will not likely be in convenient rows and columns either


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Power, Ground and Unused Pins


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Virtex-4 with power (orange) & ground (green) vias

Virtex-4 with power & ground vias removed

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Shifting Vias

If you have a set of design rules that will not allow diff pairs to be routed together, it may be helpful to shift the fanout vias


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Pushing Vias

You can push the vias at the perimeter to increase the first signal layer route density to include two additional rows.


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Fanout Patterns – Blind & Buried-Vias


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Using drilled blind & buried-vias may be a viable alternative, positioned between through-via laminated boards and micro-via HDI boards

Route density is increased in two ways:

Smaller Via Sizes – Since the layer span is only one to three layers, the drilled blind-via can be only 8th hole with a 18th pad

Additional Route Space - Connections routed on blind-via layers eliminate vias on the buried-via layers


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Smaller Via Size

In the context of high pin-count BGAs, layer reduction and higher density routing can be achieved due to the smaller via sizes compared to through-vias

However, these gains are not as significant as can be achieved with micro-via HDI methods. The gains are dependent on the size of the blind-via.

Since a minimum drilled hole size of 0.2mm (8th) applies to these blind-vias, the pad size should be 0.45mm (18th)


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mm milsBlind-Via Pad 0.45 18Buried-Via Pad 0.50 20Through-Via Pad 0.50 20Ball Pad 0.60 24

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Additional Route Space

Whenever a blind-via is used to complete a route, then the buried-via is not required on the inner core

If blind-vias are arranged in patterns, significant additional routing space can be attained on the blind-via layers


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Multiple Fanout Patterns

Using a number of different patterns works well in this context

1 – Aligned 1:2

2 – Aligned 1:3

3 – Transition 1:3

4 – Dog-Bone

Diagonal


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A blind-via pattern around the perimeter of the BGA in which the vias are shifted into columns and rows (4-6)

Results in 24% greater route density per layer

This methodology is key to success with HDI micro-vias as well

If ball pads are smaller, the vias couldn’t be shifted into a tighter column since the blind-vias are already spaced at a minimum of 0.1mm (4th)


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1,2 - Aligned

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The pins between the pins using the dog-bone via patterns and the shifted vias lack space for the fanouts

I recommend using a row around the perimeter for the transition


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3 - Short dog-bone in the transition area

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4 - Quadrant dog-bone in the center

The layer 1-2 blind-via uses a quadrant dog-bone pattern and then transitions to a buried-via

o The pins that would need to run to the bottom of the board to connect to bypass capacitors pull-up resistors, would have another blind-via between layers n & n-1

One alternative to using the blind/buried/blind vias in the center would be to just put in a through-via either in a quadrant dog-bone pattern or via-in-pad configuration

o This will simplify the fanout and since most of the pins in the center area are power & ground, it will not impact the route density in a significant manner


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The corners of the BGA are always the easiest to breakout because there are half as many pins to route to the edge, split along the diagonal

If there is a need for extremely dense routing at the corners to bring out routes from the center pins, a pattern can be used to spread the fanouts away from the diagonal


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Diagonal

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Advantages (Compared to through-hole) 24% increased route density per layer over through-

vias and un-shifted blind-vias

More room for a ground plane on the mount layer compared to quadrant dog-bone through-vias

If you route the high-speed single-ended nets on the layers using blind-vias, via stubs are eliminated and via-via crosstalk is minimized

Any signal routed on the blind-via layers, will not need to have a buried via, thus opening up route space on the buried via layers as well

Disadvantages Does not provide as much route-density as HDI


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Compared to through-hole and blind & buried-vias, the variety of stackups with HDI and smaller via sizes provide for tighter shifted column and row patterns, improved route density and greater flexibility


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When using HDI, the blind micro-vias give greater route density and fewer total layers required for routing

The fanout patterns herein use these types of HDI construction:

1+N+1 = Type II (layer 1-2 micro-vias with buried vias in laminated core)

2+N+2 = Type III (layer 1-2, 2-3 micro-vias with buried vias in laminated core)


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1+N+1 Type II 2+N+2 Type III

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Layer 1-2 micro-vias (1+N+1)

Layer 1 is assumed to be used for a GND plane

The fanouts need to be patterned to maximize layer 2 route density

The same patterns described for the blind-vias can be used for micro-vias

However, since the micro-vias are smaller, you can compact them more

Micro-vias aligned to improve route density 12% over shifted blind-vias 36% over quadrant dog-bone through-vias


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Via-in-pad micro-vias

If via-in-pad is used with micro-vias, it still makes sense to shift the vias

o Often you will lose space for one trace in one channel while gaining space for two in the other


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Advantages Smaller via size allows for greater route density

Potential for 12% increased route density per layer over blind-vias, 36% over through-vias

Dependent on the effectiveness of the fanout pattern

More flexibility for via models and patterns

Skip vias & stacked vias

More room for a ground plane on the mount layer

Routing on buildup layers eliminates via stubs

Any signal routed on the micro-via layers, will not need to have a buried via, thus opening up route space on the buried-via layers as well


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Disadvantages Potentially more expensive than laminated through-via

or blind & buried-via; however, if you consider the increased yield and reliability with fewer layers, the overall cost may be lower for a very dense board

Learning curve is greater; however, once patterns are developed for a given set of stackups of design rules, the benefits are easily justified


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Breakouts – 0.8mm Pitch BGA

Conclusion

Applying NSEW breakouts with good fanout patterns enables breakouts on large BGAs in 5-6 signal layers

With increased spacing for diff pairs, it probably could be done with 8-10 signal layers

Can maintain normal trace widths and clearances

If over 2000 pins, may have to compromise trace widths and clearances

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IPC Irvine 2010

BGA Fanouts Patterns

Conclusion

An effective set of fanout patterns developed to match the stackup significantly improve the route-ability

HDI should be considered now because in the future it will become a requirement with fine-pitch and high pin-count

IPC Irvine 2010

BGA Fanout Patterns

Charles Pfeil

[email protected]

bga fanout patterns - bokah blocks · bga breakout solutions ipc irvine 2010 if you do not use the...

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