BGA Fanout Patterns. Charles Pfeil. Engineering Director Systems Design Division

Transcription

1 BGA Fanout Patterns Charles Pfeil Engineering Director Systems Design Division IPC Irvine

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3 Charles Pfeil Background PCB Designer 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: Racal Redac ASI/Cadence Intergraph/VeriBest Original product architect for Expedition PCB An inventor of XtremePCB & XtremeAR

4 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 IPC Irvine

5 Definition of Breakouts The combination of fanouts and escape traces, having a purpose of routing out of the BGA pin array BGA Breakout Solutions IPC Irvine

6 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 BGA Breakout Solutions IPC Irvine

7 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 BGA Breakout Solutions IPC Irvine

8 Overview

9 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 BGA Breakout Solutions IPC Irvine

10 Future will bring >2000 pins,.8mm pitch Overview - Challenge 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

11 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 BGA Breakout Solutions IPC Irvine

12 Overview Practical Side Red = Difficult Green = Easy BGA Breakout Solutions IPC Irvine

13 Overview Practical Side Finding an effective fanout pattern within the context of the stackup and via models will have a significant impact on route-ability BGA Breakout Solutions IPC Irvine

14 Overview Practical Side Alignment of vias increases space BGA Breakout Solutions IPC Irvine

15 Overview Practical Side For large designs with numerous BGAs, layerbiased breakouts may be most effective BGA Breakout Solutions IPC Irvine

16 Design Rules 1mm Pitch BGA Rules used for various breakout methods BGA Breakout Solutions IPC Irvine

17 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 o o Rule Areas Space them 0.1mm (4 th ) inside the BGA with a rule area and spread them once the breakout is completed Split the diff pairs Change the via pattern and maybe add more layers to allow greater spacing 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 BGA Breakout Solutions IPC Irvine

18 Fanout Patterns Signal Integrity Diff Pair Via-Via Crosstalk When diff pairs are routed on the laminated core layers the buried-vias show insignificant crosstalk o 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. In the context of the whole circuit, this noise 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

19 Fanout Patterns - Signal Integrity Single-ended Via-Via Crosstalk Micro-vias reduce potential for crosstalk try to route single-ended nets on the buildup layers o o Buried-vias if spaced too close together will cause significant crosstalk between the single-ended nets Buried-vias can also become noisy via stubs Remove unused pads on all buried-vias

20 Fanout Patterns

21 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

22 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 1760 pins effectively reduced 41% to 1024

23 Fanout Patterns Alignment When aligning fanout vias, the relationship between diff pairs may be shifted and spread apart Pin swapping (within restrictions) can help

24 Fanout Patterns Through-Vias

25 Quadrant Dog-Bone Via-in-Pad Fanout Patterns Through-Vias 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

26 Fanout Patterns Through-Vias 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

27 Quadrant Dog-Bone Via-in-Pad Fanout Patterns Through-Vias 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

28 Fanout Patterns Through-Vias Via-in-Pad Advantages (Over Quadrant Dog-Bone) 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

29 Fanout Patterns Through-Vias Via-in-Pad Disadvantages (Compared to Quadrant Dog-Bone) 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

30 Fanout Patterns Through-Vias Via-in-Pad Disadvantages (Compared to Quadrant Dog-Bone) 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

31 Fanout Patterns Through-Vias Combine methods using both via-in-pad and quadrant dog-bone BGA Breakout Solutions IPC 2009

32 Fanout Patterns Through-Vias 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

33 Fanout Patterns Through-Vias Power, Ground and Unused Pins Virtex-4 with power (orange) & ground (green) vias Virtex-4 with power & ground vias removed

34 Fanout Patterns Through-Vias 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

35 Fanout Patterns Through-Vias Pushing Vias You can push the vias at the perimeter to increase the first signal layer route density to include two additional rows.

36 Fanout Patterns Blind & Buried-Vias

37 Fanout Patterns Blind & Buried-Vias 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 blindvia can be only 8 th hole with a 18 th pad Additional Route Space - Connections routed on blind-via layers eliminate vias on the buried-via layers

38 Fanout Patterns Blind & Buried-Vias 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 (8 th ) applies to these blind-vias, the pad size should be 0.45mm (18 th ) mm mils Blind-Via Pad Buried-Via Pad Through-Via Pad Ball Pad

39 Fanout Patterns Blind & Buried-Vias 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

40 Fanout Patterns Blind & Buried-Vias 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

41 Fanout Patterns Blind & Buried-Vias 1,2 - Aligned 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 (4 th )

42 Fanout Patterns Blind & Buried-Vias 3 - Short dog-bone in the transition area 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

43 Fanout Patterns Blind & Buried-Vias 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

44 Fanout Patterns Blind & Buried-Vias Diagonal 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

45 Fanout Patterns Blind & Buried-Vias Advantages (Compared to through-hole) 24% increased route density per layer over throughvias 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 viavia 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

46 Fanout Patterns HDI Micro-Vias

47 Fanout Patterns HDI Micro-Vias Compared to through-hole and blind & buriedvias, the variety of stackups with HDI and smaller via sizes provide for tighter shifted column and row patterns, improved route density and greater flexibility

48 Fanout Patterns HDI Micro-Vias 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) 1+N+1 Type II 2+N+2 Type III

49 Fanout Patterns HDI Micro-Vias 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

50 Fanout Via Patterns HDI Micro-Vias Many possible via patterns BGA Breakout Solutions IPC Irvine

51 Fanout Via Patterns HDI Micro-Vias Many possible via patterns BGA Breakout Solutions IPC Irvine

52 Fanout Patterns HDI Micro-Vias 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

53 Fanout Patterns HDI Micro-Vias 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

54 Fanout Patterns HDI Micro-Vias 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

55 0.8mm Pin Pitch BGA - HDI

56 0.8mm BGA Modified a Virtex-5 1mm package into 0.8mm pitch just to see the impact on fanout patterns 0.8mm Pitch 1mm Pitch BGA Breakout Solutions IPC Irvine

57 Design Rules 0.8mm Pitch BGA BGA Breakout Solutions IPC Irvine

58 Fanout Patterns 0.8mm Pitch BGA Layer 1 BGA Breakout Solutions IPC Irvine

59 Fanout Patterns 0.8mm Pitch BGA Signal uvia Thru-Via BGA Breakout Solutions IPC Irvine

60 Fanout Patterns 0.8mm Pitch BGA Signal Skip-Via Thru-Via BGA Breakout Solutions IPC Irvine

61 Fanout Patterns 0.8mm Pitch BGA Signal 3 & 4 using through-vias in center BGA Breakout Solutions IPC Irvine

62 Fanout Patterns 0.8mm Pitch BGA Signal 5 using through-vias in center BGA Breakout Solutions IPC Irvine

63 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 BGA Breakout Solutions IPC Irvine

64 Interactive Demos

65 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 BGA Breakout Solutions IPC Irvine

66 PDF from Mentor: Book from Amazon.com: just search on Amazon for BGA Breakouts

67 BGA Fanout Patterns Charles Pfeil IPC Irvine