Designing a 4-layer PCB Stackup With No Aircraft Capacitance
In a variety of earlier articles, I’ve addressed the challenges encountered when designing boards with 16-layers and above. These are the sorts of advanced PCBs present in high-end servers, switches, and the cloud titan merchandise that assist the ever-growing Web wants.
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Nonetheless, many merchandise don’t require most of these advanced boards with excessive layer depend. One instance is the tens of tens of millions of four-layer PCB motherboards manufactured annually for PCs and different comparatively low-cost merchandise, corresponding to Microsoft’s Xbox™ merchandise. At first look, it might sound that designing a 4-layer PCB needs to be a straightforward activity, however similar to some other board, it requires the proper of format and stack-up. As well as, there are specific challenges in terms of designing a four-layer PCB stackup with or with out airplane capacitance. Since excessive airplane capacitance just isn’t accessible on a four-layer PCB with out adjoining energy and floor planes, different approaches have to be utilized. This text describes how a four-layer PCB stackup is greatest designed together with how the shortage of airplane capacitance is dealt with.
The ABCs of a 4-layer PCB Stackup
Earlier than there have been four-layer PCBs, there have been two-layer logic boards, as proven in Determine 1. These boards happy digital wants fairly effectively, they usually had been extensively used earlier than issues had been quick sufficient that required the low inductance connections achieved with planes.
4-layer PCBs have been the workhorse of computer systems and video games for 40 years. From day one, PC motherboards have been 4 layers. And the necessity for four-layer boards will solely proceed to develop as extra consumer-oriented merchandise are developed.
The technical points that drive a four-layer PCB design are:
- Two good planes that distribute energy which might be steady (normally beneath the floor layers).
- Hint layers which might be near these planes in order that crosstalk and impedance could be managed.
The important thing financial and enterprise elements in making a four-layer PCB is to fabricate it in large portions (elements of tens of millions) on the lowest price attainable. These volumes are obligatory as a result of large quantities of cash need to be spent on creating the tooling that might be obligatory to construct the boards. The important thing advantages of four-layer boards embrace:
- 4-layer boards readily lend themselves to mass lamination methods that use 36” x 48” panels.
- The imaging for the within layers, that are simply planes, is completed with glass publicity plates as an alternative of movie. This makes the tooling extra sturdy and extra mechanically steady.
- After the within layers are printed and etched, prepreg and foil are positioned outdoors the laminate.
- Across the outdoors of every board on the 36” x 48” panel, there are small fiducials. The copper on the highest of the boards is skived away in order that the fiducials are seen, and the drill could be aligned to the sample that’s contained in the boards. Consequently, there isn’t any want to fret about runout errors throughout the very massive panels.
The Design Perspective
From the design perspective, the four-layer PCB stackup proven in Determine 2 is fairly easy. The weather to remember embrace:
- The 2 outer layers are sign layers.
- The 2 center layers are Vdd and floor.
- The space between every sign layer and the airplane beneath it’s set to 4 or 5 mils to manage impedance and crosstalk.
- This distance forces the 2 airplane layers to be very far aside—40 mils or extra.
- The airplane capacitance at 40-mil spacing is minuscule in comparison with what it might be if the planes had been adjoining (issue 10 decrease).
- The wanted high-quality capacitance is on-die and on-package (extra about this later).
Different elements about four-layer boards to remember embrace:
- 40+ mils thickness is greater than sufficient to make a board inflexible.
- With a four-layer board, the routing guidelines for prime velocity indicators are stricter as a result of you’ll be able to’t change layers with out danger of making an impedance discontinuity.
- There must be a workforce effort between the individuals who pin out the IC, the individuals who pin out the bundle, and the individuals who design the board. This must be negotiated between these three teams.
- All excessive velocity indicators must start and finish on the identical layer. Determine 3 is a photograph of a four-layer board part with the entire indicators operating within the high floor layer.
- The vias on a four-layer PCB are straight through-hole vias. These vias are plugged in order that no contaminates can go from one aspect of the board to a different. This sort of contamination may cause pin-to-pin leakage beneath a BGA that can not be cleaned and leads to the entire meeting being thrown away.
- Blind vias on a four-layer board are pointless as a result of they might solely be making the connection from the vias to the primary energy airplane.
- Blind vias make the price of the board go up.
What Concerning the Lack of Aircraft Capacitance?
As could be seen in Determine 4, and as famous above, the interplane capacitance on a PC motherboard with a 40-mil spacing is minuscule (about 5 pF sq. inch of airplane capacitance). However a supply of top of the range (low inductance) capacitance is required to supply present to cost up the info and tackle strains.
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This capacitance is offered by integrating massive quantities of capacitance onto the IC die itself or within the part bundle. This identical low inductance capacitance is the trail by which return currents discover their approach from one airplane to a different when a sign modifications layers. When that is absent, indicators have to be routed from level to level whereas staying on the identical sign layer.
Examples of the on-die capacitance of elements and reminiscence modules embrace:
- A DDR2 die has greater than 100 pF per I/O pin in-built to provide the cost to the strains being pushed.
- An influence PC IC has greater than 200 pF per I/O pin in-built to provide cost when driving single-ended strains.
- An influence PC IC has on the order of fifty nF in-built to provide the cost to the core because it modifications from sleep mode to all on-active.
- The customized IC processor designed for the next-generation Blue Gene supercomputer, which has a 512 bit-wide reminiscence bus, has 190 nF of on-die capacitance to assist the transient.
What About 4-layer PCBs with Extensive Buses?
Initially, the case for airplane capacitance was essential to assist massive, single-ended switching transients corresponding to these concerned in reminiscence subsystems. Nonetheless, with immediately’s vast buses, the next elements have to be saved in thoughts.
- As famous above, on a four-layer board with sign/floor/energy/sign stackup, there may be very low airplane capacitance.
- The one “return path” for indicators altering layers is the discrete capacitors that operate poorly on the frequencies concerned in quick information buses.
- Consequently, indicators should stay on the identical sign layer for the whole path size.
- ICs that don’t have on-die capacitance to assist quick switching edges will carry out poorly on four-layer PCBs, leading to programs that have “flaky” operation and excessive EMI.
- On account of the previous, a mix of on-die and on-package capacitance is required and the next apply:
- With 130 nanometer and smaller IC geometries, the present contained in the IC or core can typically be attain tens of Amperes in as brief as 15 ps.
- Present transients of this magnitude and velocity can’t be supported by capacitance on the PCB as a result of inductance of the bundle balls and vias.
- Combining ultra-low inductance capacitors mounted on a well-designed BGA bundle with on-die capacitance solves this downside.
4-layer PCBs are a mainstay of the pc and sport console merchandise industries. The profitable design of a four-layer PCB stackup, format, and routing relies on legitimate design guidelines, and there are methods to supply capacitance when inner airplane layers don’t present enough interplane capacitance. This capacitance is offered on-die on the ICs or by means of a mix of on-die and on-package configurations.
Speak to an Altium knowledgeable immediately to study extra or uncover extra about planning your multilayer PCB stackup in Altium Designer®.
- Ritchey, Lee W., and Zasio, John J., “Proper The First Time, A Sensible Handbook on Excessive Velocity PCB and System Design,” Quantity 2.
- Ritchey, Lee W., Course Slides, “2-Day Sign Integrity and Excessive-Velocity System Design,” coaching class.