Manufacturing techniques to decrease high-speed Digital Signal distortion: backdrilling is a cost effective strategy to reduce detrimental signal distortions cased by PTH via stub structures.
Plated through-hole (PTH PTH
Parathyroid hormone (PTH)
A chemical substance produced by the parathyroid glands. This hormone is a major element in regulating calcium in the body. ) via structures in high layer count printed circuit boards and thick backplanes/midplanes can significantly distort high-speed digital signals that pass through them. Often the distortion is severe enough that digital receivers can no longer ascertain whether a logical one or a logical zero was originally transmitted. As data rates increase, the amount of distortion introduced by the PTH via structure also increases, usually at an exponential 1. (mathematics) exponential - A function which raises some given constant (the "base") to the power of its argument. I.e.
f x = b^x
If no base is specified, e, the base of natural logarthims, is assumed.
2. rate considerably higher than the associated increase in data rate. For example, the distortion producing effects of a PTH via at a 6.25 Gb/s data rate is often more than double that at 3.125 Gb/s.
A dominant structure within a PTH via that introduces this undesired distortion is the via stub A small software routine placed into a program that provides a common function. Stubs are used for a variety of purposes. For example, a stub might be installed in a client machine, and a counterpart installed in a server, where both are required to resolve some protocol, remote procedure . The via stub is the conductive conductive
having the quality of readily conducting electric current.
flooring or floor covering made specially conductive to electrical current, usually by the inclusion of copper wiring that is earthed portion of a PTH via not connected in series with the circuit (FIGURE 1). Since a via stub serves no useful function in the circuit, it can be removed using a technique known as backdrilling. Backdrilling uses controlled depth drilling techniques that are compatible with conventional NC drill equipment. Essentially, a drill bit slightly larger in diameter than the one used to create the original via hole is used to remove the undesired conductive plating in the via stub region. FIGURE 2 shows a cross-section of PTH vias that were backdrilled.
[FIGURES 1-2 OMITTED]
Decreasing via stub length by backdrilling significantly reduces a particularly problematic form of signal distortion called deterministic jitter Deterministic jitter (or DJ) is a type of jitter with a known non-Gaussian probability distribution. The other major class of jitter is non-deterministic, or random jitter. . Because bit error rate (BER (1) (Basic Encoding Rules) A set of encoding rules for ASN.1 notation, which is a method for defining data structures. See ASN.1.
(2) (Bit Error Rate) The average number of bits transmitted in error. See BERT.
1. ) is strongly dependent on deterministic jitter, any reduction in deterministic jitter by backdrilling will significantly reduce the overall BER of the interconnect--often by many orders of magnitude. Other key advantages to backdrilling PTH vias include: less signal attenuation Loss of signal power in a transmission.
The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities. due to improved impedance matching Impedance matching
The use of electric circuits and devices to establish the condition in which the impedance of a load is equal to the internal impedance of the source. , increased channel bandwidth, reduced EMI/EMC EMI/EMC Electromagnetic Interference/Electromagnetic Compatibility radiation from the stub end (Mach.) the enlarged end of a connecting rod, to which the strap is fastened.
See also: Stub , reduced excitation excitation
Addition of a discrete amount of energy to a system that changes it usually from a state of lowest energy (ground state) to one of higher energy (excited state). For example, in a hydrogen atom, an excitation energy of 10. of resonance modes, and reduced via-to-via crosstalk (1) Electromagnetic interference that comes from an adjacent wire. "Alien" crosstalk is interference that comes from a wire in an adjacent cable, for example, when two or more twisted wire pair cables are bundled together. .
How Via Stubs stubs
The shares of equity in a firm that is financed almost completely with debt. Stubs are often created when firms go through a leveraged buyout or pay big cash dividends in order to fend off a takeover. Distort High-Speed Signals
FIGURE 3 and FIGURE 4 show representative eye-diagrams of two via structures, one with an intact stub and one without a stub. Comparing the two figures, one can see that via stubs introduce horizontal pedestals in the logic 0 to 1 and logic 1 to 0 transitions. These pedestals close the eye, making it more difficult for the digital receiver to ascertain whether the received signal is truly a logical one or a logical zero.
[FIGURES 3-4 OMITTED]
The "height" of the pedestal pedestal
In Classical architecture, a support or base for a column, statue, vase, or obelisk. It may be square, octagonal, or circular. A single pedestal may also support a group of columns, or colonnade (see podium). is proportional to the equivalent impedance impedance, in electricity, measure in ohms of the degree to which an electric circuit resists the flow of electric current when a voltage is impressed across its terminals. of the stub. A higher via stub impedance results in a higher, more desirable, logic 0 to 1 pedestal level (and a corresponding lower logic 1 to 0 pedestal level). An effective way to increase stub impedance is to decrease shunt To divert, switch or bypass. capacitance capacitance, in electricity, capability of a body, system, circuit, or device for storing electric charge. Capacitance is expressed as the ratio of stored charge in coulombs to the impressed potential difference in volts. by removing non-functional pads and increasing anti-pad diameters. While these techniques help, they are often not enough to reduce distortions to acceptable levels.
The width of the pedestal is proportional to the length of the stub. A short stub produces a narrower pedestal width and a correspondingly wider, more useable eye. An easy way to reduce the width of the pedestal is to decrease the overall length of the via stub by backdrilling. FIGURE 5 shows an eye-diagram of a properly backdrilled PTH. One can readily see that the residual stub length remaining after the backdrilling operation results in a significantly larger eye opening.
[FIGURE 5 OMITTED]
It should be noted that alternate construction techniques such as laser-drilled vias, and alternative stack-up arrangements where traces are moved to layers closer to the end of the via stub, can also be used to reduce stub length. But in many high-density printed circuit boards and backplanes/midplanes, these options are not always viable from both manufacturing and cost standpoints. In this case, the only option is to remove the via stub by backdrilling. Of course, stub removal is not the only signal integrity improvement technique that one can apply to the via. Optimization of the remaining portion of the via that is not backdrilled--using for example, the Sanmina-SCI patent pending Opti-Via "via-tuning" algorithm--can further reduce via-induced signal distortions.
How Much Residual Stub Length Can Remain?
Once the decision is made to backdrill, the question of how much residual stub length can remain arises. Does a thick backplane An interconnecting device that has sockets for printed circuit boards to plug into.
Passive and Active
Although resistors may be used, a "passive" backplane adds no processing in the circuit. with 17 signal layers require 17 separate backdrill depths? Or are only three backdrill depth levels equally spaced across the stack-up acceptable? The answer depends on several inter-related factors including the desired signal integrity performance and practical (cost-effective) manufacturing considerations and limitations. As a general rule, increasing the number of vias that need to be backdrilled and decreasing the maximum allowable residual stub length will significantly increase the manufacturing costs of the PCB/backplane.
From a signal integrity performance perspective, a 3D full-wave simulation of the backdrilled via structure is often required in order to ascertain the actual impact it has on the desired signal integrity performance. High performance interconnects that operate at either higher data rates and/or lower BER generally require smaller residual stub lengths. For estimation purposes, however, a rough gauge of how much stub can remain can be approximated by modeling the via stub as a lumped element impedance mismatch The difficulty of storing the many-to-many relationships of an object model in a traditional relational database. See O-R mapping. , and then calculating the percentage signal loss as a result of that impedance mismatch. TABLE 1 shows the approximate percentage signal loss due to via stub length for a typical 6.25 Gb/s backplane/midplane construction. The eye diagram in Figure 3 was measured on an interconnect (1) To attach one device to another.
(2) A physical port (plug, socket) or wireless port (transmitter, receiver) used to attach one device to another. having a residual stub length of 15 mils. As a reference point, the signal reflection loss due to a 10% trace impedance mismatch is approximately equivalent to a 20-mil stub length.
It has been our experience that most digital interconnect applications that operate at data and BER less than 10 Gb/s and [10.sup.-15], respectively, generally do not need to have their stub lengths reduced to below 10 mils. This is especially true if some form of active or passive transmit-side pre/de-emphasis or receiver-side equalization In communications, techniques used to reduce distortion and compensate for signal loss (attenuation) over long distances. is incorporated into the overall interconnect design strategy. For example, the Sanmina-SCI full-mesh high performance ATCA See AdvancedTCA. backplane only has three levels of backdrilling with the largest via stub length being 80 mils long.
Smaller via stubs are always better than larger ones, and so for the sake of expediency ex·pe·di·en·cy
n. pl. ex·pe·di·en·cies
1. Appropriateness to the purpose at hand; fitness.
2. Adherence to self-serving means: , many signal integrity engineers will dispense with dis·pense
v. dis·pensed, dis·pens·ing, dis·pens·es
1. To deal out in parts or portions; distribute. See Synonyms at distribute.
2. To prepare and give out (medicines).
3. the more time-consuming 3D simulations that were supposed to provide the answer to the maximum allowable stub length question and approach the problem from the opposite perspective: Simply require all via stubs be reduced to the smallest length possible. This often leads to an ongoing improvement mentality, where each new PCB/backplane automatically requires a via stub length that is 20% less than the last one that was designed--and a lot of lively happy-hour discussions over whether reducing residual via stubs by an additional 2 to 3 mils actually provides significant improvements in the SI performance of the total interconnect.
The smallest residual stub length one can impose during the manufacturing of a PCB/backplane without accidentally drilling out the pad connected to the desired signal layer is dependent on a number of variables. Unfortunately, not all of these are under the direct control of the PCB/backplane manufacturer. Minimum residual via stub length is dependent on two factors. The first is precisely knowing the Z-axis location of the layers (Z-axis location uncertainty). The more accurately we know the physical location of the signal layer, the closer we can backdrill to that layer. The second is limiting the residual stub length to a value slightly longer than the tolerance associated with the actual drilling operation. Of these two, the latter is generally easier to define and control than the former.
Percent copper coverage has typically shown itself to be one of the largest contributors to Z-axis location uncertainty. Localized regions where a relatively large percentage of the area is uniformly covered with copper requires a smaller resin fill "demand" than regions whose copper features are a relatively small percentage of the area. Areas with small resin fill demands are generally thicker than regions with large resin fill demands. This means the thickness of the PCB/ backplane at a particular via location, and consequently the actual Z-axis location of the layers, depends on the percentage copper coverage on the layers above/below the location of the via that needs to be backdrilled. This undesired effect can be minimized through the effective use of thieving, a process that generally requires feedback between the PCB PCB: see polychlorinated biphenyl.
in full polychlorinated biphenyl
Any of a class of highly stable organic compounds prepared by the reaction of chlorine with biphenyl, a two-ring compound. fabrication fabrication (fab´rikā´shn),
n the construction or making of a restoration. supplier and OEM (Original Equipment Manufacturer) The rebranding of equipment and selling it. The term initially referred to the company that made the products (the "original" manufacturer), but eventually became widely used to refer to the organization that buys the products and prior to manufacturing the PCB/backplane. Adding small pads (typically primary drill diameter--4 mils) in the via pad stack on layers that will be removed during the backdrilling operation will also help minimize Z-axis variations. FIGURE 6 graphically shows how localized variations in copper density can impact the overall thickness of a representative backplane.
[FIGURE 6 OMITTED]
Another significant factor contributing to Z-axis uncertainty is core and prepreg thickness tolerances. These tolerances add up, resulting in a correspondingly higher level of uncertainty the deeper one drills into the PCB/ backplane. This can be especially problematic on PCBs that have a high layer count. Utilization of low resin percent constructions can help reduce incoming material thickness variations.
One way to minimize these Z-axis uncertainties is to drill a series of test holes along the periphery periphery /pe·riph·ery/ (pe-rif´er-e) an outward surface or structure; the portion of a system outside the central region.periph´eral
1. of the panel prior to backdrilling and then do a cross section on the test holes to determine the exact location of the backdrill depth as compared to the exact locations of the PCB/backplane layers. While this technique does not fully compensate for variations in thickness across the panel, it does eliminate a significant portion of the depth uncertainty associated with the backdrilling process.
Most newer drill equipment uses a variety of sensing techniques to ascertain the exact location of the top of the PCB/backplane prior to beginning the drilling operation and, as a result, are capable of meeting a +/-0.001" depth control tolerance relative to the top of the PCB/backplane. As a general rule, Z-axis location uncertainty of a particular layer due to copper coverage and core/prepreg thickness tolerances dominate the amount of residual stub length remaining after the backdrill operation. (At the time of this writing, a high-speed drill bit outfitted with a layer sensor and associated microprocessor based A computer that uses a microprocessor chip as its CPU. Essentially, all computers today are microprocessor based. counter adjacent to the crown of the drill bit, capable of counting layers in real-time as the drill penetrates the PCB/backplane, is still a largely unfulfilled dream.)
The bottom line is that current industry-wide practice is to limit residual stub lengths to 10 mils maximum, with stub lengths less than 7 mils incurring an additional manufacturing cost.
Assembly-related considerations can also impact the overall effectiveness of the backdrilling process. For example, press-fit connector tails can act as stubs, negating the improvements afforded by backdrilling out the via stubs. It makes no sense to backdrill a via to remove its stub, and then insert a press-fit connector into the backdrilled via whose tail extends beyond the backdrilled depth (FIGURE 7a). If this is an issue, then options include replacing the long-tailed connectors with an equivalent connector having much shorter tails, as shown in FIGURE 7b, replacing the connector with an SMT/BGA equivalent, or moving critical routing layers below the connector tail so the tail becomes part of the non-backdrilled portion of the via.
[FIGURE 7 OMITTED]
Backdrilling and Routability
The backdrill diameter needs to be slightly larger than the primary drill diameter. The backdrill bit size is usually 8 mils over primary drill size, with 10 mils larger preferred. Trace and plane clearances need to be large enough that the backdrilling process does not accidentally drill through traces and planes located adjacent to the backdrilled hole. Minimum plane and trace clearances (spacings) of 10 mils is preferred.
Practical Guidelines guidelines,
n.pl a set of standards, criteria, or specifications to be used or followed in the performance of certain tasks. and Suggestions
While some design considerations need to be taken into account, the integration of backdrilling into a PCB or backplane/midplane construction is, to a great extent, an unobtrusive process. One helpful technique that can often aid in more easily identifying which vias are backdrilled to which depths is to identify each unique drill depth through a combination of unique tool numbers in NC files, unique tool symbols on CAD drawings, and specifying slightly different drill diameters for the different drill depths. For example, a backdrill to layer 13 references back drill size 0.01201, a backdrill to layer 15 references back drill size 0.01202, etc.
Backdrilling is a cost effective strategy to significantly reducing detrimental det·ri·men·tal
Causing damage or harm; injurious.
detri·men signal distortions caused by PTH via stub structures. Properly executed, backdrilling does not have a significant impact on PCB/backplane reliability or yield.
FRANZ FRANZ France, Australia and New Zealand (pact) GISIN is director of Backplane Design Technology and Signal Integrity Design in the PCB Operations division of Sanmina-SCI. He can be reached at firstname.lastname@example.org. ALEX STEPINSKI is senior engineering manager at Sanmina-SCI. He can be reached at email@example.com.
TABLE 1. Approximate signal loss for various residual stub lengths. REMAINING RESIDUAL STUB APPROXIMATE SIGNAL LOSS (PERCENT) LENGTH (MILS) 1 0.25% 2 0.5% 5 1.25% 10 2.5% 20 5% 40 10% 60 15% 100 25% 200 50%