Sunday, August 31, 2014
Friday, August 29, 2014
Sunday, August 24, 2014
Saturday, August 23, 2014
Hyperlynx / Altera - Tco
"This application note describes the output timing parameters for Altera® devices, explains how Altera defines tCO results, and presents techniques for calculating the output timing for your system. In addition, a sample DDR2 interface link is presented and analyzed for calculating output timing"
http://www.altera.com/literature/an/an366.pdf
'via Blog this'
http://www.altera.com/literature/an/an366.pdf
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Friday, August 22, 2014
VIA - Covering Recommendations
There are different and conflicting opinions when it comes to best practices for covering vias (tenting, filling or capping).
This is due to information that lingers in the cloud (internet) once it has been posted.
Here are my recommendations for reliability based on best known current practices.
click on image to view
click on image to view
Tented Vias (Not Recommended)
Why? Trapped etchant chemicals can corrode the vias over time.
"This type of via can be subject to the "micro-etch" process. This is caused by a small amount of residual etchant trapped inside a tented via. This material will crystallize rapidly, creating copper sulfate crystals. Over time, these crystals can cause long term reliability issues. In the case of a electroless nickel immersion gold (ENIG) finish, the gold and small area of exposed copper near the tent could form a galvanic cell, accelerating the etch process"
Source: TI - http://www.ti.com/lit/an/sprabb3/sprabb3.pdf see page 18.
Plugged Vias (Not Recommended)
Why? Trapped etchant chemicals can corrode the vias over time.
Same issue as the one sided tented via.
"Since it is only partially filled, chemical entrapment is a major concern."
Source: TI - http://www.ti.com/lit/an/sprabb3/sprabb3.pdf
Filled Vias (Highly Recommended)
Why ? Higher Reliability.
"Filled vias are 100% filled, usually with a non-conductive material. This process uses additional process steps. This is done to ensure 100% of the vias are covered.
There are several variations of the filled via.
A filled and covered via has a secondary covering of material (liquid or dry film solder mask) applied over the via. It may be applied from either one side or both sides."
Expensive $$.
Via in Pad VIP (Recommended)
"This is the ultimate via and is commonly called Via-In-Pad (VIP). Vias are filled with a conductive or non-conductive media, planarized and then plated over. This process allows the use of via capture pads as SMD pads.
VIP has become very common in BGA PCB designs to reduce routing issues and lower inductance associated with high speed connections. However, it does drive up the overall PCB cost. If at all possible, consider other alternatives and use VIP only as a last resort."
VIP has become very common in BGA PCB designs to reduce routing issues and lower inductance associated with high speed connections. However, it does drive up the overall PCB cost. If at all possible, consider other alternatives and use VIP only as a last resort."
Source: TI - http://www.ti.com/lit/an/sprabb3/sprabb3.pdf
Cons: Most expensive $$$.
Soldermask Relieved (Recommended)
Soldermask relieved vias have the soldermask pulled back from the drilled hole. Typically the mask open is drill + 5mils. Note finished plating in via barrel (ENIG).
Open vias allows for rinsing and cleaning of the vias to remove any trapped enchants.
Soldermask Relieved (Recommended)
Soldermask relieved vias have the soldermask pulled back from the drilled hole. Typically the mask open is drill + 5mils. Note finished plating in via barrel (ENIG).
Open vias allows for rinsing and cleaning of the vias to remove any trapped enchants.
Via Encroachment (Recommended)
Leave the via open and relief the soldermask at via barrels.
Pros: Low Cost, Good Heat Sink
Cons: Solder wicking, paste volume needs to be compensated. Solder bumps can appear on bottom side if paste volume is not properly compensated.
Fab Note:
VIA IN PAD: SOLDER MASK ENCROACHED VIAS (DRILL SIZE 8.1 MILS)
References:
Texas Instruments:
Eurocircuits Printed circuits blog
Advanced Circuits
Thursday, August 21, 2014
Wednesday, August 20, 2014
Samtec Signal Integrity Book
Connector Pin Mapping
"By assigning pins between signals as return paths (grounds), unwanted coupling between signal pins can be minimized. For high speed applications a 1:1 signal to ground ratio is typically optimal. Pin mapping for SE applications would then be S-G-S-G, while DP applications would use DP-G-DP-G.
These pin assignments provide a good performance versus density trade-off. Of course, more return paths could be added between signals to increase isolation for applications where crosstalk must be extremely low. However, this greatly reduces signal density of the interconnect."
Source: Samtec Signal Integrity Book
"By assigning pins between signals as return paths (grounds), unwanted coupling between signal pins can be minimized. For high speed applications a 1:1 signal to ground ratio is typically optimal. Pin mapping for SE applications would then be S-G-S-G, while DP applications would use DP-G-DP-G.
These pin assignments provide a good performance versus density trade-off. Of course, more return paths could be added between signals to increase isolation for applications where crosstalk must be extremely low. However, this greatly reduces signal density of the interconnect."
Source: Samtec Signal Integrity Book
Sharpen rising and falling edges | EDN
"Wherever you find a digital control signal, you find sharp rising and falling edges that need characterization. These edges occur on switching power supplies that drive power MOSFETS and they occur in logic gates. Unless your measurement system—oscilloscope and probe—have sufficient bandwidth and sample rate.
POL (point of load) switching regulators have reached 1nS edge speed and off-the-shelf high-speed logic gates, such as the NC7SZ04, have rise times of roughly 500 pSec. While many engineers measure edges on these components, they are likely getting incorrect results. The right system bandwidth will result in cleaner, more accurate measurements."
Source: Sharpen rising and falling edges | EDN:
Excellent article which demonstrates the bandwidth requirements of an O'scope to properly measure signals with 1nS or faster rising and falling edges.
'via Blog this'
POL (point of load) switching regulators have reached 1nS edge speed and off-the-shelf high-speed logic gates, such as the NC7SZ04, have rise times of roughly 500 pSec. While many engineers measure edges on these components, they are likely getting incorrect results. The right system bandwidth will result in cleaner, more accurate measurements."
Source: Sharpen rising and falling edges | EDN:
Excellent article which demonstrates the bandwidth requirements of an O'scope to properly measure signals with 1nS or faster rising and falling edges.
'via Blog this'
TxLine 2003 - Impedance Calculator
One of my favorite Impedance Calculators is TxLine 2003.
This calculator is quite good, the CBCPW calculation is the best I have found.
Here a screen shot of the microstrip calculation.
For microstrip calculations, the Effective Dielectric Constant is derived from a combination of the resin, glass and air. It does not include the effects of soldermask. However it good enough to get you within 50 ohms +/- 5%.
Phase Constant
Phase Constant sounds like something complicated, until you realize it's nothing more than angular rotation of phase in degrees of a sine wave for a selected unit of length.
In the screen shot above I intentionally manipulated the Dielectric Constant (Er) while keeping the electrical length set to 360 degrees until the phase Constant equaled 60 degrees.
As shown the Physical length of the microstrip at 1GHz wtih a dielectric constant of 5.62213 equals 60 degrees per inch. So 6 inches = 360 degrees (6 x 60 = 360). Pretty simple math.
Critical Length
We have often heard about the term critical trace length for high speed and RF designs. Most often you hear numbers like 1/7 or 1/10 of a wavelength as being the critical trace length.
The critical trace length is simply the length at which you must use a good transmission line to send transmit and receive signals and on your PCB.
Digital circuit designers are familiar with Rule of Thumb #1 Bandwidth of a signal from its rise time.
This calculator is quite good, the CBCPW calculation is the best I have found.
Here a screen shot of the microstrip calculation.
For microstrip calculations, the Effective Dielectric Constant is derived from a combination of the resin, glass and air. It does not include the effects of soldermask. However it good enough to get you within 50 ohms +/- 5%.
Phase Constant
Phase Constant sounds like something complicated, until you realize it's nothing more than angular rotation of phase in degrees of a sine wave for a selected unit of length.
In the screen shot above I intentionally manipulated the Dielectric Constant (Er) while keeping the electrical length set to 360 degrees until the phase Constant equaled 60 degrees.
As shown the Physical length of the microstrip at 1GHz wtih a dielectric constant of 5.62213 equals 60 degrees per inch. So 6 inches = 360 degrees (6 x 60 = 360). Pretty simple math.
Critical Length
We have often heard about the term critical trace length for high speed and RF designs. Most often you hear numbers like 1/7 or 1/10 of a wavelength as being the critical trace length.
The critical trace length is simply the length at which you must use a good transmission line to send transmit and receive signals and on your PCB.
Digital circuit designers are familiar with Rule of Thumb #1 Bandwidth of a signal from its rise time.
Tuesday, August 19, 2014
VIA Spacing for RF and Highspeed Designs
A good rule of thumb is 1/8 wavelength of the highest frequency of interest.
"In RF design we typically need to work with only the fundamental frequency of operation. For instance: In a 2.4-GHz RF design the goal is to have a nice 2.4-GHz sine wave on our board with low harmonics. The frequencies that we need to be concerned with are really 2.4 GHz.
In digital design, the goal is to have a nice square wave on our board. A 1GHz digital data signal needs to be square for a good eye diagram. This means that the true operating bandwidth of these traces needs to be at least five times the fundamental frequency or 5GHz. This is from the rule of thumb that for a good-quality digital signal you need to pass at least the 5th harmonic of that signal."
Critical guidelines for RF and microwave PCB Design | Embedded
Critical guidelines for RF and microwave PCB Design | Embedded:
Crosstalk is discussed in this article
'via Blog this'
Crosstalk is discussed in this article
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Monday, August 18, 2014
Signal Integrity - Guidelines
Stripline traces passing over a split plane should be at least four times farther from the signal than the reference plane. Source: Eric Bogatin
If a signal changes reference planes when passing through a via, add an adjacent ground via.
Source : Signal Consulting Inc. Dr. Johnson
Use a capacitor to AC couple the return path when reference planes are at different potentials.
Source: TDK Outline of EMC Design Methods
This article compares the propagation delay (tpd) for serpentine, spiral, and straight traces.
The results show that a signal will travel faster in a microstrip serpentine trace compared to a straight trace of the same length. And the stripline serpentine trace will lag the straight trace.
Source: A New Slant on Matched-Length Routing - Barry Olney
Serpentine Gaps should be 3 x height to nearest reference plane.
When a design requires equal-length traces between the source and multiple loads, you can bend some traces to match trace lengths (see Figure 11–20). However, improper trace bending affects signal integrity and propagation delay.
To minimize crosstalk, ensure that S ≥ 3 × H, where S is the spacing between the parallel sections and H is the height of the signal trace above the reference ground plane.
Click on images to view
Differential Pairs
D = distance between two differential pair signals; W = width of a trace in a differential pair; S = distance between the traces in a differential pair; and H = dielectric height above the group plane.
If a signal changes reference planes when passing through a via, add an adjacent ground via.
Source : Signal Consulting Inc. Dr. Johnson
Use a capacitor to AC couple the return path when reference planes are at different potentials.
Source: TDK Outline of EMC Design Methods
This article compares the propagation delay (tpd) for serpentine, spiral, and straight traces.
The results show that a signal will travel faster in a microstrip serpentine trace compared to a straight trace of the same length. And the stripline serpentine trace will lag the straight trace.
Source: A New Slant on Matched-Length Routing - Barry Olney
Serpentine Gaps should be 3 x height to nearest reference plane.
When a design requires equal-length traces between the source and multiple loads, you can bend some traces to match trace lengths (see Figure 11–20). However, improper trace bending affects signal integrity and propagation delay.
To minimize crosstalk, ensure that S ≥ 3 × H, where S is the spacing between the parallel sections and H is the height of the signal trace above the reference ground plane.
Click on images to view
Differential Pairs
D = distance between two differential pair signals; W = width of a trace in a differential pair; S = distance between the traces in a differential pair; and H = dielectric height above the group plane.
- Ensure that D > 2S to minimize the crosstalk between the two differential pairs.
- Place the differential traces S = 3H as they leave the device to minimize reflection noise.
- Keep the length of the two differential traces the same to minimize the skew and phase difference.
- Avoid using multiple vias because they can cause impedance mismatch and inductance.
MultiSIM BLUE
MultiSIM BLUE:
"MultiSIM BLUE Coming Soon! NI Component Evaluator – Mouser Edition MultiSIM BLUE allows you to scheme, simulate, PCB layout, BOM and purchase – all in one powerful tool. Even better, the entire tool chain is fully integrated. No more going back and forth between different programs. MultiSIM BLUE provides engineers an easy-to-use, seamless environment for the functional simulation of linear circuits using the Berkeley SPICE engine and advanced electronic components from the industry’s leading manufacturers."
'via Blog this'
"MultiSIM BLUE Coming Soon! NI Component Evaluator – Mouser Edition MultiSIM BLUE allows you to scheme, simulate, PCB layout, BOM and purchase – all in one powerful tool. Even better, the entire tool chain is fully integrated. No more going back and forth between different programs. MultiSIM BLUE provides engineers an easy-to-use, seamless environment for the functional simulation of linear circuits using the Berkeley SPICE engine and advanced electronic components from the industry’s leading manufacturers."
'via Blog this'
Molex Pin Map Configurator
Molex Pin Map Configurator:
Signal Integrity Support from Molex for Backplane Connectors, nice !
Video Link
'via Blog this'
Signal Integrity Support from Molex for Backplane Connectors, nice !
Video Link
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Friday, August 15, 2014
Tuesday, August 5, 2014
Why White Residue Forms on PCBs
“It’s easy to determine the type
of contamination you have on the board. Test
each patch of residue with drops
of water or alcohol (IPA). If the drops of water
dissolve the residue, its ionic;
if dissolved by the alcohol, the residue is organic.
This quickly tells you what it
will take to clean the board (either a water-based
cleaner or a solvent-based
cleaner), and can give you some indication of what
is causing the problem. In both
cases, it’s more likely to be improper techniques or incomplete
cleaning that causes the contamination problem.”
Source: http://tayloredge.com/reference/Science/solder_cleaning.pdf
'via Blog this'
Monday, August 4, 2014
Friday, August 1, 2014
Serpentine Routing - Gaps
When a design requires equal-length traces between the source and multiple loads, you can bend some traces to match trace lengths (see Figure 11–20). However, improper trace bending affects signal integrity and propagation delay. To minimize crosstalk, ensure that S ≥ 3 × H, where S is the spacing between the parallel sections and H is the height of the signal trace above the reference ground plane (see Figure 11–21).
Click on images to view
Source Altera: High-Speed Board Layout Guidelines.
Also see: A New Slant on Matched-Length Routing - Barry Olney
This article compares the delay (tpd) for serpentine, spiral, and straight traces.
The results show that a signal will travel faster in a microstrip serpentine trace compared to a straight trace of the same length. And the stripline serpentine trace will lag the straight trace.
Click on images to view
Source Altera: High-Speed Board Layout Guidelines.
Also see: A New Slant on Matched-Length Routing - Barry Olney
This article compares the delay (tpd) for serpentine, spiral, and straight traces.
The results show that a signal will travel faster in a microstrip serpentine trace compared to a straight trace of the same length. And the stripline serpentine trace will lag the straight trace.
How Via Stubs Distort High Speed Signals
Figures 3 and 4 show representative eye-diagrams of two via structures, one with an intact stub and one without a stub.
Comparing Figures 3 and 4, 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.
Source: Overview of Backdrilling - Sanmina:
Click image to view
See the link above for the full article.
More about stubs from Altera: High-Speed Board Layout Guidelines.
Daisy Chain Routing With Stubs
Daisy chain routing is a common practice in designing PCBs. One disadvantage of daisy chain routing is that stubs, or short traces, are usually necessary to connect devices to the main bus (see Figure 11–14). If a stub is too long, it will induce transmission line reflections and degrade signal quality.
Therefore, the stub length should not exceed the following conditions:
TDstub < (T10% to 90%) / 3
where TDstub = Electrical delay of the stub
T10% to 90% = Rise or fall time of signal edge
For a 1-ns rise-time edge, the stub length should be less than 0.5 inches
(see the “References” section). If your design uses multiple devices, all
stub lengths should be equal to minimize clock skew.
'via Blog this'
Comparing Figures 3 and 4, 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.
Source: Overview of Backdrilling - Sanmina:
Click image to view
See the link above for the full article.
More about stubs from Altera: High-Speed Board Layout Guidelines.
Daisy Chain Routing With Stubs
Daisy chain routing is a common practice in designing PCBs. One disadvantage of daisy chain routing is that stubs, or short traces, are usually necessary to connect devices to the main bus (see Figure 11–14). If a stub is too long, it will induce transmission line reflections and degrade signal quality.
Therefore, the stub length should not exceed the following conditions:
TDstub < (T10% to 90%) / 3
where TDstub = Electrical delay of the stub
T10% to 90% = Rise or fall time of signal edge
For a 1-ns rise-time edge, the stub length should be less than 0.5 inches
(see the “References” section). If your design uses multiple devices, all
stub lengths should be equal to minimize clock skew.
'via Blog this'