Category Archives: PCB

Laser-cut fume extractor design

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I have constantly been looking at ways to make my home workspace more convenient and safer for PCB fabrication/prototyping. I switched to all lead-free products several years ago but have been using a pretty janky setup for fume extraction: a 120mm fan with the charcoal filter attached to one side. It worked well enough, salve but I wanted to make something a little better. I wanted to have the fan out of the way, viagra sale with some hose, so it would not take up too much room on my actual table. I decided to design something in Autodesk Inventor to be fabricated out of acrylic. The idea is that a standard hose would be attached at the top and that the whole assembly can be mounted on my rack. The results are pretty good, the suction works well, with the key downside being that it is louder than commercial units which are more expensive. The files are attached, both in Inventor format and as DXF outlines. The material thickness was 0.250″ for the backplate and 0.175″ for the rest. The DXF units were in inches, so the tube OD can be measured there.

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Maker Faire 2013, here we come

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We finally have everything squared away for Maker Faire 2013 and we should be able to show off an open-source LED controller. That design is not complete, pilule however, I still wanted to be able to give something away. I settled on creating a PCB business card which doubled as an SMT prototyping board on the back. I did a few iterations and settled on something that was all surface mount. There are pads for an SMT mini-USB port, a 32qfp,80tqfp and 64 pin IDC side-connector. All of the quad footprints will accept smaller packages with the same pitch and the IDC pads are designed in such a way that the 0.8mm PCB can be soldered between the pins of a standard 2.54mm/0.1″ connector. There is also a grid of pads that have 1.27mm pitch in one direction and 0.65mm pitch in the other direction so they can accommodate a SOIC one way and TSSOP the other way. The finish is ENIG with a high performance white solder mask so it should stand up to soldering without discoloring.

Stop by the mobius.io table and just ask if you would like to get one of these. Gerbers for this card are here: mobius_io_card2013 .

On PCB mounting options

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As designers, we often give PCB function the highest priority while making the mounting/enclosure options (see above) an afterthought. This is especially problematic if the design will be constructed more than once or twice, where custom drilling/milling/manual processing will become very expensive and hard to reproduce. Having worked custom mold-injected enclosures, custom metal enclosures, and off the shelf enclosures, here are some things I have learned along the way:

  • Check with your PCB assembly house about component placing near PCB edges, a lot of places will typically want >1.27mm. This is a process issue for removing individual boards out of panels and being able to clear the cutting tool. More is better as it makes it easier to handle the board after de-panelizing without touching mounted components.
  • Label component keepout areas on the PCB print if you need to clear anything, especially when using ridges to support PCBs in plastic cases. Locations of these keepouts can be obvious when initially designing the PCB, but you will soon forget and may accidentally place components there in a future revision. Changing a PCB fab is usually a lot cheaper than changing the injection mold, so you will be redesigning your PCB in situation.
  • Make sure that your board only fits into the chassis one way. This can be done by using a non-symmetric mounting hole pattern or by having specific cutouts for mechanical pins or ridges.
  • Comply with standard drill sizes (and keepout areas) for standard mounting hardware. Tim Hausherr has a good overview here. It is a lot easier to be able to use standard fasteners instead of drilling arbitrary holes in PCBs and then trying to find something that fits.
  • Specify a torque spec and use an electric driver with torque setting when driving any kind of threaded fastening hardware. If you don’t do this, you can expect to get anything from cracked PCBs from overtightening to loose hardware/PCBs in your chassis.
  • If you expect a lot of vibration, avoid using threaded fasteners and use clips instead. This is especially true for mounting speakers to chassis. It is tempting to use threadlock, but this is a bad idea from a serviceability standpoint (see below).
  • Expect that you will get failures in the field and that you may have todisassemble units coming back from the field. This is the primary reason I don’t like using threadlock unless it is going into standoffs which are stamped into the chassis and will not turn when you try to back your fasteners out. This is also the reason to give a second thought to conformal coating/potting your circuits unless you have a specific performance reason to do this (IPXX rating, isolation). Coating your PCB/filling your case with epoxy will not stop people from being able to reverse engineer the physical aspects of your design.
  • Use 3D models if your connectors, or at least mark their outlines on an assembly layer, so you can align them with your chassis. The large gap between the USB and Ethernet connectors on the rpi makes it challenging to mount PCB in an off the shelf rectangular case.

Analog Devices’ app note on mixed signal PCB layout

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Here is a short application note (AN-404) from Analog Devices that deals with high performance analog and digital layout on the same printed circuit board.  The specific example deals with AD1845 and CS4231 codecs and demonstrates some ideas for clean power and ground plane separation, order among others. The application note provides some handy numbers, such as a “ballpark” estimate inductance of a PCB trace of 1nH/mm. Another helpful hint is that the note helps prioritize the various pins of the codec on page six to optimize noise management.

While on the subject of PCB design, here is a nice tutorial covering various dielectric materials used in printed circuit board fabrication. Its main goal is to give an overview of the various properties of the materials so the designer has a better judgement of which to use for higher performance RF boards and which is most economic for medium-speed digital designs.

( an-404.pdf )

How to do reflow-type soldering at home

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What to do when you need to mount a ball-grid array (BGA) package on a circuit board without sophisticated equipment? One popular option is to create something called a “reflow oven” which is able to control your circuit boards temperature with respect to time. The idea behind reflow soldering is that we may want to apply a thin layer of solder paste (solder with flux) over the exposed pads on a printed circuit board, then place all of the surface-mount components on that side, and then heat the board so the solder melts and the components become electrically attached. This is pretty much the only method for attaching components whose pads are completely on the underside making them inaccessible to soldering irons. The temperature profile is fairly standardized (here, here and here) and consists of first removing any excess moisture from the packages, then ramping up to the temperature required to melt the solder, then to cool off in a safe manner that prevents component or joint damage. It should be noted that these temperature profiles aim to limit the time components spend at elevated temperatures (>250C) to minimize the risk of damage due to heat.

What I am proposing is something much simpler: lets use a hot  plate to heat the PCB and achieve the same sort of reflow process. The main disadvantage is that the process is much less controlled and the dimensions of the board must be small enough to fit on the hotplate. The primary benefits are its simplicity. I am fortunate enough to have a hotplate which has a thermocouple to the surface and can measure the surface temperature with some degree of proficiency, so an alternate method will be required for other types. Some kind of infra-red measurement method would probably work well.

The idea is that we first apply solder paste to the board, when necessary. In this example, I am mounting a MICROSMD8 package where there is ample solder on the board and the chip to achieve connection. It is often a good idea to put some clean-free flux on the board in any case. Everything is first pre-heated for ten minutes at 50-80C to get rid of some of the moisture. The assembly is then heated to about 230C. At this point, the chips should already be aligned over the target pads. The reason for this temperature is that unlike the oven, the top surface of the PCB is exposed to air and thereby creates a thermal gradient. We need to control the heat on the top surface so that the solder just barely melts. This can be noted when watching the PCB under a microscope or with a magnifying glass as the solder will become very shiny when it melts.  As the solder melts on the chips and PCB, the surface tension will pull the chip into alignment. The whole assembly can then be slowly cooled and tested electrically. When populating larger projects, it is best to put on the larger chips first and then place something to act as a heat-sink on top. I have had success with larger DSP chips where I placed inverted bolts on top to radiate away some of their heat while adjusting other components. Finally, don’t forget that a cold PCB looks the same as a hot one, so be sure to avoid burning yourself.

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( an081.pdf ) ( an353.pdf ) ( xapp427.pdf )

Orcad Layout has reached end-of-life

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It is now official:

 

To our valued OrCAD Layout customers:

As demonstrated in the OrCAD® product 16.0 release, Cadence® continues to invest in providing a fully scalable PCB design solution for our customers – one that grows with you as your PCB designs grow in complexity. We’ve all seen the PCB design landscape change dramatically in recent years. In order to help customers meet current market demands and maximize productivity, Cadence continues to leverage the power of its proven Allegro® PCB technology within our OrCAD product line. This allows Cadence to offer customers unique suites and technology bundles that address current and future design challenges.

This letter is intended to communicate some important developments regarding the future of Cadence OrCAD Layout. Cadence has begun the End-of-Life process for Cadence OrCAD Layout technology based products.

Please Note: Cadence OrCAD Capture, OrCAD Capture CIS, and PSpice® technology are all integral parts of Cadence’s long-term product strategy and are not affected by this notice.

Effective July 31, 2007, Cadence will no longer sell the following Cadence OrCAD Layout based technology products:

1. OrCAD Layout (PO1410)
2. OrCAD Layout Plus (PO1420)
3. OrCAD Unison PCB (PO1510)
4. OrCAD Unison Ultra (PO1530)
5. Layout Studio (PS1430)

We acknowledge that transitioning software systems is never easy and is often a juggling act between investing in learning new technologies and meeting current business priorities. EMA is committed to ensuring we do everything possible to help minimize the impact on you, wherever possible. To help ease the transition, Cadence is providing OrCAD Layout customers with multiple paths for migrating to new technology that leverages the power of Allegro PCB Editor. Learn more about the various transition path options by visiting http://www.ema-eda.com/orcadlayout.

The products entering End of Sale will be supported thru March 31, 2009. After that date, these products will no longer be supported for hot-fixes or support calls and will not be shipped on the OrCAD CD set.

If you have any questions, or would like to discuss these changes and how it may impact you, please contact your EMA Account Manager. You may also contact the EMA technical support team at 585-334-6001, Option 5, or by email at techsupport@ema-eda.com.

We remain focused on providing solutions to ensure your ongoing and future success!

Best regards,
Manny Marcano

President and CEO
EMA Design Automation

Many people saw this coming as virtually no new features have been added to Layout in the past four years, only bug fixes. Although Allegro PCB Editor is a little bit more pricey, I think its worth it, especially for high performance designs. Finally, the Layout site gives some instructions on migrating. Layout,… we have had some great times together:

  • The great and unavoidable crashes that used to occur when the user would lock the (win32) workstation running Layout
  • All the excellent times that Layout would close your design without saving if you hit CTRL-C twice instead of once
  • Layout’s inability to recognize artwork that was placed on the Global Layer (0) when creating Gerber files

Some important aspects of PCB layout

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About four years ago, when I first started using the Orcad suite, I embarked on an ambitious project to design one of my first EEG recording amplifier and stimulator interface boards. The project was pretty simple: route twelve differential channels from the pre-amp to the EEG amplifier, track the current going out of the stimulator by way of sense resistors and interface with a National Instruments acquisition board. I did the schematic entry correctly, but I neglected to double check the footprints used for the connectors. That is, in the schematic entry, I used a generic 68 pin connector for the NI connector and selected metal can regulators instead of TO-220 and then picked the wrong polarity on all of the d-sub connectors. The result was that the 68 pin connector just didn’t fit and required massive jumpering for even basic functionality, some of the db25 connectors had to go to the back  of the board (not shown) and the power regulators had to be kludged on. The board was not becoming a truly three-dimensional object that had wires coming out from every direction and could only be secured by clamping one edge of it in a vice. Otherwise, it functioned fairly well so it was still used in some experiments. Due to its green color and protruding wires, it became known as the “christmas tree” and be came synonymous with design failure.

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How to make PCBs with a Roland EGX-300 (part two)

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After some help files and some guesswork, I think I have come up with a workable solution to engraving PCBs with EngraveLab. The previous problem was that it did not import the pads properly from a GerbTool generated HPGL file, and it didn’t do the tracks correctly when importing from a GerbTool generated DXF file. The small workaround is to import both files and then lay one on-top of the other, select both, click a single layer so they are both become the same color, and then do a “basic weld” to join the shapes into one. To be specific, export HPGL with outline selected and with a pen that is very small (0.0001″ works fine). Next, import this HPGL file into Engravelab and when it is selected, perform a basic weld to join all of the parts of the tracks together. Finally, import the DXF file and weld that to the tracks. Now a male toolpath can be created which will go around all of the traces and pads. I am still working out good speed and depth settings for 1oz copper plate and how to do the drilling, however, there is a drill toolpath editor in EngraveLab so that looks promising. As usual, more on this later. (previous post)

Another option that I have been looking at is editing the layer in GerbTool by using tools->convert->draw to pad or something like that. This way, you select all of the tracks and convert them to pads, and then when you export the HPGL with the pads-only option selected, the right looking artwork comes out. The trouble is that the toolpath is on the inside of the design instead of outside. Setting the pen size to 1mil or less makes it looked filled in, but then you still have to generate the appropriate toolpath for the contour.
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Roland EGX-300 links for 3D and PCB prototypes

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Some time ago we good an EGX-300 engraver from Roland in the hope of doing small 3D and PCB prototyping. As far as the 3D goes, try the Modella tool that comes from Roland is not too useful for any kind of complex 3D design and most people seem to use VisualMill to generate the tool paths from standard 3D files. An example guide is over at instructables. The EngraveLab software that lots of EGX-300 vendors try to push with this device has so far proven to be fairly useless for 3D work and can be considered to be a waste of 1000USD for this application. (It turns out that EngraveLab is useful once you get some details worked out.)
As far PCB milling goes, pharmacy I have had a hard time finding a commercial package that will take a Gerber file and create a toolpath to send to an engraver without using some messy intermediate step. One guide is available from UMass which relies on PCBMill web utilities by C. Scott Ananian. These are written in JAVA but have some portability issues with the compiler available for OSX (workgroup server). I have email the authors of the UMass webpage and Ananian about commercial packages and have found none so far. I will try to compile PCBMill on a FreeBSD system next and will post the results. If anyone has reasonably priced alternative software suggestions, please let me know!

( pcbmill-098tar.gz )