Blog

April 09, 2015


Building A Probing PCB

When testing and debugging components on a printed circuit board (PCB), you'll need to be able to hold the PCB and at the same time hold your testing gear in the correct spot.  Sometimes, you need to test adjacent pins on an IC.  With small pin pitches, getting the necessary probes on adjacent pins can be a challenge.  Sometime you need to test a pad which test equipment can not easily be attached to without soldering on a temporary wire.  One way to address these challenges is to build a probing PCB.  The probing PCB has pogo pins at the pitch you need and each pogo pin on the PCB is connected via a trace to a single pin of a standard 0.1" (2.54mm) header.   Below is one example of how this can be easily accomplished.  The design and pogo pins used could be changed to meet your own requirements, but the concept is the same.

Designing the PCB

You'll need to design the PCB with the number of pogo pins and pitch for your application.  In this example the PCB has been designed to connect 3 pogo pins at 1 27mm pitch to a standard header at 0.1" (2.54mm) pitch.  The PCB contains a 3mm hole so that the entire PCB fixture can be easily connected and positioned using components from the PCBGRIP system.  As shown below, directly below the 3mm hole are the holes/pads for the pogo pins, and below those are the holes/pads for the standard header.  We designed the PCBs and had them manufactured at OSH Park.  The PCB design is shared here.

Other Components

To suite our 1.27mm pitch, we used P50-B1 pogo pins.  The minimum recommended spacing for these pogo pins is 1.27mm and the PCB hole diameter required is 0.9mm.

 

As we'll explain below, a long header is required  We used a 15mm long male header available from Canada RobitixSparkfun carries a 20mm long header which would work too.

To strengthen the assembly, protect the pogo pins from pending (they are fairly fragile), and make everything easy to attach to a 3mm rod, two identical PCBs are used, with a standard M3x0.5x6mm standoff in between.  The standoff is the 'meat' between the PCB 'bread' and the standoff provides a way of mechanically securing the lower PCB while at the same time providing a place to screw in a 3mm rod at the other end.  The machine screw shown below is M3x0.5x4mm with a Philips head. 

Putting It All Together

First attach the stand off to one of the PCBs with the machine screw.

The header is then placed between the PCBs and a 3mm rod with a stop nut secures the assembly together.

With the 15mm headers we used, we had to make sure that the header pin was flush with the bottom PCB, for enough protrusion of the header above the top PCB - see the left header pin in the following picture:

A Joining Plate was used hold the assembly while the headers were soldered to the top PCB.

 

The 3mm rod was removed and the assembly was secured against a Joining Plate .  With the PCB secure against the Joining Plate as shown in the picture below, we were assured that the pogo pins would be flush with the bottom PCB in the picture (top PCB when we use it).  The pogo pins and the unsoldered ends of the header were then soldered to the top PCB in the picture (bottom PCB when we use it).

 

The 3mm rod was reattached and the assembly held so that the pogo pins could be soldered to the top PCB in the picture below.

 

Then attach your test gear to the header and place the pogo pins on the parts that need to be tested.  We are using Cylinders and Variable Tees, along with 3mm rods in the pictures below to hold the probing PCB where we want it.

March 22, 2015


Saleae Logic Anayzer Holder

We have designed a holder to hold a Saleae logic analyzer, for use with the PCBGRIP system.  The idea was to design a holder that would hold the Saleae logic analyzer against the OpenBeam used in the PCBGRIP system, providing the following benefits:

  • The relative position of the analyzer and PCB being tested would remain constant, allowing the whole testing rig to be easily moved.  Beneficial when you need to clear your bench for other projects and don't want to disassemble your setup or when you need to put your stuff away when your called away from your work;
  • In addition to using the Saleae analyzer with the mini-grippers that Saleae provides, the PCBGRIP components can be used to hold other testing fixtures, such as pogo pins.  This simplifies measurements from test pads and other surface mount components, which have traditionally been difficult to probe; and
  • When used with the PCBGRIP system, easily allows probing of both sides of the PCB.

Here is a photo of the holder holding a Saleae Logic 8:

Open source models for the Logic 4, Logic 8, Logic 8 Pro, Logic 16 Pro can be found on Thingaverse.  If you do not have access to a 3D printer, there are 3D printing services, such as Shapeways (no affiliation) which you can upload the models to and they can print them for you. 

Here is a photo of the Saleae Logic 8 being held on  the PCBGRIP system:

 

Saleae analyzer holder

 

Probing Pads And Difficult Components

The Saleae analyzers are a very well made product and are encased in a durable anodized aluminum case (we are especially partial to the red anodized version !).  The analyzers are relatively small, making them ideally suited for mounting on the PCBGRIP system.  For those situations where it is difficult to attach the mini-grippers that come with the Saleae analyzer to the location to be tested, the PCBGRIP system can be used to hold other testing components.  One of the simplest solutions is to hold a pogo pin against the contact you want to test.  To isolate the pogo pin, we first place some heat shrink tubing over the pogo pin:

Heat shrink tubing over pogo pin

The pogo pin can then be held with one of the PCBGRIP components.  In this picture, a Variable Tee is being used to hold the pogo pin:

The pogo pin can then be used to test a pad (or other component) and the pogo pin is connected to a mini-gripper (supplied with the Saleae logic analyzer) and lead from the Saleae logic analyzer:

 

Probing a testing pad

Probing a testing pad

 

Rotating the PCB and holding it vertically, easily allows both sides of the PCB to be probed.  With the holder keeping the Salaea analyzer in the same relative position to the PCB, the test leads do not become tangled and are not pulled off the PCB:

Probing both sides of a PCB

 

Saleae and The Amp Hour

In episode #237 of the Amp Hour (an electronics podcast) Joe and Mark Garrison discuss their logic analyzers and some of the manufacturing challenges.  A very interesting listen.

November 25, 2014


Work Height and Position

The optimal height and position to hold your electronics project depends upon a number of factors, including (i) your bench height relative to your seat (if you are sitting); (ii) whether you need to have access to both sides of the PCB at the same time; (iii) the physicals dimensions of the components on your PCB; (iv) the task you need to complete (soldering, taking measurements with a probe, etc); (v) whether peripheral objects, like battery packs or displays, need to be held in addition to the PCB, and (vi) most importantly individual preference, among others.  There is no single height at which to hold your work that will allow you to comfortably accomplish every different task what needs to be done.

 

There are a lot of ways the PCBGRIP system can be configured to provide you comfortable access to your work, while securely holding the PCB and what ever else needs to be held.   The following provide an overview of some of the height and position settings that can be accomplished with PCBGRIP. 

 

Height. Depending on what you are soldering, sometimes it is nice to be able to rest your hands on you bench to provide support and to help keep the iron and solder steady.  The aluminum extrusions can be easily and quickly detached from the Hinge Assembly by loosening the knurled pivot pin and the knurled index pin.  Then the whole rig can be set on your bench.  In the following setup we've used some Large Thumb Screws attached to the bottom of the OpenBeam to provide a bit more clearance between the PCB and the bench.  The thumb screws also are used to attached the Flat Spring to the OpenBeam.  Used like this, the PCB is approximately 20mm from the bench top.

 

 

The 10mm stainless steel rod can either be threaded into the M10x1.5 hole in the middle of the Base or inserted into the Rod Holder.  When the Hinge Assembly is used without the Rod Holder, the PCB is approximately 30mm from the bench at its lowest position.

When the Rod Holder is used, the PCB is approximately 115mm above the bench at its lowest position.

When the Hinge Assembly is positioned near the top of the 10mm diameter stainless steel rod, the PCB is approximately 350mm from the bench surface.

The Hinge Assembly can be positioned at any spot along the 10mm rod by simply tightening the M8 thumb screw.

 

Rotated, Twisted, or Turned.  While the Hinge Assembly is being used, the work can be rotated 360 degrees around the horizontal axis, tilted in excess of 90 degrees either up or down, or spun around the vertical axis.  While the Hinge Assembly is positively indexed at 90 degrees intervals around the horizontal axis, it can be securely locked at any angle with the M8 thumb screw.

The work can be tilted either up or down by removing the the index pin and setting it at the angle you need.

For tilting, positive indexing is possible at 7.5 degree intervals.  The 4mm index holes at 15 degrees shown below, mate with one of the 3 holes at 45 degrees in the Hinge Assembly, to provide a positive lock at lots of different tilt angles.