Neodymium magnets as logic-analyser probes

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One of the most difficult parts of examining any electronic signal on a modern PCB is the inability to get an oscilloscope probe tip or set of logic-analyser probe tips actually attached to the test points.  These test points are often the pin on a IC.  With IC packages routinely spacing pins at 0.025" (@0.7mm) apart, is often impossible to hold a probe tip onto an individual pin and operate the controls of the scope/analyser.

 

   Perhaps using thin tubes of ultra-strong neodymium magnets with a 30AWG wire-wrap leads attached would be an answer.  These tubes would be about 0.5 mm in diameter and about 3-5mm long.  Even with the strong magnetic material, the small size of the tube or pin means that it would have just enough magnetic strength to be lightly attached to an IC pin, and not fall off due to gravity.   One end of the tube/pin would be magnetically attached to the steel or copper IC pin.  [do magnets attach to copper? For argument, assume yes]  and the other end would be soldered to a length of 30AWG wire-wrap wire that would connect to the standard oscilloscope probe or logic-analyser pod.

 

   Would the magnet force holding the probe to the IC pin be enough to affect the reading of the electrical signal on the pin? I don't think so because the resistance of the pin is in milliOhms. At most it would slightly affect the rising and falling edge time of the digital signals.

 

Neodymium magnets seem to be really cheap.  1/4 inch square 'science-project' magnets sell for about $0.35 each.  But can they be made very small?

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One of my clients has modified pico probes for use on fine pitch ICs. They are neat to use, but these were a pain to position since they were originally intended for wafer probe.

 

Edit: I try to include a good quantity of testpoints on new design prototypes (sometimes a via that can steady a probe tip).

David (aka frog_jr)

Last Edited: Sat. Jul 1, 2017 - 12:12 AM
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... even a via that goes no where else.

Ross McKenzie ValuSoft Melbourne Australia

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Magnets attract ferrous metals. Chip leads are typically copper, a non ferrous metal.

It all starts with a mental vision.

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KitCarlson wrote:
Magnets attract ferrous metals. Chip leads are typically copper, a non ferrous metal.

Just to elaborate a bit, ferrous means iron. AFAIK iron is never used in chip or PCB fabrication.

Letting the smoke out since 1978

 

 

 

 

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I try to put at least a via on every MCU signal pin. On "serious" pins (SPI, I2C, UART, interrupt inputs, etc), I add a test-point which is a pad with a hole that is just big enough for the probe tip or a wire that can be soldered (when the need arises). Then, you only need to be careful that you do not snap off the pin from the probe. If your "probe" is one of those little "grabbers", a short soldered wire is enough to grab onto.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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digitalDan wrote:
KitCarlson wrote:
Magnets attract ferrous metals. Chip leads are typically copper, a non ferrous metal.
Just to elaborate a bit, ferrous means iron. AFAIK iron is never used in chip or PCB fabrication.

 

A ferrous metal is an alloy that contains iron as one of the main components. Usually, they are magnetic, but there are exceptions, like some stainless steels. Alloys containing lots of nickel and/or cobalt are also usually magnetic.

 

Actually, magnetic ferrous alloys "alloy 42" (iron/nickel) and "kovar" (iron/nickel/cobalt) were often used in lead frames when chips were encapsulated in ceramic, because they have a similar expansion coefficient. This prevents stresses inside the chip during soldering.

 

Now they are encapsulated in epoxy composites, non-magnetic copper alloys are used.

 

Here is a source for this information.

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The PDIP ATtiny85 sitting next to me quite happily sticks to a neodymium magnet, but not with any great force.  Whatever the lead frame is composed of, it is weakly ferromagnetic.  I'd guess that the force would not be enough to securely hold a tiny rare-earth magnet probe, as the relatively huge magnet I have here barely holds up an 8-pin PDIP.  Perhaps if the probe lead wire were extremely fine, it could work.  However, I wonder how hard it would be to get the magnet to stay put on the target pin without slipping and shorting against an adjacent pin.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Sliding a chip past a magnet will result in eddy currents, and related force. :) Perhaps not static attraction.

Side topic:
I use non magnetic tweezers, seems chip resistors have enough Fe, to make handing difficult for placing and soldering.

It all starts with a mental vision.

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I noticed in the race to cheapness, some of my leaded resistors must use plated steel leads (since a magnet grabs 'em tight).  Other resistors, nothing happens...nice real copper leads.

 

A lot of import junk doesn't bother with copper wire---they use some copper (or tin) plated steel to carry usb signals in $1 cables.

 

Automotive O2 sensors seem use something other than copper wire---but that is to deal with the extreme temperatures.

 

Someone makes a spring loaded "finger"/pincher  with a weighted base to hold your scope probe exactly where you want it...I lost track of where I saw that and could never find it again.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sat. Jul 1, 2017 - 08:17 PM
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What are test points for? :)

It all starts with a mental vision.

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I noticed in the race to cheapness, some of my leaded resistors must use plated steel leads (since a magnet grabs 'em tight).  Other resistors, nothing happens...nice real copper leads.

https://electronics.stackexchange.com/questions/116171/what-kind-of-metal-are-the-legs-of-resistors-made-from

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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El Tangas wrote:
Actually, magnetic ferrous alloys "alloy 42" (iron/nickel) and "kovar" (iron/nickel/cobalt) were often used in lead frames when chips were encapsulated in ceramic, because they have a similar expansion coefficient. This prevents stresses inside the chip during soldering.
and thermal cycling in satellites and avionics (engine start and shutdown)

El Tangas wrote:
Now they are encapsulated in epoxy composites, non-magnetic copper alloys are used.
Thermal cycling can be a problem for epoxy encapsulated QFN, DFN, CSP, BGA, and LGA; underfill can reduce that stress.

 


http://www.microchip.com/wwwproducts/en/atmegas128 (gold plate over nickel in ceramic QFP)

embedded.com

Underfill revisited: How a decades-old technique enables smaller, more durable PCBs

Michael Yu and Syed Wasif Ali, Nexlogic Technologies

January 27, 2011

http://www.embedded.com/design/system-integration/4235387/Underfill-revisited--How-a-decades-old-technique-enables-smaller--more-durable-PCBs

 

"Dare to be naïve." - Buckminster Fuller

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avrcandies wrote:
Someone makes a spring loaded "finger"/pincher  with a weighted base to hold your scope probe exactly where you want it...I lost track of where I saw that and could never find it again.
ezProbe QuickProbe?

ezProbe QuickProbe

 


http://www.shop.ezprobe.com/TEX-QP-QuickProbe-Probe-Holder-for-SMD-Test-Connections-TEX-QP.htm

 

"Dare to be naïve." - Buckminster Fuller