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Thing-O-Matic: HBP Connector Failure

This has been a long time coming, as the connector shell over that pin connecting the MOSFET to the heater has been getting crispier despite my attention, cleaning, and occasional DeoxIT application.

Burned-out HBP connector

Burned-out HBP connector

Notice that the burned pin now stands at a slight angle to the others. The PCB pad has no additional copper traces on that side to conduct the heat away from the failing connection, so the joint got hot enough to put the solder into its semi-liquid state, whereupon the springy connector rammed it upwards through the softened plastic shell. If the PCB fab shop used 60-40 lead solder, that’s around 188 °C. Silver solder would reach 220-ish °C. If the solder was eutectic, it would turn liquid and just drip off.

What doesn’t show: the SMD pads that pulled free from the PCB surface, fortunately only under the rightmost three pins leading to the thermistor. Repairing the pads and connector makes no sense, so I think I’ll go with pigtail leads anchored to the plywood, with offboard connectors to reduce the strain on those pads. Powerpoles will be bulky, but maybe pigtails long enough to get them onto the case might work.

As a general rule, soldering wires or connectors to SMD pads with no mechanical support is a Bad Idea and applying repeated mechanical stress to those connectors is a Very Bad Idea. Doing all that on a PCB running well over 100 °C with current right up near the connector’s absolute maximum, well…

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  1. #1 by nophead on 2012-01-06 - 08:42

    Yes those types of connectors always fail after time on my machines, even when the current is way below rating, e.g. 3A on a 156 connector like that rated for 7A. I think it is the vibration or temperature cycling they don’t like. Contact resistance rises and they soften and lose contact pressure, leading to more resistance, …

    • #2 by Ed on 2012-01-06 - 11:13

      it is the vibration or temperature cycling

      Absolutely! Combine those two with the high temperature environment and it’s about the worst case condition for a connector… I suppose if we could figure out a reason to apply salt, that’d be worse. [grin]

  2. #3 by Jason Vreeland on 2012-01-06 - 10:04

    If the HBP is BBR, you could consider using a flexible silicone rubber heat strip (or $Polyimide$) attached to a sheet of aluminum. It’s AC and would require an SSR to drive. I’ve been using PN 35765K428 from mcmaster and have zero complaints. It’s also the only part yet to fail on my printer. Given your ability to repair things regardless of value [grin], take it as food for thought. Then again, you are using cartridge heaters over resistors ;).

    Cheers!

    • #4 by sgraber on 2012-01-06 - 10:40

      What does SSR stand for?

      • #5 by Ed on 2012-01-06 - 11:01

        SSR = Solid State Relay

        The AC versions are something like a triac with a built-in driver, so you can apply a logic-level signal and it Just Works. Better than a mechanical relay with moving parts, at least for non-inductive loads like a heater.

    • #6 by Ed on 2012-01-06 - 11:10

      Just the thought of sticking a flying cable carrying 120 VAC into the guts of that contraption scares the daylights out of me… you’re a braver man than I!

      The serpentine-heater-trace on a PCB is actually a Good Idea, but running SMD parts on the back surface over 100 C isn’t. Perhaps sandwiching a PCB heater (with no circuitry) between a pair of aluminum plates would work better; the plates can handle mechanical attachment / alignment and leave the PCB free to expand as needed. I’d definitely put the connection points on a tab sticking out the side, run wires to a connector mounted on the unheated base, and hope for the best. Drill a hole in the top aluminum plate, tuck in a thermocouple, and you’d be measuring something closer to the truth, too.

      • #7 by Jason Vreeland on 2012-01-06 - 12:21

        “Just the thought of sticking a flying cable carrying 120 VAC into the guts of that contraption scares the daylights out of me… you’re a braver man than I!”

        I run the supply side through a GFCI outlet and run the leads through a power cable. Not to mention the chassis and aluminum build plate plate is grounded. It’s fairly well protected, to my standards at least :).

        I’ve used them before as panel heaters and the adhesive back is what attracted me to it as a HBP solution. Peel the backing off, slap it on a piece of aluminum and Bob’s your uncle!

        I was looking at getting the HBP originally but, there were none readily available and the cost was some what close to the flex heater. All the other parts required I had laying around.

        What you are proposing poses much less risk to average Joe Maker and would certainly work as well!

  3. #8 by dnewman on 2012-01-06 - 10:27

    I’ve wondered if they (MBI) can get a little more life out of the pads by putting “nowhere” vias on them. That is, a via on each pad which goes to the top but doesn’t electrically connect to anything top side. It would be covered with solder mask on the top, thus electrically insulating it from the Al plate. (Of course, the Al plate could be notched/cut back there as well.) The via, with its plate thru and solder plug might then give the pad a little more grip to the board and perhaps reduce the likelihood of the all to common occurrence of the connector mechanically leveraging the pads right off the PCB. Such a change to the PCB — these vias — should require no other changes by MBI. Of course, a redesign using different parts is what is really called for.

    So as to minimize my handling of that connector, I did go with a pigtail arrangement. However, I figure that it will nonetheless fail owing to issues such as you have experienced.

    • #9 by Ed on 2012-01-06 - 11:24

      putting “nowhere” vias on them

      I suspect that the mechanical stress would just rip the vias apart, so they wouldn’t buy much. The whole thing cooks the PCB at temperatures pretty close to its glass transition point and the foil adhesive isn’t really intended to withstand mechanical stress, either.

      Getting that stress off the PCB and onto an unheated connector mount is what’s really needed.

      The recent spate of X-axis cable management clips on Thingiverse shows that plenty of folks are starting to worry about those connections…

      • #10 by dnewman on 2012-01-06 - 12:31

        Ahhh, the strain reliefs. That’s a whole ‘nother problem area if you need to close up your bot’s sides in the winter: having functional but small strain reliefs that do not collide with the enclosed sides. Doesn’t matter if you have the stock HBP/ABP wire harness connector to the front or to the right of the build platform. Either orientation is an issue if you’ve put a panel over that end as well…. I’ve made several minimal profile strain reliefs but never been totally satisfied. Just using kapton tape to stiffen the wires at the connector works for about a week at which point the adhesive fails and the tape opens up allowing the wires to swim freely under the tape…..

        • #11 by Ed on 2012-01-06 - 12:57

          close up your bot’s sides in the winter

          Or, sigh, in the Basement Laboratory, which is now around 57 F and falling…

          I put acrylic sheet over the rear and right openings (it’s an original TOM with the fan port on the left), held in place with blue masking tape. The front has a blue tape hinge on the top: it pivots outward when the HBP cables poke it. I remove that panel often enough for picture-taking sessions that a more permanent hinge didn’t make much sense, but one’s on the to-do list.

          Paper skirts around the filament spindle atop the printer seal the top and the interior stabilizes around 100 F, which seems Good Enough.

          But, yeah, having a slightly larger structure would be nice. Of course, then we’d kvetch about needing a bigger build surface and … round and round it goes!

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