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I recently watched a Xbox Series X PSU repair video on YouTube and was surprised by the fact some solder joints have a star-like shape :

enter image description here

Is there any specific reason to do this ? This is the first time I see this. My guess is those joins are stronger than typical round shaped joins (which might break and create false contact after a very long time) but maybe it's something else.

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    \$\begingroup\$ If the goal was strength I don't see why you couldn't just make a larger round joint instead of a star. \$\endgroup\$
    – DKNguyen
    Jan 31 at 22:40
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    \$\begingroup\$ I think it has to do with some sort of thermal relief for soldering. A pad as big as the outer edge of the stars does not lend itself to automated soldering techniques, like solder reflow. \$\endgroup\$
    – SteveSh
    Jan 31 at 23:28
  • \$\begingroup\$ "false contact" - what's that? Did you mean "intermittent"? \$\endgroup\$ Feb 1 at 15:43
  • \$\begingroup\$ @DmitryGrigoryev : yes I mean a broken join due to many heating and cooling cycles. \$\endgroup\$
    – tigrou
    Feb 1 at 22:54
  • \$\begingroup\$ @tigrou Now you obviously mean "joint", not "join", but I suppose I should stop nitpicking. \$\endgroup\$ Feb 2 at 10:09
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This patent from 2005 claims the idea (star shaped solder pads) and gives the motivation to be the desire to simplify the manufacturing process for boards that require both SMD and leaded components. Specifically, this seems to be a pad stencil optimization for the pin-in-paste*** reflow process for leaded components.

The SMD components are soldered with a paste and reflow process but the leaded components are traditionally done with a wave soldering process. The pin-in-paste process allows leaded components to be pasted and soldered together with the SMD components in a single reflow, eliminating the need for a second wave-soldering step.

When preparing a board for pin-in-paste the stencil needs to be enlarged - bigger than the solder pad - to accommodate enough paste as the leaded components require significantly more solder than SMD devices (since the solder wicks onto the lead and down into the through-hole). Other techniques are double-stamping the paste to force more of it down into the through-hole. The star pattern may just be a way to lay out the paste in an optimized way to shave down the total amount required, cutting costs.

The star shape may encourage the solder to pull under surface tension towards the lead and through-hole in the centre, allowing a thick stack of paste out to the points which largely is pulled towards the center, minimizing the spread after melting (a large circular pad would pull more solder out to the edge of the stencil, wasting material).

It's difficult to tell whether this board was soldered with paste and reflow or traditional wave, but if the former, this is one possibility.


It's also possible that these stars are there to augment the current carrying capacity of the pour layer underneath, particularly around vias and constrictions where the current density would otherwise become too high. This may afford the designers the ability to use a thinner copper layer (reducing costs) by adding these small enhancements in key locations where current concentrates.


[***] No affiliation. Just a convenient link.

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    \$\begingroup\$ THIS really seems the real answer. Good job finding that patent. \$\endgroup\$ Feb 1 at 14:13
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    \$\begingroup\$ @zebonaut The "red dot" epoxy is also used in double-sided reflow to hold the parts on the bottom while the top side components are being placed. \$\endgroup\$
    – J...
    Feb 1 at 14:20
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    \$\begingroup\$ @zebonaut If the spikes are there for arc control, as you suggest, why would the DC side of the PSU also use the same technique on the leaded components? \$\endgroup\$
    – J...
    Feb 1 at 14:42
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    \$\begingroup\$ @zebonaut If you can defend that opinion I'm all ears. An XBox console is a loss leader - cost cutting is paramount. Eliminating wave soldering from your manufacturing process is a big way to lower costs. \$\endgroup\$
    – J...
    Feb 1 at 15:03
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    \$\begingroup\$ @zebonaut I'm still not entirely convinced. Wave soldering is hard on SOIC parts, and moreso now that everything is ROHS compliant (lead-free solder drives the working temperature up). I guess if anyone would take a board like this to 100% reflow I'd expect it to be something just like this XBox that is shaving every possible cent from the manufacturing cost. To be fair, I don't properly know the answer here either. \$\endgroup\$
    – J...
    Feb 1 at 16:15
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These are drainage shapes or solder thieves.

This patent shows something similar, as does the picture in this question. Maybe someone found the stars to work better during experiments, maybe someone just wanted to avoid the patent.

This is an area of the board where isolation matters, probably the power supply. There are even some spark gaps built using the available material (PCB traces) without additional cost, maybe underneath a common-mode choke, as they are often seen in the mains input filter (three pointy edges on opposite sides). These spark gaps do the exact opposite of the stars around the component pins: They are designed to increase the field when high voltage transients are present and the isolation gap is supposed to break down in a controlled fashion.

The stars' edges, on the contrary, do not point towards nearby traces or components.

When you look carefully, you can see how the stars' edges point towards wider areas of their own traces and not towards the designed clearance or creepage spacings or other traces. This is done to nudge the solder away from any isolation spacings.

When soldering through-hole components in what is called wave soldering, sometimes some solder will form edges or spikes when cooling off and becoming solid. To some extent, you may have noticed this effect even while hand soldering some joints. The keepouts in the green solder mask try to make the solder stick anywhere but in the designed isolations spacings.

I guess some clever process engineers have come up with this idea and have noticed how it can increase the yield - i.e. reduce the number of components that fail the visual inspection (or automated optical test) after the soldering process or even break down in the high-voltage isolation test that is done with the finished product, at least for the primary-to-secondary isolation (mains input to any low voltage circuits the user may touch).

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    \$\begingroup\$ When you refer to "isolation", are you strictly speaking about electrical isolation, or more than that? \$\endgroup\$ Feb 1 at 7:27
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    \$\begingroup\$ From question: "Xbox Series X PSU" - I'm pretty sure PSU here stands for power supply unit. \$\endgroup\$ Feb 1 at 12:10
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    \$\begingroup\$ The first paragraph here is nonsense - No way does that PCB need to care about sparks from every single connection. \$\endgroup\$ Feb 1 at 13:31
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    \$\begingroup\$ Agreed with @MikeBrockington - there is nothing in an XBox power supply anywhere near the voltages that would require field mitigation and arc suppression techniques. It's just a mains voltage to DC switching supply. Spark gaps and arcs are a problem for high voltage systems - this is not a high voltage system. Even a hi-pot test only goes to 1kV. \$\endgroup\$
    – J...
    Feb 1 at 13:42
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    \$\begingroup\$ @zebonaut Nonsense. The spikes, if nothing else, would increase the risk of arcs since they naturally create field concentrations. Suppressing arcs is done by making exposed surfaces as smooth and un-spiky as possible. Where there's a real risk, encapsulants like corona-dope, etc, are used. The spark gaps you're showing have spikes specifically because they will encourage the arc to form at a lower voltage, snubbing the hi-voltage surge more quickly. \$\endgroup\$
    – J...
    Feb 1 at 14:12

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