Surface Pro 4

The Surface Pro 4 is the fourth-generation Surface-series 2-in-1 detachable, designed, developed, marketed, and produced by Microsoft. The Surface Pro 4 was announced on October 6, 2015 alongside the Surface Book. In the U.S. and Canada, the Surface Pro 4 was released on October 26, 2015.

No Power

1. A bad Schottky Diode

If you made it here from a search engine, you’ve probably already seen this failure 10 times over.

A diode is effectively a one-way valve. It allows power to travel in one direction, and blocks it from traveling in the other. Unfortunately, a diode also causes a drop in voltage as power passes through it (known as the “forward voltage”). This is generally not ideal or efficient, and the voltage that is lost results in heat dissipation. This effect worsens the more current the diode passes.

Diodes also have a rated maximum current. Exceeding that current (say by overloading or short circuiting) can result in failure. Diodes fail in one of two ways. Either the forward voltage increases substantially (sometimes infinitely), or the diode effectively turns into a wire, allowing current to flow in both directions.

A Schottky Diode is a slightly “better” diode. These are commonly found in all kinds of electronics. Schottky diodes have a lower forward voltage compared to their silicon counterparts, are available with higher current ratings, are more efficient, and as a result allow for higher switching speeds.

The Surface Pro 4 has an abundance of Schottky Diodes on the motherboard. In reality, any of them have the potential to fail. In most cases though, one of the diodes pictured here will fail. These diodes allow either the battery or the charger to power the device. The use of diodes means that whichever input is higher voltage will feed power to the unit, without feeding power backwards into the other input. For example, with the charger connected, the charger will power the unit, without feeding the battery and potentially over-charging it. Conversely if you leave the charger connected but not powered on, the charger won’t cause the battery to drain.

If you have a lower end Surface Pro 4, you may find that the “charger” diode is not populated. This is because the dedicated charging IC has enough capacity to charge the battery, while the battery simultaneously feeds power into the motherboard. In either case, a failure of the diodes will present with the following symptoms:

• Battery: A failure of this diode will result in the device immediately turning off when the charger is disconnected. This won’t effect charging the battery, or the battery reporting its state to the operating system, but will prevent the device running from the battery. In the case of a lower end unit (without the “charger” diode), the device will not function at all.
• Charger: A failure of this diode (on higher end devices that have it) will result in the unit running from the battery, but not the charger. This may mean the device says it is charging, but the percentage decreases as it gets used. Alternatively (depending on the mode of failure), the device may not charge at all.
• Spare: Funnily enough, on all of the devices we have seen so far, Microsoft was kind enough to provide us with a spare Schottky Diode. On the boards we checked, both sides of this diode connect to ground, and as such, it does nothing at all. We aren’t quite sure what happened here. Either there are variations of the SP4 that have this diode in use, or the part was accidentally left in the pick and place queue and nobody noticed. In either case, it’s handy to know in the event of a single diode failure, there’s probably a spare waiting for you in the device.

Testing the Diodes

So if a diode is bad, how can we tell?

With the battery isolated (this is important) use a multimeter in forward voltage test mode (diode mode).

Measure across the diode leads. You should have no more than 0.250V forward voltage, and no less than 0.100V.

With the probes switched around, you should get a very high voltage (like 2.800V), or “OL“.

If you get close to 0.000 in both directions, your diode is shorted and needs to be replaced.

If you get a high reading or “OL” in both directions, your diode is blown and needs to be replaced. A burn or hole is also a pretty good indication.

To replace the defective diode(s), you may be able to use the “spare” that we have noted above.

2. A bad Fuse

If the Schottky Diodes are good, or you replaced them both and the device does not work as expected, the next thing to check is the “fuses”. There are actually a number of these on the board. So many that we can’t list them all. We’ve circled the most common “fuse” to cause no power.

There are 3 variations of these fuses we have seen. In most cases they are a 0 ohm resistor, either marked “0”, or “000”. We still call them fuses, but in reality, they are 0 ohm resistors, aka “links”, aka “jumpers”. Less commonly, Microsoft chose to populate these with actual fuses, typically marked P or L.

If you locate the fuse we have circled, whatever it looks like is likely going to be the same for all the others.

Testing the Fuses

If you’ve ever used continuity mode on your multimeter, you will be quick to grab it out and check for the “beep”, indicating a connection.

Before you do though, be warned that this method is NOT fool proof, and may lead to misdiagnosis.

While a lack of continuity would certainly indicate a “blown” fuse, a 0 ohm measurement or “beep” certainly does not indicate a working one. This actually applies for all kinds of devices, and we learned this the hard way on iPhone Backlight fuses many years ago.

To correctly test the fuse, you need to pass some current through it. Your multimeter uses a very small amount of current to test for continuity, and it isn’t enough. There is a common failure mode where the fuse will appear to have continuity, until you try to pass current through it.

To confirm operation of the fuse, we set our DCPS to 1V, and 0.05A (50mA) to 0.10A (100mA).

The voltage here really doesn’t matter, but its a good habit to use less than the circuit is designed for. This prevents damage to components should you accidentally slip and power the circuit.

• With probes connected to the DCPS, we verify the current limit by touching the probes together. The “CC” (Constant Current) LED on your DCPS should illuminate.
• Now, place your probes either side of the fuse. The DCPS “CC” LED should illuminate, and the voltage should drop down to 0V. If it does, you have confirmed your fuse can pass current.
• If the “CC” LED does not illuminate, if it doesn’t stay illuminated, or if your voltage does not drop to 0, your fuse is not working properly.

To replace the fuse, you can use a 0 ohm resistor, an iPhone/iPad backlight filter/fuse, or even a piece of wire (although we would advise against it).

3. Damage from improper battery/logic board installation or removal

The Surface Pro 4 uses a flat flexible cable with exposed contacts adhered to the rear case to connect to spring pins on the back of the motherboard. If the motherboard is installed or removed in a way that causes the positive power to be bridged to the communication SMBus pins, the device will no longer power on or charge. Replacing the battery charger IC with another does not appear to resolve this problem. Further troubleshooting has not been performed on this particular unit, but it should be noted that before the motherboard is removed, it should be tilted vertically without sliding it or angling it in the xy plane, and a thin insulating sheet inserted carefully to insulate the power terminals, being careful not to bend the spring pins behind the motherboard.

Where to from here?

If it’s not the Schottky Diodes, and it’s not the fuses, what’s next?

Unfortunately, this is where the availability of information on the internet ends. It would seem that without schematics, nobody has worked out (or documented) what causes no power if its neither of the aforementioned.

We had a few devices that fit this criteria, so we set to work reverse engineering them to find out why.

To begin with, we connected the device to a DCPS directly and prompted the SP4 to boot with the power button. We figured if there is a short somewhere, injecting power and looking for heat dissipation is a good way of finding it.

We got no response at all.

This is unusual. Without schematics, we don’t really know what is responsible for the power on sequence, what IC is responsible for what rails, if any of our rails are powering up upon prompt to boot etc.

We set our sights on the power button. The first step to powering on the device, is obviously here. We were expecting to find a regulated 1.8V, 3.3V, or Battery Voltage at the power button. This is because the power button either needs to start “HIGH” and get pulled “LOW” (power button GPIO has a pull-up resistor), or start “LOW” and get pulled “HIGH” (power button GPIO has a pull-down resistor).

Upon probing around, we find that we have none of the above. Instead, we have a curious 0.157V. That’s bizarre. We appear to have located an issue.

Now to find out whats causing it..

Luckily, we have multiple devices in our queue, some of which are now working. We quickly probe the power button on a working device and find that we have 3.3V. Now we need to find out where this 3.3V rail goes, where it comes from, and why we don’t have it on some devices.

We found a 3.3V regulator on the board, but unfortunately, its generating 3.3V no problem. There must be more than one. We started probing around and found our mysterious 0.157V in multiple locations, next to multiple IC’s.

The secret answer. A missing 3.3V rail.

While investigating our missing 3.3V rail, and mapping out all of the locations we have a mysterious 0.157V, we noticed a suspicious mark on the IT8528VG.

Just next to this is a capacitor where we expect to find 3.3V, but instead have only 0.157V

We cleaned the IT8528VG to see if it was an impurity sitting on the surface. Nope. That’s a very, very small hole. Huh…

At this point, we were thinking this IC might be responsible for our 3.3V output. A hole, and subsequently internal damage, would certainly explain why the output is low. We removed the IT8528VG and tested again, expecting to have 0V. Our 3.3V is back!

It turns out, the 3.3V rail is actually incredibly low current. So much so that a short on the rail, or excessive current draw, doesn’t appear obvious when powering the device from a DCPS. We assume this rail is a reference for digital I/O. We still don’t know where it comes from, but we now know why it was low. Our IT8528VG has failed, and developed an internal short.

We just happened to have a donor device, so we promptly pulled the IT8528VG from it. Without a stencil, we had to manually re-ball the IC.