Why precisely does the trackpad cable fail?

Just like a peeping Tom, there is a limit on what we can see through the window we are looking through into Apples world ;-}

Apple is not very open as they once were. Even the Apple Stores and authorized service centers Apple has are often kept in the dark! While some employees have a brain (and use it) Apple does what it can to dumb down the staff! Hardware expertise is within the service depot which sees the bulk of the repairs. Apple uses buyers to test their stores employees to make sure they don’t disclose anything to the customer here in the states (they can loose their job). Being in Australia gives you a bit more access as I’m sure Apple is not being as aggressive checking on their employees there.

OK, so what I can tell you is this issue is complex! Lithium Ion batteries breath like your lungs expanding and contracting as the battery is charged and discharged and at rest. All of the keyboard and trackpad/touchpad units face the same issue some more so than others.

The older models place the cable underneath and due to this design the battery would press against it like a vise. In your model the cable was placed on top with the idea the compression issue would be less of an issue as there is more free space to the bottom cover.

Now we need to add some grit (sand) into the equation as we all know it finds its way deep into areas which can be an irritant (bathing suit). Here the grit can wear the insulative coating on the ribbon cable and the grit can also be conductive enough to create a short! So over time with the constant cycles of the battery swelling up the cable will get damaged.

But wait! You said the cable going over the top didn’t face this! Well it does! When the battery gets older even it will press on the bottom cover and when the battery cell fails fully to the point of rupture it really gets pressed! But in normal conditions that’s not the issue and looking at your pics your battery is not puffy so we know thats not your issue.

But! expansion is still a factor here! In your case the length of the cable comes into it and how to apply the adhesive points down. The best way to think this is think how a balloon which has writing on it stretches out the letters as you inflate the balloon.

Here we have something that is not very flexible in its length so as the battery swells it can pull on the cable actually disconnecting it from the connectors! So the trick is making sure you mount the cable so there is slack on both sides. To help out place the cable (with the adhesive sections still covered in place) take a strip of tape on both the cable and battery so you can draw an index mark across at the ideal placement of the cable. Now carefully remove the adhesive covers placing the cable down so your index marks line up.

But I did jump over one step here! The folds! You don’t want sharp folds you want a nice radius at each bend. I use a cut piece of coat hanger (smooth and clean) to help me form the bends needed here (I use a BIC pen ink straw for the SATA cables).

OK, so thats what I can tell you on what I see and do. I do use the correct cable per the system. Some sellers cheat! They sell you the 15” cable for the 13” models (overly long) and might ship you the under battery version instead of the over battery version which is different too!

As to why you encountered a problem with haptic feedback. I’ve never encountered that with the cables I use. There is nothing magic here within the cable, it has no hidden chips. But, its wiring could be different depending on the series it was designed to work in. I’ve not made any study of the pinouts so I can’t tell you if that is what you faced here.

The last part of your question is this a design issue. I can tell you there is a slight length difference in the newer version (about 1.5 mm longer) of the cable so Apple did note the expansion factor tended to pull on the connectors.

I can’t blame Apple here as each system’s battery will expand at a different rate depending on how deep the battery was discharged and how much charge it needed as well as its cycle count. They did address the length issue quite quickly so people who hit the problem got the better cable. More often I saw the newer cable was not properly installed and the battery was already at the point of failure. I’ve also seen damaged connectors on the touchpad and the logic board. Often the act of removing the cable can damage it or the logic board connector if you are not careful!

@softwaredev - I'm aware on many of these comments. Sadly, some of it is in error. To start with pure Copper is very soft to make it stronger it's often hammered or drawn through dies as in the case of creating wire. Annealing is not really required as it does not get brittle (not drawn enough or alloyed with other metals to make it brittle). The issue with ribbon cables is a bit different!

Try this simple experiment: Take three exact same sheets of paper 2" by 8" Staple the short dimension of all three sheets so the sheets are in book form. Now roll the the sheets starting at the staple end. You'll note what started out as equal sheets are now changing as you roll and the tighter you roll! Now the gap between the sheets grows!

So what you have seen is the diameter of the sheets won't fit the same circumference as the sheet below it.

So why does that have bearing here? Well, the issue is the same when you start bending these ribbon cables unlike here in our experiment the sheets slide across the sheet below it, here the copper center sheet is adhered to the upper and lower insulating sheets, this creates stress when it is bent! So the copper kinks up just like how a copper wire when bent becomes narrower at the point of the bend unlike the wire which is much thinker the thin copper foil fatigues to the point of failure! Thats the killer, bending beyond what the copper foil can handle. That was the point in this paragraph:

“But I did jump over one step here! The folds! You don’t want sharp folds you want a nice radius at each bend. I use a cut piece of coat hanger (smooth and clean) to help me form the bends needed here (I use a BIC pen ink straw for the SATA cables).”

If you read many of the SATA cable issues you’ll see I’ve talked at length as being one of the P1 points! The other is isolating the cable from the rough aluminum uppercase using a strip of electricians tape.

Here the issue is different! the act of compression against grit or stretched pulling the cable out and/or damaging the connectors with the keyboard and trackpad/touchpad cables.

There is no stresses within the copper sheet traces only by our mishandling.

If you take the time to review how a logic board is made in mass production you can see ribbon cables are made in the same way.

References:

• How motherboards are made: a miracle of modern electronics
• Wikipedia -Flexible electronics