Analog Angle Blog

Engineer’s ordeal with tiny PCB pads in a laptop repair

My low-end “traveling” laptop PC died after nearly five years. The symptom was obvious: it wouldn’t start up when I pushed the “soft” power button on the side. The fact that it had finally gone to laptop heaven wasn’t much of a surprise, actually. This unit was old by laptop standards and had been dragged around quite a lot, even though it was primarily for checking email and basic web work when away from my workhorse desktop PC. And I was careful not to save anything important on it.

However, the specific cause of death wasn’t so obvious. Perhaps the battery had shorted internally or there was some other hard fault in its ICs or connections. Even though such faults are hard to spot and harder to fix, I figured “what the heck, maybe there’s something I can see and do,” plus I was just plain curious.

A quick check on the web showed that it was fairly easy to open this unit—Dell Inspiron 11—just seven small screws on the bottom and that was it. Once opened, the actual cause of death was immediately obvious: the tiny power momentary-contact pushbutton—less than 2 × 2 × 1.5 mm high—was no longer attached to the small PCB on which it resided along with the up- and down-volume pushbuttons (Figure 1). Not only was this switch detached, but as it pulled off, it also tore off the nearly invisible PCB lands (pads) to which it was soldered. Reattaching this switch was not going to happen.

Figure 1 This 2 × 2 mm momentary-contact switch, detached from its host board laptop, disallows power-on booting up, and that was enough to turn the laptop into a brick. Source: Bill Schweber

By looking at the physical implementation of this design more closely, the true root cause was an obvious mechanical design weakness. The two tiny pads used for connections—and there was possibly a third dummy one as well—were also the sole support for the switch. The user action on this switch naturally pushes it sideways, which initiates a shear force, and it may also induce a rotational moment using the switch rear edge as a pivot (Figure 2). Alone or paired, these forces eventually broke the switch away from its minimalist moorings to the board, even though the switch actuator force is quite low. Materials science shows that even very small repeated actions do have a cumulative and often detrimental effect.

Figure 2 The sideways force from pressure of the user’s finger on the switch (via a small plunger) creates shear and even rotational stress (moment force) between the switch and solder pads, leading to metal fatigue, crack growth, and eventual separation. Source: Bill Schweber

I have opened up other laptops in the past to see how they are built. In one laptop, the switch was also side pushed, but it did have a support bracket attached to the PCB with a tiny screw. In another, the switch was mounted top-side along the keyboard, so pushing the switch induced a downward force onto the connections rather than a sideways one. In the latter case, I suppose the repeated depression cycles could flex the board and eventually induce tiny cracks due to metal fatigue. However, as the switch was in the corner of its board, the actual flexing would be a lot less than if it were sited mid-board or simply cantilevered.

I was actually surprised that the switch attachment lasted as long as it did, but I was also frustrated. To discard an otherwise still-useful laptop due to a barely visible and broken physical connection seemed very wasteful; it’s similar to tossing out a product only because its power-supply electrolytic capacitors have failed. A little more thought on the integration of the mechanical design with the electrical components would likely keep this laptop going for a while. Since this laptop is otherwise adequate for its intended application, it seemed a waste to toss it out solely due to a severed and non-repairable on/off switch connection to the PCB.

I was able to confirm that shorting the two-board tracks, where the switch was mounted, would replicate the switch action and turn the laptop on. But despite my soldering experience and suitable tools, there were only hair-thin tracks less than a millimeter apart and about one millimeter long available as solderable connection points. I wasn’t able to solder super-thin wires to them with the intention of adding an external pushbutton switch. Even if I succeeded, they clearly would not stay there for long. Yes, I did try.

I decided to take a chance and spend a few dollars to try to revive this laptop—even if only for a while—as I looked for a replacement. I ordered a replacement board with three tiny switches and attached thin, flat cable for under $20 from Parts-People, an Austin, Texas-based company specializing in Dell replacement parts (Figure 3).

Figure 3 This “simple” replacement board with three switches and tiny, 4-lead flat cable allowed me to revive the defunct laptop. Source: Parts-People

The good news is that after some delicate “surgery,” assisted by tips from Dell’s online service manual, and some luck, I was able to install the new switch board and boot the laptop. Even this took some additional finesse, as the alignment of the new switch board was not absolutely perfect with respect to the plunger. This kept the power switch continuously depressed, a situation which initiated constant shutdown/reboot cycles. I fixed this by shaving a fraction of a millimeter from the plunger tip using an X-Acto knife and new #11 blade.

Now I can look for a replacement laptop PC without “need it right now” pressure. However, I still wonder about the “engineering” of this unit. Is this marginal mechanical design only a problem on the lower-end units in a given family, meaning those laptops with less memory, slower processor, and lower cost? Or is it across all units in a product line and unrelated to the cost, but instead is just a function of a particular design team’s perspective and expertise? As I’d have to do a lot of laptop autopsies to find out, the answer will have to wait for another time.

Have you ever had a decent electrical design that was intermittent or unreliable due to marginal mechanical support because of your “blind spot” or overly optimistic assumptions? Was this a matter of cost pressure or was it just not understanding the use situation is the field? Have you ever been told to cut back on structural integrity and not to worry too much about it?

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