The symptoms are all too familiar - the buttons of a remote control require very hard presses to be recognized, and the problem only gets worse with time. Many, many, millions of remotes are replaced and/or discarded because of this problem. Upon investigating the cause, I found the presence of an oily substance between the rubber keypad and the printed circuit board (PCB) it makes contact with. At first, I thought it was the result of a spill or perhaps even a buildup of hand oil (and there are technicians who can never be convinced otherwise). However, the oil did not seem to be petroleum-based as it was highly resistant to detergent. It reminded me a lot of DOT 5 silicone brake fluid. And in fact, that is exactly what it turned out to be - silicone oil. Here is a quote from section 7.0 of the Silicone Rubber Components Manual of the Danish company J.D. Friderichsen A/S (which does not exist any more as it was purchased by the Danish company Fritz Schur Teknik in 1999, and the manual is now available only as an internet archive), which among other things, manufactured silicone rubber keypads:
It is a matter of confidence to buy silicone rubber components, because the purchaser has to has be certain that the silicone oil is baked out of the keypad, in order to ensure that quarts don't form on the circuit board and thereby disrupt the connection. Most of us have probably had this experience with a remote control at home. (However an incident such as this can also happen in situations where the OEM are using conductive ink instead of contact pillars in order to reduce the cost). The only way to control if the silicone is baked out properly is to check if the keypad has lost some weight after it has been baked. The current problem is that many far east manufacturers are used to manufacturing components for cheap products such as one dollar calculators, and are having difficulties recognizing the requirements set by western manufacturers. They might not "forget" it for the first few supplies, but maybe later. The result will surface a few years down the line, so you are required to know your supplier well. It is equally important that the printed symbols is baked into the keys, to ensure they don't wear off. A large variety of qualities are available to the buyer on the market, and you will probably experience that our price is DKK 0,25 higher than our competition. You are however welcome to test our quality with an eraser.
I posted about this issue on December 8, 2000 in the thread "Do remote keypads sweat silcone oil?" (dang it, I misspelled "silicone" in the title) to the Usenet group sci.electronics.repair and have received email about that post years thereafter. The solution is easy enough - to clean and degrease the internals of the remote. And this will have to be repeated every few years, although the interval will become longer as the amount of oil trapped in the rubber keypad decreases. The remote for my old Dish Network 3000 satellite receiver (shown at the upper right) was a major offender and required cleaning every year. My more recent Dish Network 3900 remote, requires cleaning every 3-5 years. Based on my experience, most remotes with silicone rubber keypads cannot go a decade without needing to be cleaned. The pressure generated by pressing a button rather than time itself appears to be the trigger by which oil is released. So, all things being equal, a remote that is used more often releases more oil. In theory, it should be possible to bake the oil out yourself as silicone rubber can take high temperatures (spark plug boots are made out of it). However, I don't know what temperature is needed or how well the conductive rubber contacts can withstand this heat without an oxygen-free environment.
Now, it is true that silicone oil contamination is not the only cause of unreliable remotes. Other common problems include bad solder connections (especially at the LED and battery terminals), worn contacts on the keypad or PCB, or microbreaks of the PCB traces. While you have the remote apart, be on the lookout for these other problems. If spotted, bad solder connections are easy enough to fix by reflowing the solder with a soldering iron. Broken PCB traces can be really tough to find, but fortunately this normally does not happen unless the remote has been abused. Worn keypad contacts are tough to fix. There are conductive rubber repair kits for this purpose, but they can involve careful cutting and gluing. Sometimes fine (1000 grit) sandpaper can be used to clean oxides off of the contact surfaces, but it is all too easy to destroy the conductive rubber contacts and does not seem to be necessary, so I don't recommend this. You could try it on stubborn keys if you've tried everything else and have nothing to loose if it ruins the remote.
Figure 1: An open Dish Network remote showing silicone oil.
Figure 2: A closeup view of the oil.