Pad printing, referred to by Cherry as Tampo printing after the original manufacturer's name, is by far the most common form of keycap printing. It is less expensive but also less durable than other methods.
Pad printing has been the most popular form of applying legends to keycaps since the 1990s. Pad printing uses a rubber pad to transfer ink from a printing plate to the keycap. This method has the advantage of being cheap and flexible (the process can be adapted easily to print any character in any colour required) but the ink is prone to wear. Manufacturers will often coat the ink with a thick transparent coating, but this does not prevent legend wear.
Advantages and disadvantages
Pad printing is primarily chosen for its low cost. Its machinery, being similar to traditional print machinery, is less specialized and expensive than that for other methods. It is less complex, faster to set up and use, and requires less training and experience.
Pad printing can imprint any colour, on any part of the keycap, on any colour of plastic. For this reason, it is often used to add secondary colours to double-shot keycaps, to avoid the cost and complexity of triple-shot moulding (although triple- and quadruple-shot caps do exist).
Pad printing also allows more detailed typography than can be achieved with moulding. For example, it enabled Diatec to use Futura as the font for their Filco keyboards. Dye sublimation also permits such typography, but it is limited to creating dark characters on light-coloured keycaps.
The ability to print in any colour and in fine detail also gives pad printing an advantage over laser-based methods. Historically, laser marking could not achieve white legends, although this is now possible.
Pad printing is less durable than most other key-inscribing methods. Because the ink doesn't penetrate the key and become part of it, pad-printed legends can be worn away by abrasion.
On some keyboards, only the legends on a few most-used keys may become worn. Other keyboards may end up with more legends missing than present, or with no significant wear at all. This depends on:
- How much the keyboard is used
- The hardness and thickness of the ink and protective coating (if present)
- The quality of the ink's adhesion to the key material, and of any coating to the ink
- The user's typing habits (lighter/stronger touch; amount of friction when pressing; tendency to rest hands on keys when not typing)
- The user's skin chemistry (more acidic skin can accelerate wear)
- The cleanliness of the user's hands
Since dust and dirt are abrasive, most users can avoid unnecessary wear on pad-printed legends by washing their hands before they begin typing.
Some pad-printing is thick enough to be felt. (The legends on Filco Majestouch keyboards can be felt through the coating that covers their keys.) Some users find this uncomfortable.
Pad-printed keyboards from the 1990s often used a thick, gloss ink with a raised look and feel; examples include Chicony, NTC and Datacomp. Uncoated legends on some modern keyboards, particularly E series Dell Latitude laptops from the 2010s, are similarly thick but not glossy. According to Tai-Hao, this was simply "normal ink"; the specific composition remains unknown. Tai-Hao expected such legends to last at least 2–3 years.
Datacomp DFK192 legends (1991); the thick, glossy nature of the ink can be seen
NTC KB-6251EA legends (1990)
Clear-coated thin ink
Most people's perception of pad printing is the so-called "decal" look found on most modern desktop and laptop keyboards. Despite the appearance, the letters are not decals; after the legends are printed, a clear coating is applied over each legend, and this coating follows the contours of the legend. Signature Plastics refer to this as "selective image" coating. Clear-coated legends have been the norm since the 2000s, but it was already in use in the 90s; the specific chronology of this change is not yet documented.
Tai-Hao offered two possible explanations for their change from thick ink to clear-coated thin ink as of the 2000s:
- Their ink supplier changed the ink formulation to meet RoHS legislation requirements, leading to a less-sticky ink
- RoHS legislation mandated a change in raw materials for the keycaps themselves, and this may have led to reduced adhesion by the ink
Some keyboards use all-over coating to reduce or hide the presence of the clear coating. The most notable example is the Filco Majestouch line from Diatec; with these keyboards, the underlying ink is still laid on thick, and can be felt under the coating. Cherry G84 Series keyboards are amongst the few pad-printed keyboards where the lettering does not have a raised feel at all.
Cherry G84-4400 pad-printed legends (1999)