Description
Matrix keypads are  kinds of keypads usually seen on cell phones, calculators, microwaves ovens, door locks, etc. 
However, in DIY electronics, they are mostly used to let users interact with your project .They are often used to navigate menus, punch in passwords and control robots.
They do come in sizes of 4×3, 4×4, 4×1 etc. Regardless of their size, they all work in the same way. They come with an adhesive backing so you can attach it to nearly any surface. You just have to pull off the backing paper  and then you place it on the surface.
 
Specifications
Pad Size: 77 x 70 x 0.8mm
Transparence cable length: 86mm
Connector: 7 pins (Pitch 2.54mm)
Mounted: Adherence
Maximum Circuit
Rating: 35VDC, 100mAa
Insulation Resistance: 100M Omh, 100V
Dielectric Withstand: 250VRms (50-60Hz, 1min)
Contact Bounce: <=5ms
Life Expectancy: 1 million closures
Operation Temperature: 0 to +70 centigrade
Humidity: 40 centigrade, 90%-95%, 240 hours
 
Arduino Interface