Bicycle Mechanics - Frequent inner tube flat

I have a mountain bike with original mountain bike tires. I use my bike mostly on asphalt / sidewalk.
My rear inner tube frequently flat (about every one or 2 months).
The holes/puncture/torn are in different places, but mostly in inner area ( against the frame ).
The flat varies, from small punctures to big tears. I checked the inside wheel area. There is nothing sharp, nothing protrude and the tape is not damage, the tape is made of cloth.
I am not sure where else to check. I never have problem with the front tire.
I need help please.

Thank you much.

My first instinct is to write this off as snake bites, (the tube getting crushed when the tire is folded back on itself. The cause is either lots of sharp obstacles, like rocks, curbs, deep potholes, etc. Underinflation is the major contributing factor, so the first step is to maintain higher pressure, especially in the rear. The rear wheel and tire sees rougher service for a variety of reasons. Among them is steering the front around obstacles while the rear goes through them, more axle weight, less or no suspension, and failure to shift weight off the saddle over hard bumps.

That was and is my first instinct, but there's something else I see from time to time, but that depends on the spicific rim/tire combination, so repost details of the widths of the tire and rim. Don't give me model info, I don't want to look things up. Instead measure the actula overall outside width of the tire and rim. If you don't have a caliper cut a gauge (U-shaped slot) out of a piece of cardboard, keep trimming until it's the same width as the rim or tire, then measure the gauge with a ruler.

What's the theory, FB?

Big tears (over 1/4 inch or so) are almost exclusively blowouts, caused by the tube escaping from the tire, so completely different cause than punctures or small tears. Blowouts are due to poor mounting, a weak tire bead, or a hole in the tire large enough for the tube to pass through. It is very important to determine very accurately what each problem in the tube looks like and where it is located - both in relation to rim, sidewall and road but also where in relation to valve stem, spokes, etc.

Move the tire to the front (swap them) and see if problem persists on rear or moves to front.

If the OP does that and the problem stays on the back it will have done nothing but eliminate the tire as the cause of the problem, and that is the least likely cause.

What's the theory, FB?

OK
This applies mainly to narrow rims, most often when combined with wide tires. A key factor is the inside width of the rim and the thickness (not width) of the tire at the bead. Thick beads on narrow rims leave only a narrow gap between them inside the rim, so the air chamber is sort of hourglass shaped with a circular section in the tire, the pinch, and another space below it within the rim.

When you inflate the tube, it first fills the space inside the tire, then the narrow section at the bottom blows down past the pinch filling the area in the rim. That means that this section of the tube may stretch 2-4 times more than the rest of the tube, hyperstretching it and often splitting or tearing it. You'll often see the results of this hyperstretching along the underside of the tube.

There's no easy fix, but these have been known to help.

Use the largest tube that fits, so there's more room to stretch
Rub talc on the inside of the tire, and outside of the tube, to allow the tube to shift more easily.
Inflate the tire only to a few psi and massage it to try to let the tube shift down.
Inflate the tube outside the tire to about double it's size (look for quality tubes that expand evenly, rather than crappy ones that inflate in small sections the way long balloon do). Let the tube stay this way overnight to relax the material a bit.
Inflate part way and leave the wheel alone for a while before coming to full pressure.
Hand stretch the tube, as many do with balloons to relax the material.
Warm the tube slightly (in the sun, or under a heat lamp) to make it stretchier.

I make no claims to precise science here, but I've helped lots of friends with similar issues, and with one or more of the above all have managed to put this behind them. Some may only be voodoo, but if it solves the problem I don't worry about exactly why or how. Part of me believes that the biggest factor is the tubes overall quality, and the rest may be like banging sticks together to keep the lions away.

Thanks FBinNY, I believe you have explained a something that I have seen but never been able to understand - splits in the tube next to the tyre, without visible abrasion.
I had wondered if it was chain oil leaking in and weakening the tube, or old or supercheapo tubes getting weakened at the lengthwise fold during storage or just failing at a manufacturing joint.

Back to baby powder for me then.

Thank you all for the responses. Really appreciate it. :)

Thanks FBinNY, I believe you have explained a something that I have seen but never been able to understand - splits in the tube next to the tyre, without visible abrasion.
I had wondered if it was chain oil leaking in and weakening the tube, or old or supercheapo tubes getting weakened at the lengthwise fold during storage or just failing at a manufacturing joint.

Back to baby powder for me then.

I forgot to mention that you can usually tell if this is the issue because there'll usually be sags, or other distortion from the over stretch running along the bottom of the tube. If you see this kind of stretching and have otherwise unexplained punctures or tears then your problem is what I've described.