[Android Q&A, Help & Troubleshooting] harddisk_wp:
So, here is the firmware flash guide for B15Q. It assumes either a Win7 x86 machine or a x64 with driver signature verification disabled (but Win8/8.1 in any variant or W7x64 is NOT RECOMMENDED).

I assume no warranty for bricked devices, especially not if you manage to kill your PRELOADER or DSP_BL. Double and triple check before downloading.

0) Driver and toolkit setup

Get the driver set, scatter file and spFlashTool from this thread: http://xdaforums.com/general/general/stock-rom-cat-b15q-rom-development-t2988774

Remove back shell from phone, remove battery (!)

Attach phone via USB. Windows should now, approx. once every 2-3s, make a sound similar as if you put in a USB stick and then pull it out again.

Start=>Run=>devmgmt.msc, in the View menu choose "Devices by connection"

Click yourself through the tree until you find an unknown device (MT65xx Preloader or similar)

Rightclick on the device and press "Install drivers"; you shall find the drivers in "MTKUsbAll_0.9.2\New inst. win 7&8x64" folder.

Launch "SP Flash Tool v5.1352.01\flash_tool.exe"

In the tab "Download", click on the "Scatter loading" button and select the downloaded file "MT6582_Android_scatter.txt". spFlashTool usually remembers this across restarts.

Unplug the phone at the computer side if you want to work with it later (the MicroUSB ports don't like too many inserts, they wear out physically).

A) ReadBack for backup of existing firmware/userdata

This will involve a ****load of typing for the first readback.

Open the scatterfile using Notepad++ or any editor capable of handling UNIX line endings, NOT notepad

You will see a lot of blocks like

- partition_index: SYS0
partition_name: PRELOADER
file_name: preloader.bin
is_download: true
type: SV5_BL_BIN
linear_start_addr: 0x0
physical_start_addr: 0x0
partition_size: 0xC00000
region: EMMC_BOOT_1
storage: HW_STORAGE_EMMC
boundary_check: true
is_reserved: false
operation_type: BOOTLOADERS
reserve: 0x00

In spFlashTool, select the "Readback" tab.

For all the blocks you see in the scatterfile (IDs 0-23), repeat the following:

Click "Add"

Double-click on the newly appeared row

Choose a filename (e.g. preloader.bin for the first block; some blocks will have name = NONE, use the partition_name here for the filename) and click SAVE

A window "Readback block start address" will appear

Choose Type = "hex". Copy (Ctrl+C,Ctrl+V, and take care to exactly select the hex value! Do NOT type the hex values by hand!) the value from linear_start_address (with the 0x) into the "Start address" box and the partition_size value in the "Length" box, press OK.

Unless you are at the SYS23 partition BMTPOOL, proceed to the next block, repeat from above.

The SYS23 partition can not be backed up, because it has invalid lengths. Do not enter it into spFlashTool.

CHECK THE VALUES FOR START ADDRESS AND LENGTH IN THE TABLE. CHECK THEM ANOTHER TIME.

Press "Read back" button at the top.

Plug in the phone with the battery removed(!) and wait. This process will take time and consume approx. 3-4GB of disk space.

Archive the files you created somewhere safe. Do NOT distribute anything to other people except the BOOTIMG, RECOVERY and ANDROID partitions, because the other partitions contain data that is hardcoded to your board (e.g. IMEI/MAC addresses, sensor calibrations,...) or your private data and app data (SYS22/USRDATA).

If you want a full backup to disassemble by hand lateron (aka you trust in nothing going wrong and don't want to do the hard work except when you need it), just create one readback section with start=0x0 and length=0xE5720000 - this backups everything in one file.

B) Download for flashing new firmware

In spFlashTool, select the "Download" tab

In the dropdown box below the scatter file, keep it at "Download only" or set it if this is not the case

Uncheck all the boxes in the table

Double click on the "Location" column of the BOOTIMG, RECOVERY, ANDROID or USRDATA rows which you want to flash, select the appropriate image file.

Check that you selected the correct images for the correct partitions!

CHECK THAT ONLY THE ROWS WHICH YOU WANT TO FLASH ARE CHECKED. ESPECIALLY, NEVER EVER CHECK THE PRELOADER, MBR AND EBR PARTITIONS. YOU HAVE BEEN WARNED.

Press Download

Plug in the phone with the battery removed and wait. spFlashTool will tell you when it's done.

Unplug phone from computer (!) and put in the battery.

If you get an error "PMT changed for the ROM; it must be downloaded", reboot your machine.

When you have a running ROM on it, you may also try to keep the battery in the phone during flashing; just press Download on the PC, then select Shutdown in the Android menu that appears after holding Power pressed. Once the phone has shut down, it will vibrate shortly and then be detected by spFlashTool. I also recommend using a high-quality USB cable and not a worn out one because USB cables with worn out plugs may lead to issues during transmission.