For back pain do i use heat pack or cold pack first?

Heat or cold: For acute back injury with tissue swelling, use ice to bring down the inflammation. But heat and ice both shock the body into breaking the pain spasm cycle. So both are useful for back pain. Heat relaxes the muscles, dilates blood vessels and helps bring more blood to the area along with oxygen and nutrients, and removes waste products, which help heal the tissue.

Often heat, but...: If you've acutely injured your back (strain, contusion/bruise, etc) then cold compresses for the first few days will probably help. If this is a chronic recurrent problem, pain related to muscle spasm and tension then often a heating pad, warm showers, massage can be helpful. Don't forget proper posture, gentle stretching and core strengthening as well.