I am going to Phnom Penh flying with Air Asia from Chiang Mai via Swampy in April. To avoid the long queues at Swampy could l go through Chiang Mai Immigration as a transit passenger on the outward flight and do the same on the return flight. Will Air Asia allow a straight through boarding pass even though it is 2 different flights with them. Thanks

If you have two separate bookings, you will have to collect bags in BKK and check in again, and go through immigration in BKK.

Air Asia is a point to point carrier so you have to collect bags at Bangkok and re check in again and go through immigration there.

If one has no baggage, just carry-on?

My guess would be the domestic Air Asia flight won't let you go through Immigration, and you'll get dumped out in the arrival hall, then go through the Swampy immigration for the international flight.

With carry on bags, you won't have to stop at the baggage pickup, go directly to the immigration queue.

You will have to go through Bangkok Immigration immediately followed by Security to even get into the departure hall for yor Phnom Penh flight.

Nothing can be done at the Chiangmai end.

^... so make sure you have a minimum of 4 hours between your arrival and departure at BKK; say 45 minutes from landing to getting your bags, 15 minutes to get upstairs and in the queue for check-in. That gives you the recommended 3 hours to get your arse down to the gate.

Chiang Mai immigrations only allow certain airlines as acceptable transit connections in Bangkok (i.e. allowing you to clear CIQ in Chiang Mai). You can see the list of approved airlines at the THAI check-in counter at Chiang Mai airport. I'm pretty sure that the airlines themselves need to make and pay for this arrangement. AirAsia is guaranteed to not be on the list...