All the international border crossings should be accesible in that direction.
You write "Northern Thailand" but be aware that in Thailand they are dividing up that area by northern and northeastern.
There are 8 interenational border crossings between Laos and Thailand.
In northern Thailand there are two provinces that borders Laos namely Chiang Rai and Nan.
Chiang Rai have one border crossing - the one Hemuli mentioned: Chiang Khong - Huay Xay. A much used border crossing.
Nan has also one border crossing to Laos namely Muang Ngeun/Huay Kon. A little used border crossing, but I have been using it once and it was completely hazzle free and easy.
In northeastern Thailand (very often called Isaan) there are 6 more border crossings.
Here is a list from Wikitravel of the different border crossings Laos - Thailand:
There are Eight border crossings open to all between Thailand and Laos. From north to south:
Huay Xai/Chiang Khong: Fourth bridge under construction. Usual route to/from Luang Prabang, easy bus connections to Chiang Rai and points beyond on the Thai side.
Muang Ngeun/Huay Kon: Visa on arrival. 40 km from Pak Beng.
Nam Hueng/Tha Li: Easily reached via Loei on the Thai side, but 378 km of dirt road away from Luang Prabang. No visa on arrival.
Vientiane/Nong Khai: The first Friendship Bridge and the busiest of crossing of them all. Direct trains from Bangkok now available.
Paksan/Bueng Kan: No visa on arrival.
Tha Khaek/Nakhon Phanom: Third bridge under construction.
Savannakhet/Mukdahan: The Second Thai-Lao Friendship Bridge.
Vang Tao/Chong Mek: On the route from Pakse to Ubon Ratchathani
http://wikitravel.org/en/Laos
The Vientiane - Nong Khai bc is the main border crossing between these countries. Can be very busy at times.
In the south of Laos you have the Vang Tao/Chong Mek bc which also is a main bc between these two countries. Comes natural to use this one if you intend to drive Laos from north to south. But you will be entering Thailand at well....not exactly the most intersting spot.