Can anyone give me a ran down process on CSP? Is it like BGA process that needs reflow? what oven do I need...vapor, nitrogen or convection? is under fill a necessity? when and why do i have to under fill? when not to underfill and why? etc....
CSP is similar to BGA process in that you need to undergo a reflow process to properly solder the chip. Also, a soak period is desirable to promote outgassing.
You do not need nitrogen. We are building CSPs in a regular convection oven without nitrogen.
Underfill is a tricky one. It is generally used to strengthen the bond of the component to pcb. Often CSPs cannot withstand the thermocycling process for as long as other components, and underfill helps to stabilize the joint.
The "do I" or "don't I" or underfill is up to you. Here are some questions that might help you decide: Are you building to a certain customer spec.? Or industry standard? Is this small quantity or large? What is the expected lifetime of the card? Or are you simply building a card for an in house process?
I suggest you read some of the previous postings on SMTnet. Also, the magazine Advanced Packaging had an article on BGA, CSP, and flip chip underfill that might help in June 2002. http://www.apmag.com I'm sure some of the other industry magazines also have informative articles.