My recommendation comes in two parts. First is bottom heat. Localized heating from the top heater, and from the local bottom heater if applicable, is causing thermal stress to the board.
It is important that you provide bottom heat to the entire board and that the board reach a uniform 150C or more (lead free).
Second is board support. It is critical to support the board in the immediate area of the rework site.
The bottom side support should not interfere with bottom heating but must be close enough to the site to avoid the board sagging under its own weight (something that is likely at these elevated temperatures).
VJ Technologies, Inc.
Don is the General Manager of VJ Technologies, Inc., a leading manufacturer of X-ray Inspection and Rework equipment for the electronics manufacturing industry. He has more than 20 years experience in development, manufacturing, and support of a wide range of capital equipment.
Board warpage is avoidable ... if a PCB can enter and exit a reflow oven without warping, the same should hold true for rework. Pre-heating is extremely critical in the process.
Here's the wrong scenario: If the PCB is not properly heated/saturated from the pre-heater, then as the top heat is activated, it is absorbed into the copper (thermal conductor) of the PCB as quickly as it's supplied.
As a result, the top heater is not heating the solder of the socket, it's heating the PCB. Therefore, the top heater is used more aggressively and the absorption rate of the PCB is compromised. That is what is likely causing board warpage.
Typically, in a lead free environment, top side heat does not begin until the board is approximately. 120°C. Then the recommended reflow profile from the solder paste vendor should be followed. The end process should look similar to the reflow oven profile.
Understanding the pre-heating phase could eliminate board warpage. If that doesn't solve the problem, take a close look at the nozzle design and make sure the airflow and heat uniformity are okay.
Other than having a specially designed nozzle to direct hot air/N2 to the solder, socket rework is no more difficult than reworking a BGA.
Neil O'Brien has worked in the field of electronic manufacturing equipment for over fifteen years and is currently Sales Director for Finetech, a manufacturer of precision rework systems and die bonders.