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Part Removal

The objective of this step is to remove the component while minimizing the impact to the PCB. The most common PCB defects that should be avoided include lifted pads and site warping.

The basic requirements of this operation are the following:

  • Preheat the entire PCB between 75º C and 125ºC
  • Insure that all joints have a temperature greater than 190ºC
  • Make sure that all joint temperatures are less than 220ºC.
  • Insure no solder is smeared on the part bottom or edge which might aid in part reclamation
  • Maintain adjacent component temperatures to less than 180ºC in order to minimize the heat impact on these parts

*Note these temperatures assume standard Pb/Sn soldering temperatures

Course: BGA Rework - A Primer - Removal

Develop Profile

Thermal profiling methods.
Several Profiling Methods

There are several methods available to correctly identify and build a PCBGA rework profile. The method used is a function of the availability of a sample or test profile PCB and component to be reworked. One method assumes that a profile PCB and part is available in order to establish the BGA rework profile, while another assumes that no dedicated profile PCB exists. Whatever the method used it is critical to read amongst other temperatures the BGA solder joint temperature. Floating air thermocouples or thermocouples attached next to the part being reworked are NOT accurate solder temperature indicators rather a temperature probe installed inside a solder ball to be reflowed is the proper location.

Profiling Using a "Test" PCB

A thermocouple embedded in to the solder ball is the preferred profiling method as it provides the most accurate BGA older joint temperature readings, serves to reduce the chances for damage to nearby components on the boards being reworked, while reducing the chances for board damage or warpage. In this method the thermocouple is attached to the solder ball either on the side of the BGA ball using conductive epoxy or into the BGA joint by drilling through the backside of the BGA and filling it with thermally conductive epoxy. The thermocouple is then properly routed between the balls or on the underside of the PCB.

profileFigure 4 Representative Profile of Hot Air BGA Rework System

Profiling Without a Dedicated Profile PCB

In this method it is critical to measure the temperature under the center of the BGA to be reworked. Insulated thermocouples are pushed underneath the center of the BGA to approximate joint temperatures in order to formulate the best BGA rework profile. It has been shown that thermocouple readings are affected by hot air nozzles of rework stations if they are placed near the outside rows of the of the solder joints. Thus the center BGA solder joints, which tend to be lower in temperature than the outside rows, need to be measured. In this technique the BGA rework profile is established during the removal process.

In both profiling methods there are several points which should be monitored:
  • BGA part center joint temperature
  • BGA part edge joint temperatures
  • Adjacent component nearest body temperature (if applicable)
  • PCB temperature

SOLDERING TIPS

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