By Zhen Feng, Juan Carlos Gonzalez, Sea Tang, Murad Kurwa, Evstatin Krastev, Nordson Test & Inspection.
Figure 1. BGA DMN-8802 dual encoder processor.
The electronic manufacturing industry needs non-destructive techniques to identify BGA opens and cracks. Currently X-ray and time domain reflectometry (TDR) are most widely used.
This whitepaper reports the results of a comparison of the following techniques: TDR, automatic X-ray inspection (AXI), transmission X-ray (2DX), cross-section/SEM, and Dye & Pry. The first three are non-destructive, while the cross-section/SEM and Dye & Pry are destructive techniques.
A correlation among the various techniques in finding opens and cracks in BGA joints were looked at. The experiment included:
• Testing thirty pins on a particular BGA from ten different boards using TDR, AXI, and 2DX.
• Further examining eight of those boards with cross-section/SEM, and the remaining two boards with Dye & Pry.
These studies helped gain a good understanding of TDR, AXI, 2DX, cross-section/SEM and Dye & Pry techniques.
Introduction
More BGA and area array devices are appearing on PCBAs as product/functional complexity increases. Furthermore, to achieve good signal integrity, more I/Os are packed in smaller areas within the available real estate. Therefore, engineers need non-destructive techniques to identify BGA defects when ICT or FT calls for a faulty device. Identifying BGA cracks is not an easy task with the available tools including 2DX. Automatic X-ray inspection (AXI) systems are used for identifying BGA opens in electronics manufacturing, however, it is challenging for AXI Laminography to detect BGA defect size less than 4 mils.
Time Domain Reflectometry (TDR) has the capability to identify BGA full cracks (opens) using impedance measurement data. The 2DX has been widely used to identify BGA defects because of its clear image. Engineers often use cross-section/SEM or Dye & Pry to identify defective BGA pins; however, the boards are destroyed during these tests. Because of this, non-destructive techniques were studied to look for a correlation with the cross-section/SEM and Dye & Pry methods.
AXI has more than 90% coverage for PCBA, and is an effective tool for collecting real time data for SMT process improvement. Engineers usually use 2DX to verify critical defective BGA pins found with the AXI. Both AXI and 2DX are non-destructive techniques, and so is TDR. Thus, TDR, AXI and 2DX was chosen for this study. The objective was to find the correlations between the different testing methods:
• TDR, SEM, and Dye & Pry;
• 2DX, SEM and Dye & Pry;
• TDR and 2DX.
Equipment used for the comparison work were a LeCroy WE100H mainframe with ST-20 TDR module, an Agilent Laminography 5DX, a Dage XD7500 transmissive X-ray, and a JEOL Scanning Electron Microscope. These are TDR, AXI, 2DX and SEM, respectively. The BGA DMN-8802 Dual Encoder processor has defects based on ICT and FT testing, and was selected as the object of the experiments.
The BGA is on a board which is a Set-Top Box bundled with user-controlled broadcast, frame enhancement, and network access. The fab, which has six layers, has 388 pins with a pitch size of 0,75 mm. Thirty BGA pins were selected for the experiment including A24 and C26. Solder joint problems have been reported previously for A24 and C26 using SEM and Dye & Pry.
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