It can be difficult to section electronic components for quality control or failure analysis. This is because it is necessary to be careful to ensure that the true structure is revealed without preparation-induced artifacts. Traditionally, sectioning processes which caused great damage to the sample were used. These could induce artifacts which could be easily misinterpreted as service failures or manufacturing defects. In order to avoid this, it was necessary to carry out sectioning far away from the region of interest (ROI). Additionally, long polishing and grinding stages were needed in order to recover induced damage.
In the past, the four most common methods of sectioning have been by router, punch, bandsaw and precision saw. Each of these methods has its advantages and disadvantages. Although punch and press are quick, they are limited by coupon size and are liable to cause significant deformation strain on multi-layer boards.
Although routers and bandsaws can be set up in order to handle different sized parts, they can produce heat, behave aggressively towards populated boards and cause shear strain on multi-layer boards. Damage is significantly reduced by precision cutting, however this method is limited by the size of the board and traditionally required slower sectioning of parts when compared to the other methods discussed above.
Technological advances in the manufacturing of precision saws have made the fast sectioning of larger populated or unpopulated boards possible. Increased X, Y and Z axis movement, higher torque motors, and a greater cutting chamber capacity have allowed for increased flexibility when handling samples. Combining automated three-axis movement, a large open working area, a 2 kW motor, laser alignment for visual confirmation of cuts, and a stored methodology, the IsoMet High Speed Pro is shown in Figure 1.
Figure 1. Stored methodology can be utilized to ensure cutting in the same ROI if routine work is to be done.
In order to determine the damage induced from varying sectioning methods, cross sections of a 12-layer MIL-SPEC unpopulated board were made. In order to ensure that superior edge retention was retained during preparation, each of the coupons was cast in EpoKwick FC as shown in Figure 2.
Figure 2. EpoKwick FC epoxy provides excellent edge retention, low shrinkage and low viscosity. This ensured that there was no board delamination due to sample preparation during the grinding and polishing steps.
Shown below, the standard SumMet method for General Electronic Components was used to prepare samples. This can be found on the Buehler website under the ‘Solutions’ page.
In order that the surface left from each of the sectioning methods – and the sub-surface damage induced – could be easily examined, the samples were then ground and polished perpendicular to the sectioned edge.
Figures 3-6 present the photographs and measurements of the samples which show the damage induced by removal.
Sectioning methods can induce damage into board material. Consequently, they subsequently require grinding and polishing steps to remove. Depending on which equipment is chosen for removal, damage can be over 2.5 mm and requires operators to “grind and find” in order to ensure the revelation of unaltered material. Samples can become uneven when removed in this fashion, or they can go beyond ROI if sampled too closely. Precision saws such as the IsoMet High Speed Pro can ensure that they get near to the ROI without inducing damage.
Figure 3. Damage from bandsaw sectioning.
* Note board cracking perpendicular to layer, delaminating extending ~1500 microns (1.5 mm) from the edge of the cut. Glass fiber chipping noted at 2500 microns (2.5 mm) from the leading edge.
Figure 4. Damage from punch and die sectioning.
* Note board delaminating within the last 4 layers, distortion of copper layers extending ~1200 microns (1.2 mm) from the edge of the cut. Glass fiber chipping noted at 1600 microns (1.6 mm) from the leading edge.
Figure 5. Damage from router sectioning.
* Note distortion of copper layers and slight delamination of last two layers with glass bundle chipping extending ~400 microns (0.4 mm) from the edge of the cut.
Figure 6. Damage from IsoMet High Speed Pro sectioning with IsoMet 15HC blade.
* Note copper and board layers are parallel with little damage to the glass bundles. Damage extending ~20 microns (0.02 mm) from the edge of the cut.
This information has been sourced, reviewed and adapted from materials provided by Buehler.
For more information on this source, please visit Buehler.