Targeted micro-sectioning for microscopic analysis of samples is essential, especially in the electronics industry, for quality control and failure analysis, as well as in the research and production control purposes. If the surfaces of the samples are flat enough or the features to be hit are on a similar plane, a sophisticated micro-sectioning system capable of handling multiple samples in high volume is preferred. One such system is the Buehler PWB-Met system for quality control of through-plated holes.
Smarter Sample Preparation Method
Low sample throughput, inconsistency in results and cross-contamination are some of the drawbacks of conventional sample preparation methods. These issues can be addressed with an improved method that follows a relatively direct way of preparation. The distance between the bottom of the mounted sample and the final level of interest is first defined, followed by manual grinding until reaching a level close to the level of interest. The next step is the automated polishing, wherein the parameters set on the machine controls the material removal. The schematic of this smarter sample preparation method is shown in Figure 1.
Figure 1. Preparation levels of the sample
Since the first step is often relatively fast, it won’t take much time even though it is performed manually. The rest of the procedure is actually time-intensive and is performed automatically in the improved method, meaning that multiple samples at a time. The following are the advantages of automated preparation:
- Shortened total processing time per sample
- Significantly improved sample quality and uniformity of the results
- Improved results irrespective of the level of experience of the operators
- Significant improvement in the preparation capacity
Here, this improved method is used to prepare Chip Scale Package (CSP) samples.
Chip Scale Package
For CSP, the duration of the first manual step relies on the judgment of the operator. A coarser 180 (P180) or 240 (P280) grit CarbiMet SiC paper could be used if the distance between the target and the bottom of the mounted sample is too far. For comparatively shorter ‘known distance,’ 320 (P400) grit CarbiMet SiC paper could be used. It takes roughly 2-3 minutes normally for grinding. The drawing and overview of the sample are illustrated in Figures 2 and 3.
Figure 2. Schematic drawing of preparation levels of CSP-BGA
Figure 3. Overview of the sample. A known distance is shown on a CSP-BGA; best stopping level at the level of 0.7mm from the edge.
Once this level is reached, a distant market of the ‘next row’ will be there on the existing grinding plane, indicating the closeness of the target (Figure 4). The 600(P1200) grit CarbiMet (Figure 5) is normally utilized in semi-automated grinding with multi samples. The four semi-finished samples (Figure 6) are loaded into the single-force holder of the Metaserv 250/Vector grinder/polisher and the grinding cycle is started. The six single-force fingers of the EcoMet/AutoMet 250/300 can prepare six samples simultaneously. Prolonged grinding times can lead to loss of target.
Figure 4. The target is almost revealed by 320 (P400) grit CarbiMet SiC paper, a vague contour of the solderball is shown underneath the current grinding plane.
Figure 5. Decent scratches pattern from Grit 600/P1200 CarbiMet SiC paper; 50x
Figure 6. Four Semi-finished samples in single-force holder (L); automated grinding by Metaserv 250/Vector grinder/polisher (R).
The next step is the automated polishing using 3µm MetaDi Diamond Suspension on the VerduTex cloth or TriDent cloth (Figure 7). This process takes roughly 3 minutes to complete. The final polishing is performed with MasterPrep on a ChemoMet cloth for roughly two minutes (Figure 8). The intermetallic layer is depicted in Figures 9 and 10. Table 1 summarizes the complete sample preparation method and parameters.
Figure 7. VerduTex 3um finishing: 100x(L); 200(R)
Figure 8. Final Polishing with MasterPrep on ChemoMet (L) Final Polishing Result; 200x
Figure 9. Intermetallic whiskers at the solder joint; 1000x
Figure 10. SEM Micrograph of intermetallic whiskers
Table 1. Preparation Method for CSP
|Cold Mounting Consìmables & Accessories
|EpoThin Epoxy & 25mm SamplKup
|Metaserv 250/Vector with Single Force Holder: 4 x 25mm & 10 ” platen
||Time per Step (Min)
||Time per Sample* (Min)
||**Time per Sample (Min)
|320(P400) grit CarbiMet
|600(P1200) grit CarbiMet
|VerduTex 3ìm Metadi Diamond suspension
|ChemoMet 0.05ìm MasterPrep polishing suspensions
*Vector single-force holder allows for auto preparation flexibility up to 4 samples
**AutoMet 250/300 grinder/polisher allows for auto preparation flexibility up to 6 samples
From the steps discussed above, adequate practice and experience is required in the first step to achieve the known distance prior to the development of a full successful recipe. A 320 (P400) grit CarbiMet is typically used for the known distance over 100µm to perform grinding for 2-3 minutes to bring the grinding level close to the level of interest. The 600(P1200) grit CarbiMet is preferred for the known distance of below 60µm, though its removing rate is fairly slow.
If any known distance is below the foregoing range, targeting has to be performed carefully to prevent overgrinding. If the device is composed of some very fragile materials such as GaAs, it is recommended to use a finer SiC paper to minimize severe damage. Monitoring the grinding level in every 10 seconds using an optical microscope is always a safe practice.
Although the rest of the procedure is performed automatically, some initial work is required to identify the correct parameters. However, this is facilitated by the single-force mode that enables the operators to stop the places and analyze the samples. And once done, it is much easier to repeat the procedure, thus saving at least 10 minutes per sample and producing more uniform samples with increased capacity at a lower cost.
This information has been sourced, reviewed and adapted from materials provided by Buehler.
For more information on this source, please visit Buehler.