barcode scanner code in c#.net SELECTING BASE MATERIALS FOR LEAD-FREE ASSEMBLY APPLICATIONS in Software

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SELECTING BASE MATERIALS FOR LEAD-FREE ASSEMBLY APPLICATIONS
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Application recommendation Material generally recommended for typical applications of this type Material may be acceptable for applications of this type, but is not generally recommended Not recommended
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FIGURE 11.2 Color code key.
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processes can also impact the performance of the finished PCB. So although these tools are based on considerable data and experiences from a number of sources, they are intended to serve as a general guide for typical applications, and as such, it remains the user s responsibility to confirm acceptability of any material recommended. This is particularly true with respect to long-term reliability requirements. For example, the field reliability requirement for a cell-phone PCB is going to be very different for a very complex high-end computer or telecommunications infrastructure PCB. The intent in developing this tool was to come up with a simple method for dealing with this multitude of variables in PCB design and assembly. Figure 11.2 shows the basic color-coding selected for this. Figure 11.3 shows an example of the actual chart format. In the horizontal axis it divides PCBs into thickness categories, and in the vertical axis it differentiates them by the number of reflow processes. This format forces definition of a typical PCB design for each range of thickness shown on the x-axis. Although this is very difficult and exceptions to the rules are evident as soon
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Example material application suitability Peak reflow temperature range: XXX Please consult the PWB Design considerations section before using this tool 6
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5 4 3 2 1 mm 0.80 Inches 0.031 1.60 0.062 2.40 0.093 3.20 0.125 4.00 0.157 4.80 0.188 5.40 0.212 6.20 0.244 7.00 0.275
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FIGURE 11.3 Example of a chart format.
PRINTED CIRCUITS HANDBOOK
Layers Micro vias Cu Wt (oz)
2 6 Yes <2
2 8 Yes <2
2 14 No <2
2 18 No <2
6 22 No <2
10 26 No <2
10 30 No <2
14 34 No <2
14 40 No <2
14 50 No <2
RC % Aspect ratio Retained Cu %
35 55 <3:1 <50
35 55 <5:1 <25
35 55 <8:1 <25
35 55 <10:1 <25
45 60 <10:1 <25
45 60 <10:1 <25
45 60 <10:1 <25
45 60 <10:1 <25
45 60 <10:1 <25
45 60 <10:1 <25
mm 0.80 Inches 0.031
1.60 0.062
2.40 0.093
3.20 0.125
4.00 0.157
4.80 0.188
5.40 0.212
6.20 0.244
7.00 0.275
PWB Thickness
PTH Cu (mm) Surface finish Lamination cycles Mixed materials Blind and buried vias External planes
Ni/Au, Silver, Tin, OSP
1 No No No
mm 0.80 Inches 0.031
1 No No No
1.60 0.062
1 No No No
2.40 0.093
1 No No No
3.20 0.125
1 No No No
4.00 0.157
1 No No No
4.80 0.188
1 No No No
5.40 0.212
1 No No No
6.20 0.244
1 No No No
7.00 0.275
1 No No No
PWB Thickness
FIGURE 11.4 Typical PWB features for the selection tool.
as a typical definition is provided, these definitions represent fair descriptions of a broad range of products. In addition, you will see that an attempt is made to define a procedure for accommodating the exceptions. Figure 11.4 outlines the typical features for PCBs in each thickness range in the charts. To accommodate the exceptions to these criteria, a method was developed to adjust the selection tool based on specific design features or processing conditions. The basic concept for these adjustments is shown in Fig. 11.5, and the specific adjustments are shown in Fig. 11.6. For the purposes of illustration, the materials outlined in Table 10.5, presented here as Table 11.3. Figure 11.7 shows the charts for product A with two different assembly temperature ranges, 210 to 235 C for tin-lead assembly and 235 to 260 C for lead-free assembly.
SELECTING BASE MATERIALS FOR LEAD-FREE ASSEMBLY APPLICATIONS
Example material application adjustments Peak reflow temperature range: XXX Please consult the PWB Design considerations section before using this tool 6
Number of reflow cycles
Higher thermal demands
5 4 3 2 1 mm 0.80 Inches 0.031 1.60 0.062 2.40 0.093 Lower thermal demands 3.20 0.125 4.00 0.157 4.80 0.188 5.40 0.212 6.20 0.244 7.00 0.275
Lower mechanical demands
Typical demands
Higher mechanical demands
FIGURE 11.5
Concept for adjustments to material selection.
As you can see when these charts are presented side by side, the range of designs for which this product is deemed suitable decreases as the assembly temperature increases. This should be expected based on the earlier discussion of material properties. The other key point is that there may still be a group of products, albeit more limited, where standard 140 C Tg material may be adequate and the most cost-effective option. This may help clarify some of the confusion about whether standard FR-4 materials are lead-free-compatible or not. For specific designs with specific requirements for reliability, the answer is yes. For other designs, applications, or reliability requirements, the answer is no. The value of this tool is that it attempts to define the range of PCB designs where specific materials should be considered. Figure 11.8 presents similar charts for product C, which is the higher-Tg conventional FR-4 material. This is where the conversation gets very complicated. In Table 11.3, you can see that the decomposition temperature for this product is 310 C, which is the lowest of the materials described. On the one hand, the higher-Tg of this product helps reduce the total amount of z-axis expansion and therefore stress on plated vias. On the other hand, the lower decomposition temperature makes this material the most sensitive to higher assembly temperatures. In fact, the temperature range of 235 to 260 C is a very broad one for this product. Limited success may be seen when assembling at the low end of this range, but as temperatures increase, especially toward the high end of this range, this product is simply not recommended, due to the potential for resin decomposition and resultant defects. In fact, for these reasons and the potential interactions with PCB fabrication and assembly processes, not to mention the potential for moisture absorption to further impact performance, standard high-Tg FR-4 materials should not be considered at all for lead-free applications. Products such as B, D, and E are significantly more robust in these applications and offer attractive cost-performance relationships. In summary, you should exercise extreme caution when considering a conventional high-Tg FR-4 for a lead-free application and should discuss these issues with your laminate material supplier. In contrast, consider the charts for products B, D, and E in Figs. 11.9, 11.10, and 11.11, respectively.
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