Circuits Assembly - February 2008 - (Page 30)

RoHS Compliance detector-based systems, with their generally smaller spot sizes (0.1 to 2 mm diameter), were able to identify noncompliant joints, with all eight systems tested giving correct identifications (Figure 3). Screening resistor joints. Spot size also played a part in the analysis of R1206 solder joints. The latter are 3 x 2 mm, filling around 85% of the typical measurement window of a portable system. Although no lead (<0.1%) was present in one sample, six systems recorded lead at levels of 0.1 to 0.8% in that sample (i.e., RoHS-noncompliant). The majority of these systems had larger spot sizes, and consequently, the measurement window extended beyond the joint area and included part of the component and PCB base (Figure 4). The lead signal therefore included some contribution from the resistor, which was known to contain lead in the passivation layer of the resistive element. Thus, incorrect RoHS noncompliances were generated. Screening BGA joints. Several assembled BGAs were tested to determine if the XRF systems could detect and measure the lead content of the assembled BGA balls, when measured through the top of a BGA. This approach might permit nondestructive testing of completed assemblies, as may be required on imports into the EU. Only one system, using a Co57 x-ray source, recorded any lead signal sufficient to indicate noncompliance. However, the measured RoHS Screening of Solder Joints Figure 2. Removing other materials such as inner values from this system were significantly For the much smaller SM solder joints, the cores of cables may be necessary for accurate results. below the actual sample values (1.98% for measurement window size (or spot size) 40% lead sample, and 0.32% for a 10% is important. For accurate determination lead sample). Hence, if lead contaminaof lead content, samples need to fill the tion levels were low, none of the systems measurement window. Solder joints by could be relied on to determine RoHS their nature tend to be thin, and the signal compliance. recorded may contain contributions from Incorrect identifications of cadmium. other materials beneath the solder joint. Again, the detection of cadmium during Screening SOIC joints with large spot sizes. joint testing was difficult. Six systems gave With Si-PIN/SiLi detector-based systems, 19 (of 121 measurements) false indications the spot size for portable systems tended to of RoHS noncompliance. All the recorded be greater than for the benchtop machines Figure 3. Accurate nondestructive analysis of small values were less than 0.08%. (at least 3 mm diameter). With the toe joints requires small spot sizes. Solder joints and proportional counterof the SOIC solder joints tested approxibased systems. Performance of propormately 0.65 x 1 mm, such joints filled only tional counter-based systems was variable. Only two systems provided a around 8% of the measurement window of a typical portable system. full set of measurements, neither of which proved adequate for identifyA further complication is that the joint areas examined were not of ing RoHS-noncompliant joints. constant thickness due to the shape of the fillets. Although one of the portable systems did indicate noncompliance for lead for a sample RoHS Screening of Other Components SOIC joint containing ~3% lead, the recorded values were still very low Screening components/solder pastes in packaging. At incoming inspection, at <0.2% lead. Similarly, for a SOIC joint containing ~11% lead, three portable systems indicated noncompliance, but gave recorded lead there are distinct advantages in being able to test components/materials levels at least 10 times lower than the actual value. Thus, for identifying without removing them from secondary packaging (tapes, reels, sticks, samples containing high lead levels (40% or more), the majority of the etc.). This may avoid deterioration after opening, as in the case of solder handheld systems are adequate, but the systems tested could not be pastes, or prevent waste, as in the case of components in reels. Handling relied on to identify noncompliant joints where lead levels were below damage in removing components from sticks can also be avoided. 3%. Such systems may be suitable for analyzing joints if sufficient joints When testing resistors on the reel, five of the 11 XRF systems evaluwere removed from the assembly and collected together to fill the meaated (including one proportional counter-based system) did not record surement window. a sufficiently high level of lead to identify the RoHS-noncompliant Screening SOIC joints with smaller spot sizes. The benchtop Si-PIN/SiLi components. Of the remaining systems, four recorded significantly bulk alloy samples tested, the Si-PIN/SiLi-based XRF systems were able to detect lead levels to around 500 ppm. Some systems even achieved good repeatability at 50 ppm lead. All the systems indicated noncompliance in all instances of lead present in tin at levels of 2000 ppm. At 1000 ppm lead, 11 of 12 systems indicated noncompliance for lead, or within 10% of RoHS limit. Thus, all Si-PIN or SiLi detector-based systems proved suitable for screening bulk solder samples for RoHS compliance for lead, detecting lead at or above 2000 ppm. For levels between 500 and 2000 ppm, additional techniques are recommended if accurate elemental analysis is required. Incorrect indications of noncompliance were again recorded for cadmium in SnPb. Ten Si-PIN/SiLi systems were used to analyze cadmium levels, and of these, 50% gave false indications of noncompliance. All the proportional counter-based systems were able to detect lead in bulk solder samples at 0.2% or above. But, only one system was able to detect lead to 0.1%, and none was able to detect less than 0.1% Pb. Some proportional counter-based systems were not able to detect less than 0.2% lead in bulk alloy samples; therefore, care should be taken when using them for RoHS compliance screening. 30 Circuits Assembly FEBRUARY 2008 circuitsassembly.com http://circuitsassembly.com

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Circuits Assembly - February 2008 Contents Caveat Lector Letters Industry News Market Watch Global Sourcing Better Manufacturing Maximizing Lean Copper As a Viable Solution for IC Packaging Embedded Active Components for High-Rel Products Cover Story: XRF Equipment As a RoHS Screening Tool Tech Tips Selective Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - February 2008

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