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September, 2021 PARTNERING


Qualifying a PCB Fab in the Lab: Cleanliness, DSC and Microsections


By Stanley L. Bentley, P.E., Technical Director — Americas, ICAPE Group


chain partner, running samples of PCBs at the new supplier, and having the samples analyzed by a qualified laboratory to the rele- vant IPC specifications as well as internal requirements. In the first article on labora-


T


tory analysis we covered vision inspection to IPC-A-600, electri- cal testing (bare board test), and XRF measurement of the surface finish. In this article we discuss cleanliness testing in µg/in.2 (base level testing does not in- clude ion chromatography), DSC (differantial scanning calorime- ter) measurement for laminate Tg, and microsection analysis.


Cleanliness testing is critical due to the rise of no-clean flux.


here are three steps quali- fying a new PCB fabrica- tor: choosing a supply


When no-clean flux is used dur- ing assembly, there is no addi- tional cleaning process to remove any residual ionic contamina- tion. Any ionic flux residue left on the surface by the PCB fabri- cator process can be activated by moisture. These random resis- tive paths can cause erratic oper- ation or contribute to dendritic growth and metal migration. The standard cleanliness


test is to immerse the sample in a 75/25 percent mixture of DI water and anhydrous iso- propanol. This mixture is a very effective solvent for most of the ionic chemistries used in PCB fabrication. Dissolving the ionic substances from the surfaces of the PCBA increases the conduc- tivity of the DI-IPA solution. The solution is cleaned up


The material matters in material handling


by the tester and the results con- verted to a measurement system that is easily understood. The software converts the measure- ments into a standard of micro- grams of equivalent chloride per square inch or square centime- ter. This means how much salt (NaCl) by weight would be need- ed in the DI-alcohol solution to show the same conductance. For reference, the IPC Class


II and Class III recommendation is 9.8 µg of equivalent chloride per square inch of exposed PCB surface area.


Using a DSC is one of only two means of determining the glass transition temperature (Tg) of the laminate. The Tg is critical to the long-


term performance of the PCB substrate. This may not have much of an effect in an office envi- ronment, but under the hood of an automobile controlling its an- tilock braking system, for exam- ple, it is vitally important. You cannot determine the difference in the Tg by a visual examination. The Tg is only one of many


critical characteristics of the laminate. However, many of the other critical characteristics are related to the Tg. For this rea- son, a Tg measurement is usual- ly sufficient to determine if the correct laminate was used. One example of the importance of the Tg, is the effect on the laminate during reflow. The melting point of typical


lead-free solder is between 424 and 442°F (218 and 228°C) and normal double-sided laminate has a Tg of 302°F (150°C). All epoxy-based laminates exhibit z- axis expansion once they exceed the Tg. This expansion places enormous stress on the plated barrels of the PCB. Epoxy lami- nates cool slowly. However, lead- free solder solidifies quickly. The ceramic parts exhibit no expan- sion at fusion temperatures, so quick cure of the solder can in- duce stress and cause latent frac- turing in the components.


Microsection analysis allows the examiner to look inside the structure of the PCB. The IPC has defined 42 different defects


that can be found in just one mi- crosection and because of this, it is considered to be the most im- portant of all laboratory tests. If we select only one meas-


urement from a microsection, it would be the thickness of the in- hole copper plating. Beyond the obvious electrical properties, the in-hole plating has a critical function when the laminate tem- perature exceeds the Tg. Above the Tg the laminate expansion goes to over 350 ppm/°C, which induces stress in the cylindrical copper barrel in the plated- through holes. The in-hole copper must be


thick enough and ductile enough to withstand this z-axis stress. The IPC has defined an in-hole average copper thickness of ap- proximately 20µ for Class II and 25µ for Class III (these figures have been rounded up). If the copper is thinner than these min- imums, then there is the proba- bility that the plated barrel will fracture. This failure will usually manifest as a field failure, not an assembly test failure. Once the laminate returns


to its original state, the copper fracture will make a mechanical connection. This connection usu- ally has enough conductivity to pass initial testing. However, it will oxidize in the field and tran- sition to a high resistance con- nection, resulting in intermittent operation. ICAPE USA routinely per-


forms these tests in its North American analytical laboratory and these should be the mini- mum requirement for PCB vali- dation.


Equally important is advice


and assistance from PCB and as- sembly professionals who under- stand the significance of each test for all stakeholders — OEM, assembler, and end user. Availability of experts is a


crucial part of selecting a labora- tory. Remember, data without analysis and interpretation is only a spreadsheet full of figures. Contact: ICAPE USA, 8102


Zionsville Road, Indianapolis, IN 46268 % 317-405-9427 E-mail: sales@icapeusa.com.com Web: www.icape-group.com r


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