August, 2015
www.us-
tech.com
How Clean Is Clean Enough? Continued from previous page
increase in conductor spacing could realize a decrease in SIR stringency. For this study, 0.005-in. and 0.015- in. spacing located at the center of the array was measured. High-impedance meters meas-
ure resistance levels greater than that of polyimide or FR-4 circuit materials. For the cleanliness exper- iments, a representative voltage was applied across the test traces, and current was measured, to determine
Reflow Soldering: With or Without
Vacuum? Continued from page 54
file guidance and a stable reflow process. The measurement of a tem- perature profile with the vacuum process switched on shows that despite a very low vacuum of 10mbar, all profile settings have been fulfilled. With the help of heat- ing elements integrated into the chamber, the temperature of the components inside the vacuum unit can be adapted to the settings of the most common standards. This refined solution ensures a time-effi- cient and stable production process. Both condensation and convec-
tion soldering processes are available with or without vacuum. To identify a suitable combination of condensa- tion or convection soldering and vac- uum technology, the individual goals to be achieved must first be precisely analyzed. Despite the common main
requirement of reducing voids in sol- der joints, condensation and convec- tion soldering procedures differ in There are several important features of the two vacuum soldering proce- dures. Considered to be crucial are the type of component to be proces - sed, the availability of the vacuum processes, and the productivity. Production, component, and cus-
tomer-specific aspects always play a primary role in the selection of the optimum combination of soldering and vacuum technology. At the Rehm Technology Centre in Blau beuren, Germany, customers have the oppor- tunity to test both processes and to ascertain the optimum process for a given component to be manufactured. Since the market launch of the VisionXP+ Vac reflow solder system, several systems have been placed suc- cessfully on the market. Customers have been impressed
by the significant increase in the reli- ability of the soldered joints when using the vacuum process and the flexibility of a reflow soldering sys- tem that allows the switching off of the vacuum process if desired. Contact: Rehm Thermal
Systems GmbH, Leinenstrasse 7, 89143 Blaubeuren-Seissen, Germany % +49 (0) 7344-9606-0 fax: +49 (0) 7344-9606-525 E-mail:
info@rehm-group.com Web:
www.rehm-group.com r
the equivalent impedance. Repre - sentative voltages in a digital circuit tend to be 2.5-+5.5VDC, so that the applied voltage should be equivalent. At +5.5VDC, a 1 MW resistor draws 5.5µA current, which is easily meas- ured by standard test equipment. Three solder pastes supplied by a
common flux manufacturer were selected as low, medium, and high activity flux compositions. These des- ignations were provided by the flux manufacturer based on knowledge of the flux formulation. Three circuit boards with each flux were cleaned using an aqueous cleaning agent in an in-line washer at 15 percent cleaner concentration at +150°F. The cleaning agent selected was known from experi- ence to be capable of removing all of the flux from underneath these com- ponents. The belt speed for the in-line
washer was varied for different clean- ing levels: 1ft./min for complete clean- ing of flux residues, 5ft./min for partial flux removal, and 8ft./min for virtually no cleaning. After cleaning, the resist- ances between four sets of adjacent traces and four sets of alternating traces were measured. Measure ments were performed at ambient conditions of +20 to +25°C and approximately 30 percent relative humidity. Due to capacitive effects, a
stepped DC voltage waveform was applied. The +3.3VDC bias was applied for 15s prior to a current measurement. The voltage was then switched to –3.3VDC for 15s before another current measurement was made. This was then repeated a sec- ond time yielding a total of four resistance measurements per sensor trace pair. The resistances for each sensor distance were averaged for each circuit board, with lower resist-
Page 59
ances evident for cleaner circuits. For the species testing, one drop of the ionic species at the desired concen- tration was placed on the sensor traces. The PCB was then placed in a +110°C oven for 30 min and allowed to cool in a desiccator for 1h. A set of IPC specifications cur-
rently drives industry compliance for circuit cleanliness, but these tests do not provide the resolution needed for measuring dense devices using BTCs. Improved test methods are needed for this purpose and to quan- tify the issues and problems associat- ed with residues under BTCs. This experimentation can be considered a contribution towards a better model for making more accurate cleanliness risk assessments. Contact: Kyzen, 430 Harding
Industrial Dr., Nashville, TN 37211 % 800-845-5524 or 615-831-0888 Web:
www.kyzen.com r
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