Page 56
www.us-tech.com Getting PCBs Really Clean By P.J.Duchi, Anne-Marie Laugt, Marie Verdier, Inventec Performance Chemicals, Bry sur Marne, France
not do a good cleaning job on today’s electronics because most flux formu- lations have changed. Cleaning should be adapted to the job’s requirements while maintaining sta- bility in time, efficiency, quality and performance. The world of cleaning is composed of two parts: organics and inorganics. Cleaning perform- ance is affected by three main crite-
Y
esterday’s chemicals — CFCs, HCFCs, brominated solvents, detergents and glycols — can-
ing product cannot make contact with the contaminants under or around components, the contami- nants cannot be dissolved. The third criterion consists of such physical parameters as temperature, mechan- ical activity, and the duration of the process.
Proper cleaning wisdom man-
ages all of these parameters while dealing with such high-tech chal- lenges as ultra-miniaturization and environmental care issues, that include RoHS and REACH. This wisdom of cleaning should always bring innovation to the cleaning process in elec- tronics.
Miniaturization Today’s miniatur-
As components get smaller and smaller, it becomes more difficult to clean under them after reflow.
ria. The first involves the solvency power of a product also known as the Kauri Butanol Index (KB Index). The
Miniaturization has raised some design problems, but also some practical aspects such as handling and proper cleaning to meet finished specs.
second is surface tension, expressed in mN/m. This parameter must be considered because when the clean-
ization has taken our industry a hundred light years beyond what was common in the 1980s. Today’s component size
is down to 0.1mm — just a dot on your metric ruler, possibly the size of a period on this page. The reliability of these components must always be maintained — miniaturization should not become a source of insta- bility and unreliability. To achieve this, the PCB assembly’s cleanliness should be pass all required tests to meet the product’s specifications.
Contaminants The contaminants on a circuit
board are mainly composed of organ- ics such as natural and/or synthetic resins, ions, acids, solder balls, fin-
Cleaned without ultrasound.
gerprints, and particulates of PCBs. Because lead-free alloys require higher soldering temperatures than the standard Sn/Pb, there are signif- icant changes in the fluxes and sol- der pastes that are used. These fluxes are for the
most part more active and must respond properly to higher reflow temperatures. They pres- ent more risks than the ones formerly used, and the tempta- tion is high to choose production parameters that produce shiny solder pads. The ionic cleaning of the PCBs thus becomes more critical than ever before, but these cleaning requirements will also influence the assembly process that precedes the clean- ing. The ionic contamination is a good quality indicator for the long- term reliability. Every end user has his own
specifications which depend on his own production needs, or those of his customers. For this study, the PCBs for trials were produced in large
quantities to triple the cleaning results. Each trial contains 30 components. All residues must disappear, including the con- taminants under the compo- nents. No fingerprint, particle nor mote of dust should remain; and there should be no residues of cleaning products. The com- ponents, the resins, the under- fill and the substrate should not be damaged by the cleaning process. The parts should be dried at the end of the washing step, and the ink markings
should be unaffected by the cleaning. Cleaning Products Available The most important part of the
job is to remember which chemical
April, 2012
Cleaned with ultrasound.
families are available in the market. The cleaning products available can be classified in six different families: detergents, light petroleum distil- lates, formulated hydrocarbons, brominated solvents, glycols and flu-
Continued on next page
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96