February, 2013 Continued from page 57
savings with selective coating is sig- nificant; a higher volume of products is possible during a shift. In addition, the manual masking process increas- es the opportunities for failure because of the additional board han- dling during the assembly phase. Selective coating equipment only has the initial capital equipment invest- ment that is depreciated over the 19- to-15-year lifetime of the 24/7 capa- ble equipment; the only significant reoccurring cost is that of the coating itself. Using automated equipment eliminates operator-to-operator vari- ation, further increasing yield over manual processes.
Optimized Coating Processes For optimized coating processes,
the conformal coating material and the equipment should be thought about collectively. Manu facturers commonly select a specific conformal coating based on the cured properties of the material. This is a logical first step as the conformal coating is in a cured state when the electronics are in their final use model; however, the material is applied in a liquid state that is subsequently cured by thermal, ultraviolet, moisture, and other solidi- fying processes. For a high yield process, a manufacturer also needs to strongly consider the material’s liquid properties and to characterize the coating process in conjunction with the design phase to minimize the PCA lay- out while respecting process toler- ances. An understanding of the specif- ic material’s rheology allows for auto- mated coating equipment selection and defining process capabilities. When considering design for
Materials Testing
Continued from previous page
temperature, Tg. At temperatures well below Tg, they are elastic and brittle. Above Tg, their behavior depends on whether or not the poly- mer chains are cross linked. Linear (unlinked) polymers tend to be vis- cous or visco-elastic, depending on temperature. In cross linked poly- mers, the individual molecules are restrained and these polymers gener- ally exhibit some form of visco-elastic beaver.
components were processed within their thermal specs.
AOI cannot see the leads underneath a BGA or PoP body. Batch X-Ray typically inspects less than 1% of the PCBs.
www.us-tech.com Selective Conformal Coating
manufacturing, specific properties of the conformal coating are the materi- al’s viscosity, chemical composition and associated work time, as well as environmental considerations. The viscosity is important because it deter- mines how the material will flow once applied to the board as well as the type of applicators needed to dispense the material. Low viscosity coatings (typi- cally less than 100 centipoise) can be applied with high speed, highly selec- tive non-atomized film coating tech- nologies that can offer keep-out zones less than 2mm in production.
Higher Viscosities Higher viscosity coatings require
the use of atomizing coating technolo-
gies that are slower in coating rate and have keep-out zones typically greater than 3mm. Jetting or needle dispensing technologies can be used to decrease the keep-out zones to the 1mm neighborhood and are generally used to precisely coat specific parts rather than for coating an entire board. The addition of a second coating valve to increase selectivity can decrease process throughput. Considering the need for confor-
mal coating, applying the required process tolerances for the material, and evaluating conformal coating equipment and characterization at the design phase greatly aid in man- ufacturability. It makes the manu- facturing process less difficult, less
Page 59
time-consuming, and less costly, and avoids having to use specialized equipment, applying additional con- formal coating, or using masks after the fact due to unplanned-for parts or areas that have zero tolerances.
Pot Life The chemical composition is
also very important as often materi- als have finite work times, commonly referenced as pot life; the hardware must be cleaned to avoid curing inside the equipment. The presence of hazardous solvents is common, so consideration needs to be given regarding process controls and the environmental impact of these mate- rials. This specifically applies to the health of the employees, potential for
Continued on page 61
Your AOI image of BGA and PoP components
Your Batch X-Ray image of 99%+ of your BGAs / PoPs
HBP with magnified view. Adhesives are really a subset of
plastics. Much of what has been said about plastics applies to adhesives as well, but care must be exercised in understanding the rather special stress condition that exists in the narrow joint between the bonded plates.
Contact: Nordson DAGE, 48065
Fremont Blvd., Fremont, CA 94538- 6541 % 510-683-3930 Web:
www.nordsondage.com r
See at IPC/APEX Booth #2915. 1
See us at IPC/APEX Booth 1314
KIC World Headquarters 858.67
858.673.6050 sales@
sales@kicmail.com www.kicthermal.com
KIC International Sales - Europe +39.342.147.9460
europe.sales@
kicmail.com www.kicthermal.com
KIC International Sales - Singapore +65.6744.4998
asia.sales@
kicmail.com www.kicthermal.com
KIC International Sales - China +86.512.6763.5171
asia.sales@
kicmail.com www.kic.cn
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