|Ask the Experts|
July 20, 2011
Special Cleaning Needs for BGA and QFN Components
Our products now us very standard components and all have adequate space to easily clean under them using our in-line cleaner. Should we make any changes in our cleaning process as we add BGA and QFN components since the spacing under the devices is so tight?
|Expert Panel Responses|
With the introduction of new components and the decreasing spacing beneath those components impacting the ability to properly clean off the flux residues, flux evaluations must be conducted. The bottom line being if the flux cannot be removed then if and when it is left behind, it has to be benign so not to impact the form, fit and function of the product. This why no clean fluxes can and do apply in this situation. After reflow they are theoretically benign and could be left beneath the component with no long term detrimental effects on the product. I say this quite quickly realizing I used the word theoretically. Testing is a must to verify the flux is in fact benign and this can be done with a Surface insulation Resistance (SIR) testing, and lab evaluations. The second test I would recommend is the thermal profile analysis, to make sure all the fluxes on the board are in fact heated to soldering temperatures, especially beneath components. If the flux does not reach soldering temperature beneath the components, it will still be active and at times conductive, creating electrical leakage beneath the components. I cannot emphasize this enough as this can and will cause product failures in the field. Using these new component configurations required proactive evaluations of both the flux materials and the thermal profiles throughout the product. This work must be done up front to prevent chasing failures in the product at a later date. If more information is needed please don't hesitate to contact me at your convenience.
Vice President, Technical Director
S.P. you make no reference to the flux or solder alloy currently being used so I will make a general statement on this subject. Component miniaturization decreases the spacing between conductors. During solder reflow, flux under fills the bottom side of the component. The distance from the board surface to the bottom side of leadless components is consistently less than 2 mils. For cleaning to occur, the cleaning agent must first wet the residue. To sufficiently wet the residue, the cleaning process must break through the flux dam under the component to create a flow channel. Cleaning under low gap components is increasingly more difficult due to the higher molecular weight flux composition and heat exposure during the soldering process. Clearance gaps under components of less than 2 mils create a highly difficult cleaning challenge. The critical differentiator for removing higher molecular weight flux residues is the cleaning agent. The ideal cleaning agent is formulated with the greenest environmental properties within performance limitations; rapidly dissolves polar and non-polar soils; and is easily rinsed leaving an ionic cleaned assembly. Since flux residues are a composition of rosin, resins, organic activators, rheological additives and reacted ionic salt forms, the cleaning agent requires a composition of materials that remove dispersive and polar soils, and non-polar resins. To remove flux residues under component gaps on leadless components, cleaning equipment designs must deliver the cleaning agent to the source of the flux residue. The combination of fluid flow, capillary forces, pressure at the component gap, and dissolution are needed to create a flow channel under the component. Dependent on the forces applied, changes in your cleaning process will likely be required. This could be as simple as a temperature change or adjustment of your belt speed to allow the cleaning agent increased wash exposure time. If you are currently using only water as the cleaning agent on OA flux and water alone is not providing 100% clean due to the factors described earlier, now you have to investigate the addition of an aqueous cleaning agent in order to effectively clean under these components. I hope this has clarified or given you direction on your process question and if you have any further questions please do not hesitate to contact me.
Technical Expert Sales Support
Please do not use Water Soluble (OA)Flux under tight components, also IF you HAVE TO CLEAN then do not use anything with wetting agents (surfactents) as these will normally be very hygroscopic, conductive in nature and difficult to rely on Water alone to rinse off under devices so they can themselves cause issue's left un rinsed. So the use of a good old Rosin type Paste and leave it on IF you have to clean then use a Pure Solvent cleaner only.
Technical Sales Manager
BLT Circuit Services Ltd
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