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poor underfill flow is a common concern in flip chip assembly and is due to several issues. the predominate problem is contaminates left behind post reflow assembly. the primary contaminate is flux residue. although you may clean the assembly post reflow, the likelihood of flux left behind is high. most fluxes will polymerize during the reflow process in air or high o2 ppm level (>500 o2 ppm). current cleaning processes may not be efficient enough to remove all the residue under the flip chip. the key is to use an inert atmosphere, such as nitrogen or argon, to eliminate high o2 levels. the high o2 levels will cause the flux to polymerize and be difficult to clean.
poor underfill flow is a common concern in flip chip assembly and is due to several issues. the predominate problem is contaminates left behind post reflow assembly. the primary contaminate is flux residue. although you may clean the assembly post reflow, the likelihood of flux left behind is high. most fluxes will polymerize during the reflow process in air or high o2 ppm level (>500 o2 ppm). current cleaning processes may not be efficient enough to remove all the residue under the flip chip. the key is to use an inert atmosphere, such as nitrogen or argon, to eliminate high o2 levels. the high o2 levels will cause the flux to polymerize and be difficult to clean.

best process practice is to use an o2 ppm level of around 100 ppm. this will give you two benefits: 1) reduce the chance of flux polymerization and allow for good results post cleaning, and 2) allow the flux to stay active longer, increasing the fluxing properties to insure good wetting and provide a reliable solder joint.

air products can assist you in evaluating your processes and offer solutions to your process in this area.

this is a question that has been asked in the past and free air ball (fab) formation using forming gas (5% h2/95% n2) is very common for copper wire bonding.  fab using copper has been researched and although most people believe that copper will not oxidize; however, it does.  if fab is done in air, you will experience a thin oxide layer on the ball and will require greater force to form the bond between the wire and integrated circuit (ic) interconnection pad.  this greater force may cause microcracks under the interconnect pad that is difficult to observe and seen in some copper wire bonding processes. to prevent this oxide layer from forming, forming gas is highly recommended.

air products can provide several supply options for the forming gas, from pre-mixed cylinder bundles to onsite blending systems.  we have the knowledge and expertise to safely provide solutions for your ic assembly processes and improve your process.

dross is a waste product, although it can be reclaimed, costs are high.  dross will cause equipment maintenance issues and reduce the uptime of the equipment. there are a variety of dross inhibitors that can be used on the surface of the solder pot but be careful in selecting the correct material for your process. one needs to understand the chemistry used and whether it will be detrimental to the machine and the people working the system. these chemicals are placed over the solder in the solder pot and form a layer over the molten solder to reduce metal oxide formation.

the chemicals will need to be replaced periodically. covering it with a dross inhibitor is one way to protect the pot, yet the solder coming off the wave generator would be exposed to the air and that will create some dross and in turn may attach itself to the component leads and form shorts. another potential issue in using a dross inhibitor powder is that particles may collect on the bottom side of the board, leading to defects and aggressive cleaning processes.

another method is to use a nitrogen inerting gas that can be introduced via a inerting cover over the wave area or a fully inerted wave solder system. the use of nitrogen can reduce dross effectively by displacing air over the solder pot and forming a gas cloud over the wave generator, reducing micro dross.

beyond the dross reduction advantages of using an inert atmosphere, one can gain benefits such as improved wetting of the solder into the plated through holes (pth) or barrel fill. 

there are several reasons for insufficient fill of the pth during wave soldering. the major reason is poor wetting that can be due to oxidation on the leads, oxidation in the barrel, not enough flux, etc. one way to increase the barrel fill is to use an inert atmosphere, such as nitrogen, which will increase the wetting of the solder into the barrel by displacing air and, in turn, allow your current flux chemistry to work more efficiently. you can also use a more aggressive flux which will require intensive cleaning post wave and add to your costs.

another benefit is one can convert to a less active flux chemistry to reduce the volume of flux required per board.  defects from the wave soldering process will require intensive manual rework at a cost.  by using a nitrogen blanket in the wave solder process, one can effectively reduce the most common defects observed.

using an inert atmosphere in the wave soldering process will reduce several defects including bridging, icicles and will improve the voiding issue. through hole fill will also be improved.  reduction will be dependent on the o2 ppm levels and the type of inerting system that is used. our work in this area has seen overall reduction of total defects of over 50%. dross reduction is also a benefit that we have experienced over the years with our technology. here is a chart that one of our customer provided on defect reduction using an inert atmosphere for sac 305 in wave soldering.



air products has developed a 3rd generation nitrogen inerting kit, nitrofas ™ inert wave soldering (iws) that has been successfully implemented at major ems and oem companies with over 200 systems installed.  our iws units can reduce dross > than 85% at low nitrogen flow rates to minimize the overall cost of ownership.

let air products assist you in lowering your manufacturing costs, improving productivity, reducing your environmental impact and increasing the quality of your assembled boards.  through our innovative equipment and knowledgeable team of industry experts, we can develop the best solution for you and your process.