Electronics FAQs

I am experiencing oxidation of my copper lead frames post molding final cure. I do not want to subject my assemblies to a rigorous acid clean and increase my costs. Are there any alternatives?

This is a common issue that integrated circuit assembly companies have experienced. Post mold, final cure, and bake out are accomplished in a batch process in an oven. Temperatures for this process can exceed 200^oC, and when in an air atmosphere, will lead to oxidation of the copper lead frames.

The best alternative is to use an inert atmosphere within the post mold curing oven. Most standard ovens for this process are not designed to use an inert atmosphere. There are several vendors that manufacture inert atmosphere ovens for this process. The ideal O₂ ppm level for this process is <1000 ppm. At these O₂ ppm levels, one can achieve good final cure and eliminate the copper lead frames for oxidizing. The initial cost to replace the ovens may be slightly prohibitive; however, your overall cost of ownership will be reduced, and the quality of the assembly will be enhanced, not to mention the elimination of potential environmental issues with copper oxide removal via acid etch and deionized water cleaning.

Air Products has the offering, Intelligent Nitrogen Control System (INCS), that can monitor the O₂ ppm levels in the curing oven and maintain a consistent O₂ ppm level while controlling the nitrogen gas consumed. Ask Air Products to evaluate your processes and assist you in converting to an inert atmosphere post mold curing process.

We are experiencing poor underfill flow in our flip chip assemblies. What is the possible cause and is there a solution that can be implemented to improve our underfill process?

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.

We are currently evaluating the use of copper wire in place of gold wire in our wire bonding assembly process. In our conversion from gold to copper wire, we implemented the use of forming gas (5% H2/95% N2) form ball formation. Do we need forming gas for copper wire?

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.

We are currently evaluating the use of copper wire in place of gold wire in our wire bonding assembly process. In our conversion from gold to copper wire, we implemented the use of forming gas (5% H2/95% N2) form ball formation. Do we need forming gas for copper wire?

What is the best way to reduce the dross formed in a Lead-Free (SAC Alloy) wave soldering 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.

PCB assembly chart

Air Products has developed a 3^rd 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.

Concerned about reflow issues and looking for a wider processing window?

HDI (high density interconnection boards) assembly with small geometry components can pose many assembly issues. With the smaller components, most likely a Type 4 (30–38, micron ball size) or Type 5 (15–25, micron ball size), solder powder paste will be used. This allows for an increase in solder volume for smaller footprints and provides an improved solder joint. The flux chemistries will vary; however, most assembly houses use a no clean formulation.

When using the small micron solder powder, there is a tendency for the powder to oxidize at a faster rate during the reflow process. This is due to the increase in surface volume and less oxide dissolution into the solder mass. In an air atmosphere reflow process for lead-free solder, the flux chemistries begin to polymerize and lose fluxing capacity, leading to poor wetting, insufficient solder joints, and other defect issues.

Using a nitrogen atmosphere in the reflow furnace, with a maximum oxygen level of 1000 ppm in the reflow zone, provides a wider processing window and less assembly processing issues. The nitrogen atmosphere reduces the polymerization of the flux, allowing for enhanced solder wetting and improved solder joint quality. Another added benefit of using nitrogen is that the solder powder will not oxidize and allows for improved wetting.

If you are interested in understanding how the use of nitrogen in your SMT (surface mount) reflow process can improve your HDI assembly process and reduce costs, please contact Air Products for an evaluation of your assembly process and learn how our team of experts can assist you.