Selective soldering flux


Interflux® LMPA-S is an absolutely halide free no-clean soldering flux designed to reduce micro solder balling on solder ball sensitive solder masks when selective soldering with the low melting point alloy LMPA-Q.

LMPA S 10L front

Suitable for

  • Selective soldering is a soldering technology in electronics manufacturing, typically used for PCB designs with mainly SMD (Surface Mount Device) components for reflow soldering and only a few through hole components that cannot pass through the reflow soldering process. These are usually thermally heavy mass components like e.g. big transfo's or thermally sensitive components like e.g. film capacitors, displays,  connectors with sensitive plastic bodies, relays, etc... The selective soldering process allows to solder these through hole components without protecting or affecting the SMD components on the bottom side of the PCB.  The selective soldering process is very flexible as the parameters can be programmed for each solder joint separately. The main limitation of the process however is the throughput or the production capacity. This can be considerately improved when using a low melting point alloy that allows for faster soldering speeds increasing production capacity up to 100% (double). The process starts with the application of a liquid flux that will deoxydize the surfaces to be soldered. This flux is applied by a micro jet or drop jet fluxer that shoots little drops. The correct calibration and programming of this fluxer is essential to get good soldering results. A common mistake is that flux is applied outside of the contact area of the soldering nozzle. This flux will remain as an unconsumed flux residue. For some fluxes and sensitive electronic circuits this can lead to increased leakage currents and failure in the field. It is advisable to use fluxes that are specifically designed for selective soldering and that are absolutely halogen free. The IPC classification for fluxes allows up to 500ppm of halogens for the lowest activitiona class but also these 500ppm can be critical, so absolutely halogen free is the key word. The next step in the process is preheating. This process step evaporates the solvents of the flux and provides heat to support good through hole wetting of the solder. Soldering is a thermal process and a certain amount of heat is needed to make a solder joint. This heat is needed from the bottom as well as from the top of the through hole component to be soldered. This heat can be provided by the preheating and by the liquid soldering alloy. Some basic machines do not have preheating, they will have to apply all heat through the liquid soldering alloy and in general they use higher temperatures for soldering. A preheating unit is usually a short wave IR (infrared) unit that applies the heat from the bottom side of the PCB. In most cases, the time and power of the preheating can be programmed. For thermally heavy boards and applications, top side preheatings exist. Usually they are hot air (convection) units where the teperature of the air can be programmed. When using this unit, it is important to know if there are any temperature sensitive components on the top side of the board that might be affected by this preheating.  Several systems for soldering exist. The one where the PCB board is standing still and only the soldering nozzle is moving is definitely preferred as all G-forces should be avoided when the solder solidifies. In the soldering step, a liquid soldering alloy is pumped through a soldering nozzle.There are different nozzle sizes and shapes available, wide nozzles, small nozzles, long nozzles and short nozzles.  Depending on the components to be soldered, one is preferred to another. In general wider nozzles and shorter nozzles give better heat transfer and are preferred. Smaller and longer nozzles can be used for situations with limited accessibility. Wettable nozzless are preferred to non wettable nozzles as they give a much more uniform flowing of the solder and more stable soldering results. Nitrogen flooding of the nozzle is advisable to have a stable flowing of the solder. The nitrogen is preferrably preheated because when not, it will cool down the solder and the PCB. The optimisation of the soldering program is essential for optimisation of the throughput/capacity of the selective soldering machine. This will focus on finding the minimal times and maximal speeds that give good through hole wetting in combination with no bridging.

Key advantages

  • Absolutely halogen free soldering chemistry contains no intentionally added halogens nor halides. The IPC classification allows up to 500ppm of halogens for the lowest 'L0' classification. Soldering fluxes, solder pastes and solder wires from this class are often referred to as 'halogen free'. Absolutely halogen free soldering chemistry goes one step further and does not contain this 'allowed' level of halogens. Specifically in combination with lead-free soldering alloys and on sensitive electronic applications, these low levels of halogens have been reported to cause reliability problems like e.g. too high leakage currents.  Halogens are elements from the periodic table like Cl, Br, F and I. They have the physical property that they like to react. This is very interesting from the point of view of soldering chemistry because it is intended to clean off oxides from the surfaces to be soldered. And indeed halogens perform that job very well, even hard to clean surfaces like brass, Zn, Ni,...or heavily oxidized surfaces or degraded I-Sn and OSP (Organic Surface Protection) can be soldered with the aid of halogenated fluxes. Halogens provide a great process window in solderability. The problem however is that the residues and reaction products of halogenated fluxes can be problematic for electronic circuits. They usually have high hygroscopicity and high water solubility and give an increased risk on electro migration and high leakage currents. This means a high risk on malfunctioning of the electronic circuit. Specifically with lead-free soldering alloys there are more reports that even the smallest levels of halogens can be problematic for sensitive electronic applications. Sensitive electronic applications are typically high resistance circuits, measuring circuits, high frequency circuits, sensors,...That's why the tendency is to move away from halogens in soldering chemistry in electronics manufacturing. In general when the solderability of the surfaces to be soldered from component and PCB (Printed Circuit Board) are normal, there is no need for these halogens. Smartly designed absolutely halogen free soldering products will provide a large enough process window to clean the surfaces and get a good soldering result and this in combination with high reliability residues. 

  • Solder balls are small balls of soldering alloy that remain on the solder mask of the PCB (Printed Circuit Board) after wave, selective or reflow soldering. They are non desirable but often present. They are usually caused by more parameters. In wave soldering the biggest parameter is the solder mask. The tendency of a solder mask to 'generate' solder balls depends on its surface structure which is a property of the solder mask itself. Furthermore the correct curing parameters of the solder mask in PCB (Printed Circuit Board) manufacturing needs to be respected. Poor curing may result in more solder balls. A second parameter is the flux. Some fluxes have more tendency to solderballing than others. In general, the higher solid content fluxes and fluxes from the 'RO'- classification generate less solder balls. Water based fluxes in general generate more solder balls than alcohol based fluxes but special versions of water based fluxes exist that provide lower solder balling than alcohol based fluxes like PacIFic 2009MLF and PacIF 2009MLF-E. In the process it is important to have correct flux application setting in combination with the right preheating setting to minimise solder balling. Too much flux, or flux that has been pushed in between the carrier and the PCB can be hard to dry off in the preheating and can generate solder balls upon wave contact. Too low preheating settings in this matter can also be problematic, certainly with water based fluxes. A hot air convection preheating can help to evaporate flux solvents. Another parameters is the solder wave. Turbulent waves generate more solder balls. Turbulence can be caused by the type of wave former itself (like e.g. a chip wave or a Wörthmannwave) or by bad settings or dross pollution in the wave former. The physical construction of the PCB board and carrier can also create extra turbulence. PCBs with a lot of components on the solder side and carriers with small and deep pockets will create extra turbulence. In selective soldering also the solder mask is the main parameter for solder balls and the differences between fluxes are similar to wave soldering. In this process the miniwave itself is turbulent and often is used to solder connectors who create an extra turbulence. This results in the fact that in general the selective soldering process is even more sensitive to solderballs than wave soldering. In reflow soldering, the main cause of solder balls is the solder paste printing process. If solder paste ends up outside the wettable solder pads this can result in solder balls after reflow. The reason for this can be numerous: The horizontal positioning of the PCB (Printed Circuit Board) underneath the stencil was not correct, the vertical alignment of PCB and stencil was not correct (not parallel). The pressure of the PCB against the stencil was not high enough, the squeegee pressure was too high, the printing speed was too low, there was no stencil aperture reduction, there was a deviation in the PCB, the temperature in production was too high (>30°C), accumulating residues due to too long intervals for stencil cleaning, a solder paste that slumps after printing, An oxidized solder paste,... Some solder pastes can be more sensitive to generate solder balls when they are outside the wettable pad than others. Another cause of solder balling can be the Pick and Place unit. When the vertical force when placing the component is too high, this can result in the paste being squashed and ending up outside of the wettable pad. Unfortunately, not all Pick and Place machines are easily adjustable in this matter. The soldering profile might als contribute to solderballs. Soak zones between 100-150°C are known to cause some solder pastes to slump and end up outside of the pad. This however can be very different from one solder paste to another. Vapor phase ovens in general are also a bit more sensitive to create solder balls as the liquid that condensates on the vapor can cause the solder paste to slump. Also here, there can be quite a big difference between one solder paste and the other. Another phenomenon where a solder ball is sticking to the side of a chip component is called solder beading or mid chip solder balling. This is mainly caused by too much solder paste and the non wettable part of the component lead that is contacting with the solder paste. The excessive solder paste will remain as a solder ball sticking to the side of the chip component. A thinner stencil, a higher stencil aperture reduction and a special stencil aperture design are used to solve the problem of solder beading.

  • Alcohol based soldering fluxes are liquid fluxes  that have alcohol(s) as their principal solvent(s). The majority of liquid fluxes used in electronics manufacturing are still alcohol based. The main reasons are their historical use and hence market share and their in general larger process window compared to water based fluxes. Water based fluxes have numerous advantages to alcohol based fluxes, like lower consumption, no VOC (Volatile Organic Compound)-emmissions, no fire hazard, no need for special transport and storage, lower smell in the production area,...However a lot of electronic manufacturers seem to prefer the larger process window of alcohol based fluxes to the advantages of water based fluxes. Alcohol based fluxes in general are less sensitive to the correct spray fluxer settings to get a good flux application on the surface and in the through holes. Furthermore they are more easily evaporated in the preheating and give less risk on remaining solvent drops creating solder balls, solder splashes or bridging upon wave contact. Another parameter that is complicating the implementation of water based fluxes is that changing a flux in some cases can be a time consuming and costly process. It usually involves homologation testing and approval of end customers. Specifically for EMS (Electronic Manufacturing Servivces = subcontractors) this can be a challenge. Some countries have already implemented legislation that limits the VOC-emission of factory chimneys or imposes taxes on VOC emissions. This appears to be an extra incentive to change to water based fluxes. A recent development forced a lot of manufacturers to look into water based fluxes. The COVID-pandemia in the beginning of 2020, suddenly increased the demand for alcohol based desinfectants to that extent that at a certain moment the availability of alcohols on the market was pretty much non existing. Luckily the industry that produces alcohols was able to ramp up their volumes just in time to avoid electronic manufacturers to fall without fluxes to operate their soldering machines.