Flexsol 903

Suitable for

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  Lead-free soldering

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  Rework & repair

  Rework and repair on an electronic unit can be performed on defective electronic units that return from the field but can also be necessary in an electronic production environment to correct defects in the assembly and soldering processes. Typical rework and repair actions involve the removal of solder bridging, adding of solder to poor through hole filled components or adding missing solder, replacing wrong components, replacing components that are placed in the wrong direction, replacing components that have defects related to the high soldering temperatures in the processes, adding components that were left out of the process due to e.g. availability or temperature sensitivity. The identification of these defects can be done by visual inspection, by AOI (Automated Optical Inspection), by ICT (In Circuit Testing, electrical testing) or by CAT (Computer Aided Testing, functional testing). A lot of repair operations can be done with a hand soldering station that has a (de)soldering iron with temperature setting. Solder is added by means of a solder wire that is available in several alloys and diameters and contains a flux inside. In some cases a liquid repair flux and/or a gel flux are used to make the hand soldering process easier. For bigger componnets, like BGAs (Ball Grid Array), LGA's (Land Grid Array) QFNs (Quad Flat No Leads), QFPs (Quad Flat Package), PLCCs( Plastic Leaded Chip Carrier),...a repair unit can be used that simulates a reflow profile. These repair units are available in different sizes and with different options. In most cases they contan a preheating from the bottom side that is usually IR (Infrared). This preheating can be controlled by a thermocouple that is placed on the PCB. Some units have a pick and place unit that facilitates the correct positioning of the component on the PCB. The heating unit is usually hot air or IR or a combination of these two. With the aid of thermocouples on the PCB, the heater is controlled to create the desired soldering profile. In some cases the challenge is to bring the component to soldering temperatures without remelting adjacent components. This can be difficult when the component to be repaired is big and has small components near to it. For BGAs with balls made of a soldering alloy, a gel flux can be used or a liquid flux with higher solid content. In this case the solder for the solder joint is provided by the balls. But also the use of a solder paste is possible. The solder paste can be printed on the leads of the component or on the PCB. This requires a different stencil for each different component. The BGA can also be dipped in a special dipping solder paste that first is printed in a layer with a stencil with one large aperture and a certain thickness. For QFNs, LGAs QFNs, QFPs, PLCCs,...solder needs to be added to make a solder joint. In some cases QFPs can be hand soldered but the technique requires experience so the use of a rework unit is preferred. QFPs and PLCCs have leads and can be used with a dipping solder paste. QFNs, LGA's QFNs who do not have leads but flat contacts cannot be used with a dipping solder paste dipped because their bodies would contact the solder paste. In this case the solder paste needs to be printed on the contacts or on teh PCB. In general it is easier to print solder paste on the component than on the PCB, especially when a so-called 3D stencil is used that has a cavity where the position of the component is fixed. Replacing through hole components can be done with a hand (de)soldering station. This is usually done by placing a hollow desoldering tip over the bottomside of the component lead that can suck away solder from the hole. The desoldering tip will have to heat all the solder in the through hole until it is fully liquid. For thermally heavy boards this can be very difficult. In this case, also the top side of the solder joint can be heated with a soldering iron.  Alternatively the board can be preheated over a preheating before the desoldering operation. Soldering the through hole component is usually done with a solder wire that contains more flux or alternatively extra rework flux is added to the through hole and/or on the component lead. For larger through hole connectors, a dip soldering bath can be used to remove the connector. If accessibilty on the PCB is limited a nozzle with its size adapted to the connector can be used. The use of flux in this operation is recommended.
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  Hand soldering

  Hand soldering is a technology in electronics manufacturing that uses a hand (de)soldering iron to make a solder joint or to desolder a component from a PCB board. The process is mostly used in rework and repair but also to solder single components that have been left out of the bulk soldering process (reflow or wave soldering). This can be due to the availability or the temperature sensitivity of these components. The soldering iron usually is part of a soldering station that has a power supply that controls the temperature of the soldering iron. This temperature can be set according to the used soldering alloy and usually is between 320°C-390°C. The soldering iron has an exchangeable soldering tip that can be chosen according to the component to be soldered. For optimal heat transfer the biggest possible soldering tip is recommendable, certainly when soldering (heavy thermal mass) through hole components. For soldering thermally heavy components and boards, the power of the soldering station is also important to keep the set temperature of the soldering tip. In rework and repair, changing the soldering tip for every different component is not realistic and only a few soldering tips are used. Soldering tips exist to solder several surface mount solder joints in a row like for e.g. SOICs (Small Outline Integrated Circuit) and QFPs (Quad Flat Package). To promote heat transfer and flowing of the solder, the soldering tips are wettable, meaning that they make an interaction with the soldering alloy. During soldering these tips will oxidize and they can loose their wettability which will obstruct heat transfer. This can be avoided by cleaning the soldering tip with e.g. a tip tinner. After some time the soldering tips will also wear out and will need to be replaced. The life time of the soldering tip can be optimised by avoiding the use of abrasive or agressive soldering tip cleaners or by avoiding mechanically cleaning the soldering tip with e.g. steel wool or sand paper. The use of an absolutely halogen free tip tinner is advisable.  In hand soldering, the solder for the solder joint is usually provided by a solder wire. A solder wire is available in several diameters and several alloys, and has a certain quantity of a certain type of flux inside.  The alloy is usually the same or a similar alloy as the bulk soldering process (reflow, wave or selective soldering). The diameter is chosen according to the size of the solder joint. The flux content in the solder wire is usually determined by the thermal mass of the component and board to be soldered. (Heavy thermal mass) through hole solder joints need more flux. More flux content will also give more visual flux residue after soldering. Sometimes extra flux is needed which in most cases is a liquid rework and repair flux but also can be a gel flux.  The type of flux/ solder wire is determined by the solderability of the surfaces to be soldered. With normal solderability of electronic components and PCB boards an absolutely halogen free 'L0' type of flux/solder wire is advisable. In general a hand soldering operation is performed like this: Set the temperature of the soldering tip according to the used soldering alloy. For lead-free alloys, the advised working temperature is between 320°C and 390°C. For more dense metals like Nickel, the temperature may be elevated to 420°C. The use of a good soldering station is important. Use a soldering station with a short response time and with enough power for your application. Choose the correct soldering tip: to reduce the thermal resistance, it is important to create a large as possible contact area with the surfaces to be soldered. Heat up both the surfaces simultaneously. Slightly touch with the solder wire, the point where soldering tip and the surfaces to be soldered meet (the small quantity of solder ensures a drastic lowering of the thermal resistance). Add subsequently without interruption, the correct amount of solder close to the soldering tip without touching the tip. This will reduce the risk on flux spitting and premature flux consumption!

Key advantages

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  Low spattering

  Spattering of a solder wire is caused by evaporation of flux chemistry contained within the solder wire. Flux chemistry substances passing into the gas phase create a pressure that at a certain moment is released as a small explosion that can be accompanied by some hot spatters of flux and alloy. These spatters are unwanted because they can create pollution and even short circuits on the electronic unit. Specifically lead-free solder wires are sensitive to this phenomenon because they have high melting points. This means that the temperature they need to reach to get to the liquid state is high and the higher the temperature, the higher the vapor pressure of the evaporating soldering flux. Some solder wires have been specifically designed to minimise this spattering phenomenon. They can be very interesting for robot soldering and laser soldering processes where spaterring is a known issue. But also for normal hand soldering of electronic units, a solder wire with low spattering properties can be interesting.
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  Rosin based (colophony)

  Rosin also known as colophony is a natural product coming from trees. There are many kinds of rosins with very different properties but some general properties apply.  As a part of soldering chemistry, like soldering fluxes, solder pastes and solder wires, in general, rosin provides a large process window in the soldering process. This means that in general it is able to withstand longer times and higher temperatures than e.g. a resin.  An advantage of the rosin in a liquid flux is that in general it tends to leave less solder balls on the solder mask after wave or selective soldering. Furthermore the rosin residue will give a certain protection against atmospheric moisture. This can provide an extra chance to pass climatic reliability tests. This protection capacity however degrades in time.  On the other hand, rosin contained in a liquid soldering flux can also have some disadvantages. It increases the risk on blocking the spray nozzle or jet nozzle of wave and selective soldering machines. The residues left in the machine and on carriers are quite hard to clean off. Residues left on the PCB board can interfere with electrical pin testing (ICT, In Circuit Testing) and create a contactproblem causing a false reading/false error. In some cases this can lead to obstruction of the production flow. When some of the rosin containing flux spray accidentally ends up on contacts of e.g. a connector, a switch/relay/contactor with a partial open housing or on carbon contacts or on contact pattern on the PCB, this can also lead to contact problems. Rosin residues in general have poor compatibility with conformal coatings. After thermal cycling the conformal coating can start showing cracks where atmospheric moisture can penetrate and condensate. Considering all the above, weighing the advantages of rosin in liquid soldering fluxes against the disadvantages, there is an ongoing tendency to chose for liquid fluxes without rosin. 'OR' classified fluxes do not contain rosin.  Rosin is very often used in solder wire because of its wide process window in time and temperature.  The disadvantage is that rosin tends to discolor with temperature and leave visually heavy residues. When the solder wire is used for reworking electronic PCB boards, this residue is for some electronic manufacturers non desirable, as they do not like their customers to see that rework has been done on a PCB. Cleaning of these rosin residues requires special cleaning agents and is a time consuming process. In this case manufacturers can chose for an RE classified solder wire like IF 14. The residues are minimal and can be brushed away with a dry brush. Rosin is also used in solder pastes. Beside giving a good process window in time and temperature, it also provides a good stability of the solder paste on the stencil. This will facilitate a stable printing process and hence stable soldering results and defect rates. The discoloration of the rosin in reflow soldering is not so prominent as it is with a solder wire because the temperatures in reflow soldering are lower than in hand soldering. Still the rosin residue has poor compatibility with conformal coating and in time after thermal cycles it might show cracks or detatching of the conformal coating. Although most manufacturers will apply the conformal coating over the solder paste residues, for optimal results it is advisable to clean off the solder paste residues. Giving the benefits of colophony described above, most solder pastes contain colophony.
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  Absolutely halogen free

  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. 
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  Available in lead-free alloys

  Lead-free alloys are soldering alloys without Pb used to connect electronic components to PCB (Printed Circuit Board) boards in electronics manufacturing. In 2006 legislation restricted the use of lead (Pb) because of the risk that end-of-life electronics in land fills would pollute ground water and Pb would be introduced in the eco-system. When Pb is taken in by the human body, it is very hard to be removed and it is known to cause all kinds of (long term) health issues. In 2006 the use of lead (Pb) was restricted by legislation. For that reason, the industry was forced to look into alternatives without Pb. In the end, the industry standardised on Sn(Ag)Cu based soldering alloys. These alloys provided acceptable useability in the existing soldering processes in combination with sufficient mechanical reliabilty of the solder joints and good thermal and electrical properties. The main disadvantage of Sn(Ag)Cu alloys is their quite high melting points (or melting ranges) that result in quite high operating temperatures. This induces thermo-mechanical stress on the electronic unit in the soldering processes that can result in damage or predamage of some temperature sensitive PCB materials and components. Typical soldering temperatures in wave soldering are 250-280°C, in selective soldering 260-330°C and measured  peakT° in reflow 235-250°C. The most popular alloy is the Sn96,5Ag3Cu0,5 alloy with melting temperature around 217°C, often referred to as SAC305. Other versions are SnAg4Cu0,5, SnAg3,8Cu0,7, SnAg3,9Cu0,6,...The differences in melting point between these alloys and the differences in terms of mechanical, electrical and thermal properties are for most electronic applications and soldering processes non significant. Because of cost reasons, the one with lowest Ag-content has the preference and that is the SAC 305. Also because of cost reasons, there is a trend towards low Ag SnAgCu alloys like e.g. Sn99Ag0,3Cu0,7, Sn98,5Ag0,8Cu0,7,... often referred to as low SAC alloys. These alloys have a melting range in between 217°-227°C. This in most cases will require higher working temperatures in the soldering processes up to 10°C, which for some temperature sensitive components can be significant. The mechanical, electrical and thermal properties of the low SAC alloys differ a bit more from the standard SAC alloys. In general they have a lower thermal cycling (fatigue) resistance but for most electronics applications this is not significant. The required 10°C higher working temperature however is often a problem in reflow soldering because most electronic units will have one or more temperature sensitive components. Furthermore, in general SMD (Surface Mount Device) solder joints are weaker than through hole soldered solder joints and SAC alloys in general have rather poor thermal cycling resistance, specifically on thin solder joints. Considering all these paremeters, in most cases the choice will be made for the standard SAC alloys and not the low SAC alloys for reflow soldering. For wave soldering the story is a bit different. The wave solder bath with a lead-free soldering alloy generates quite a lot of oxides becuase of its high working temperature. This is why a lot of manufacturers chose for closed nitrogen machines. This however requires investment in infrastructure which not every manufacturer is willing or able to do. The oxides generated, in genral are being sold back to the manufacturer of the soldering alloy where they are being recycled. The total cost for the electronics manufacturer in this matter is quite high, cetainly with the high Ag soldering alloys like SAC305. That's why there is a tendency towards the use of low SAC and even SnCu alloys (without Ag). Also here the higher melting point will require an increase in operating temperature to get acceptable through hole filling. As in most cases the heat is applied from the bottom side and to the leads of the components, the temperature sensitive components on top of the board in general do not suffer too much from this. In terms of mechanical reliability of the low SAC and  SnCu alloy, this is less an issue because through hole soldered solder joints in general are much stronger than SMD joints. When (glued) SMD components are wave soldered on the bottom side of the PCB this can be different. Also when thermally heavy applications need to be soldered, the higher melting points can give a problem with good through hole fill and cases are known where the working temperature had to be raised so much that PCB material and some components from the top side were damaged. In those cases a low melting point soldering alloy is a good solution. Low melting point alloys that are SnBi based were never considered a viable alternative in the changeover from Pb containing to Pb free alloys because of there incompatibility with Pb and in the transition phase where still a lot of components and PCB materials contained Pb, it was impossible to use them. However since a couple of years the industry is starting reconsider the low melting point alloys because they have a lot of advantages and the risk on Pb contaminationhas become extremely low. A low melting point soldering alloy like e.g. LMPA-Q requires much lower operating temperatures than the standard lead-free soldering alloys. In reflow soldering it requires a peak T° of 190°C-210°C, in wave soldering the bath temperature typically is 220°C-230°C and in selective soldering, the working temperature typically is 240°C-250°C. This substantially reduces the risk of damaging temperature sensitive components and PCB materials and even facilitates the use of cheaper components and materials that are temperature sensitive. In reflow soldering the low melting point alloy also gives lower voiding on BTCs (Bottom Terminated Components). In general low melting point alloys have lower than 10% voiding where lead-free SAC alloys typically have 20-30% of voiding. In wave soldering the low melting point alloy allows for faster production speeds up to 70% and in selective soldering where the soldering of connectors can be done up to 50mm/s the total process time can be reduced by half, increasing the machine capacity with 100%. Furthermore the low melting point alloy does not have problems with good through hole fill on thermally heavy components. The use of nitrogen for wave and reflow soldering is possible but not required. The thermal, electrical and mechanical properties of the LMPA-Q low melting point alloy are sufficient for most electronic applications. Given all these advantages, many see the low melting point alloys as the future of electronics manufacturing.
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  No-clean - no post cleaning needed

  When a soldering product is labelled No-clean, this means that  soldering product has passed reliability testing like a Surface Insulation Resistance(SIR) test or an electro(chemical) migration test. These tests are designed to test the hygroscopic properties of the residues of the soldering product under elevated temperature and high relative moisture conditions. No-clean is an indication that the residues can remain on the electronic unit after the soldering process without being cleaned. This will apply for by far most of the electronic applications. For very sensitive electronic applications, which are typically high resistance electronic circuits, high frequency electronic circuits, etc... it is possible that cleaning of the electronic unit is necessary. It is always the responsibility of the electronic manufacturer to judge wether cleaning is necessary or not.
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  RoHS compliant

  RoHS stands for Restriction of Hazard Substances. It is a European directive: Directive 2002/95/EC. It restricts the use of some substances that are considered Substances of Very High Concern (SHVC) in electrical and electronic equipment for the territory of the European Union. A listing of these substances can be found below: Please note that this info is subject to change. Always check the website of the European Union for most recent information: https://ec.europa.eu/environment/topics/waste-and-recycling/rohs-directive_nl https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32011L0065 1. Cadmium and cadmium compounds  2. Lead and lead compounds  3. Mercury and mercury compounds(Hg)  4. Hexavalent chromium compounds(Cr)  5. Polychlorinated biphenyls (PCB)  6. Polychlorinated naphthalenes (PCN)  7. Chlorinated paraffins (CP)  8. Other chlorinated organic compounds  9. Polybrominated biphenyls (PBB)  10. Polybrominated diphenylethers (PBDE) 11. Other brominated organic compounds  12. Organic tin compounds (Tributyl tin compounds, Triphenyl tin compounds)  13. Asbestos  14. Azo compounds  15. Formaldehyde  16. Polyvinyl chloride (PVC) and PVC blends  17. Decabrominated diphenyl ester (from 1/7/08)  18. PFOS : EU directive 76/769/EEC (not allowed in a concentration equal to or higher than 0.0005% by mass) 19. Bis(2-ethylhexyl) phthalate (DEHP)  20. Butyl benzyl phthalate (BBP)  21. Dibutyl phthalate (DBP)  22. Diisobutyl phthalate 23. Deca brominated diphenyl ester (in electrical and electronic equipment) Other countries outside of the European Union have introduced their own RoHS legislation, which is to a great extent very similar to the European RoHS. 

Physical & chemical properties

Flux content

2,2% and 3,5%

Compliance

RO L0 to EN and IPC standards

Documents

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  TD Flexsol 903 (PDF)

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