IF 14

solder wire

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!
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.
Lead-free soldering
Lead-based soldering

Key advantages

Colophony, also called rosin, is a substance derived from trees that is typically used in soldering fluxes. It can be used in liquid fluxes as well as in gel fluxes. Colophony containing fluxes can be identified by the denomination 'RO' in the IPC classification. Colophony in general provides a good process window in time and temperature but has a number of disadvantages depending on the application that the colophony containing flux is used in. In liquid fluxes for wave and selective soldering, the colophony will give an increased risk on blocking the nozzle of spray and micro jet flux application systems, resulting in more maintenance and higher risk on bad soldering results. The residues of a rosin (=colophony) based flux in the soldering machine and on tools and carriers are quite hard to remove and a solvent based cleaner is usually needed. When the flux with colophony accidentally ends up on the contacts of a connector or contact comb structures like for a remote control or in electro mechanical contactors / relays / switches, it is known to give contact problems and malfunctioning of the electronic unit in the field. Furthermore the residues of the flux that remain on the board can give contact problems with electrical pin testing ( ICT= In Circuit Testing) which can result in delays in production because of false errors. This usually requires cleaning of the PCB and/or the test pins. These expensive test pins are rather fragile and sensitive to be damaged by cleaning. Furthermore the residues of a rosin flux are known not to be compatible with conformal coatings in time. The rosin residue forms a separation layer between the PCB and conformal coating that in time can cause detaching of the conformal coating and also cracking, especially when the electonic unit experiences a lot of temperature cycles (warming up and cooling down). For those reasons fluxes without colophony and more specifically fluxes from the 'OR' classification are generally used for wave and selective soldering. Colophony can also be used in solder wires. Although the colophony provides a good process window in time and temperature, it is very sensitive to discoloration when heated. The discoloration will depend on the type of colophony and the temperature it has seen. As soldering tip temperatures are usually quite high, the colophony in the solder wire will give quite heavy visual residue formation around the solder joints. This will distinguish them from the other solder joints made in reflow, wave and selective soldering. When this is not desirable a cleaning operation needs to be performed. Furthermore the fumes of a colophony containing solder wire are considered hazardous. A fume extraction is mandatory but anyway advisable for any hand soldering operation. Colophony containg wires are still being used quite a lot but colophony free solder wires and more specifically solder wires from the 'RE' classification are gaining importance. Colophony 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.
Residues after soldering are inherent to the soldering process. Some soldering products will leave more residues than others. In general low residue soldering products have the preference. Residues are usually undesirable for more potential reasons. One of those reasons is esthetical. When the end customer receives his boards, obviously he likes them to be as clean as possible. Residues can also interfere with electrical pin testing, like ICT (In Circuit Testing) or flying probe. They might create contact problems and false readings that can obstruct production flow. Residues can also assemble on the test pins where they need to be cleaned off. These test pins are pretty fragile and the risk on damaging them during cleaning is real. Residues of the soldering process might also interfere with high frequency signals of sensitive electronic applications. Residues created by rosin and resin usually have poor compatibility with conformal coatings. Furthermore they are known to cause contact problems when they end up on connector contacts, (carbon) contacts of remote controls, contact surfaces of switches, relays, contactors,...and cause field failures. When the soldering product is classified as 'No-clean' it is an indication that the residues of these soldering products can remain on the electronic unit. This is based on passing reliability tests like Surface Insulation Resistance (SIR) tests and electro (chemical) migration tests. There are many standards worldwide that specify such tests. The most accepted standard is the IPC standard. In these reliability tests a test board with a comb pattern is soldered with specified parameters with the soldering product. The test board is submitted to high humidity and elevated temperature conditions over a defined period of time during which the surface insulation resistance is monitored. This surface insulation resistance cannot drop below a defined value and boards are also visually inspected with a microscope for anomalies like e.g. electro (chemical) migration.
A brushable residue left by a soldering product can be moved with a dry brush without the aid of a solvent. Most residues of soldering products can only be moved by dissolving them with a suitable solvent or cleaning liquid. The advantage of a brushable residue is that the cleaning operation is much quicker and easier. This quality is very much appreciated in visual control and rework and repair after the soldering process in electronics manufacturing.
Soldering tip contamination can be caused by residues of the soldering flux contained within the solder wire or by repair fluxes. This contamination inhibits heat transfer from the soldering tip to the surfaces to be soldered. A certain amount of heat is needed in order to make a solder joint and when heat transfer is inhibited this will slow down or even prevent the making of that solder joint. Some solder wires tend to create more contamination of the soldering tip than others. In this matter the flux quantity contained within the solder wire is an important parameter, logically the higher the flux content, the higher the contamination. However, the type of flux inside of the wire in many cases is the most important parameter that will determine the soldering tip contamination. Some solder wires give substantially lower soldering tip contamination than others. Especially in high volume hand soldering operations and robot soldering applications solder wires that give low soldering tip contamination are advantageous as they will increase throughput. They will provide optimal heat transfer and reduce soldering tip cleaning operations.
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.
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.
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.
Lead-based alloys are the traditional SnPb(Ag) based alloys that were used to connect electronic components to PCB (Printed Circuit Board) boards in electronics manufacturing before 2006. In 2006 legislation restricted the use of lead (Pb) because of the risk that end-of-life electronics in land fills would pollute groundwater and Pb would be introduced in the ecosystem. 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. For this reason electronics manufacturing introduced lead-free soldering alloys. As the long term reliability of the lead-free alloys was at that point (2006) not yet established, some critical branches of the electronics industry like e.g. automotive, railway,medical, military,... were allowed to temporarily continue using the SnPb(Ag) alloys. But also in these branches the use of lead-based alloys is gradually being phased out. The most typical alloys for wave soldering were Sn60Pb40 and Sn63Pb37 with melting point around 183°C. This facilitated operating temperatures around 250°C. The oxydation behavior of the alloys was considered acceptable and the use of a closed nitrogen atmosphere like for lead-free alloys was not necessary. For reflow soldering, the most typical alloy was Sn62Pb36Ag2 with melting point around 179°C. The addition of Ag gives extra mechanical reliability to the SMD (Surface Mount Device) solder joints who are typically less strong than through solder joints. The alloy facilitated (measured) peak temperatures in between 200-230°C. The use of nitrogen in reflow was existing but certainly not as widespread as with lead-free alloys.