Lead-free solder alloy with high tin content is gradually replacing the traditional alloy solder Sn63/37, with the introduction of the lead-free soldering in the manufacture of the electronic products. The Sn content of the lead-free solder is much higher than that of the traditional tin-lead solder. Therefore it is easier to oxidize, produce more oxidation slag (SnO2). The problem of the oxide slag is aggravating, notably the waste rate reaches to 30-50% or above, with the extensive application of the lead-free solder. The quality and reliability of the products soldered are also considerably influenced. How to minimize the formation of tin oxide slag becomes a compulsory course in the electronic manufacturing industry!

　　lHazards of tin slag

　　1. It affects the flow-ability of molten tin and the height of the tin surface and the soldering quality.

　　2. It attaches to the surface of the board and causes defects such as tin spherules on the board surface, and directly affects the reliability of the electronic products.

　　3. Loose oxide slag facilitates the air staying inside the molten solder, and aggravates oxidation of the solder.

　　4. Useful metals can’t be completely utilized due to the blockage of the tin oxide slag, which will lead to tremendous waste.

　　5. Extra problems in the management on the disposal and transportation of the tin oxide slag that will have an environmental impact to a certain extent.

　　lFormation of tin slag

　　1>The oxidation of static molten solder：

　　According to the theory of liquid metal oxidation, the molten metal surface will strongly adsorb oxygen which are decomposed into the oxygen atoms and become oxygen ions after obtaining electrons under high temperature, then combine with metal ions, forming metallic oxides. The whole oxidation of the surface of the molten metal exposed to the air can be completed instantly, forming a layer of single molecule oxidation film, further oxidization needs electronic movement or ions to transfer through oxidation film, the speed of oxidization of static molten solder decreases gradually;

　　In addition, the standard free energies of formation of d PbO and SnO2 under different temperature are different; the former can form easier thanks to its lower free energy of formation, which can explain to a certain extent why more tin slag forms easily when the solder are lead-free. Commonly the oxide film on the static molten solder is the mixture of SnO2 and SnO.

　　Oxides can partially dissolve into the molten solder as per distribution law, meanwhile the dissoluble difference drives the metal oxides diffuse internally, the gradually increasing oxygen content of the internal metals deteriorates the quality of the solder, this can explain in a certain extend why the metal alloys that refined by high temperature (or called reduction) oxidize easily and generate more slag; The differences in composition and structure of the oxide film, will affect its growth rate and mode, and distribution factors of the oxide in molten solder will have big difference, which is closely related to the compositions of the solder. In addition, oxidation also relates to parameters such as temperature, partial pressure of oxygen in gas phase, absorption of oxygen by the molten solder and decomposing speed, chemical reactivity of superficial atom and oxygen atom, compactness of surface oxide film, dissolubility and diffusivity of the products etc.

　　2>. Dynamic oxidation of molten solder

　　Double waves are widely used in wave soldering, i.e. the first wave peak is the turbulent peak, with narrower width, molten solder flowing faster; the second wave peak is laminar flow wave that looks like flat mirror, stable and slow in flow of the molten solder. New molten solder on the surfaces of the wave continuously contact with oxygen, forming oxide slag as the molten solder flow rapidly. The oxide slag that formed dynamically, have the following three forms and have big differences compared with the static oxidation:

　　a. Superficial oxidation film, oxidation of the molten solder is undergone in the tin stove at high temperatures via its surface exposed to air interacting with oxygen. The tin oxide film, an epithelial membrane mainly composed of SnO, is primarily formed in the relatively static surface of the molten solder in the stove. As long as the molten solder surface is not destroyed, it can play a role in insulating the air and prevent further oxidation of the internal solder within the protective layer. The superficial oxidation film accounts 10% of the oxide slag.

　　B. Black powders These powders are big in size and generated in the surfaces of molten solder and in the boundary of the axle shaft, they are roundly distributed and buildup near the shaft. The temperature of the molten solder near the shaft is not higher than other areas thanks to the better heat-transferability of the molten solder although there are frictions between high-speed rotation shaft and molten solder. The formation of black powders are not caused by temperature rising generated by frictions, but caused by vortex of molten solder around rotational shaft, the oxides move with the frictions and spherulize. Meanwhile the frictions can increase the superficial energy of the solder particles and aggravate oxidation that generate oxide slag; accounts about 20% of the total.

　　C. Oxidation slag Severe mechanical mixing effects exist in the mechanical pump wave generator. Acute vortex movements are being formed in the groove of the molten solder, which are aggravated by rolling motion due to unreasonable design. These movements of vortex and rolling continuously absorb the oxygen in the air and facilitate it entering the molten solder. A lot of silvery sand type oxide slag (or shoddy soybean curd residue) is formed within the molten solder due to limited oxygen absorption that can not fully oxidize as the surface oxidation. More slag formed and oxidation occur within the molten solder, then float to the surface of the molten solder or even occupy most parts of the groove, obstructing pump chamber and channels and then continuously lower the height of the peaks or cause damages in impellers or shaft of the pump. Another type of the slag is formed by the enhanced contacts between solder and oxygen in the air and oxygen absorption generated by acute vortex inside the molten solder swirled by the wave peaks refluxing to the groove. The said two types of slag account 70% of the total and cause huge losses. Application of lead-free solder will generate more oxide slag, and slag from SnCu solder are more than that of SnAgCu, with a typical structure of 90% metal and 10% oxide.

　　Japanese scholars Tadashi Takemoto <3> et al had carried out tests on SnAg3.0, SnAg3.0Cu0.5 and Sn63Pb37 etc, and found that the weights of slag of all three solder were increased linearly and the growth rate of the said three slag were nearly the same, that is to say the growth rates had less correlations with the compositions of the solder. The formation of oxide slag related to molten fluid flow, solder fluid instability and waterfalls effects may cause absorption of oxygen and rolling of molten solder etc that can make oxidation of oxides slag becoming more complicated. In addition, from the angle of the process, factors influencing the formation of oxide slag include the height of wave peak, soldering temperature and atmosphere, the wave soldering interference, types and purities of alloys or types of soldering agents, the multitude of wave passing through PCBA and the quality of original solder etc.

　　lStructure of the tin oxide slag

　　The tin slag we usually talk about is mainly composed of oxide SnO2 (i.e. tin grey) and Sn wrapped in the tin oxides and little carbonized substances. The tin wrapped in the tin oxides accounts at least 50% or above 90% of the total (the specific contents depend on the slag removed).

　　The oxide tin in the tin slag (i.e. tin grey) is usually SnO2, grey powder, tetragonal, hexagonal or orthogonal crystals, with a density of 6.95 g/cubic centimeters, melting point of 1630 degrees Celsius; its structure: O:SnO, molecular weight: 150.69, it sublimes from 1800-1900 degrees Celsius; it is hardly soluble in water, alcohol, dilute acid and alkali; it can be slowly soluble in hot strong alkali solution and decompose, and co-melt with strong alkali forming stannate; it can be soluble in strong sulfuric acid or strong hydrochloric acid. Tin content of the slag: 70-90% or up.

　　lMeasures of reducing tin oxide slag

　　Many research works on reducing the tin oxide slag in the wave peak soldering, have been carried out by domestic and overseas scholars and enterprises. The works are mainly focused on the following aspects:

　　1>. Nitrogen gas protection

　　Nitrogen gas protection: is an effective measure that uses nitrogen gas to isolate the oxygen in the air (normally the oxygen content in air is 27%) from the molten solder so as to reduce the formation of tin slag. It becomes one of the common technologies in lead-free soldering.

　　The lead-free solder hardly oxidizes and the quality of the soldering is better when the oxygen solubility is lower than 50ppm or less. The tin slag can be reduced to about 85 - 95% when the oxygen solubility ranges from 50 to 500ppm.

　　Nitrogen gas protection also brings disadvantages; it mainly forms tin spherules on the surface of PCBA and increase operation costs. The value of the tin saved can’t offset the cost of purchasing liquid nitrogen or the operating and maintenance cost of the nitrogen generator.

　　2>Use of anti-oxidation solder:

　　Japanese scholars Tadashi Takemoto et al. had carried out studies by adding Phosphorus and Ge elements into the solder such as SnAg and SnAgCu. The equipment is a little wave peak tin stove than can hold 15 Kg tin and the test temperature is 250 °C. Test results show that: the weight of tin oxide slag increases with time linearly; the weight of the oxide slag can be effectively reduced after adding a small amount of the Ge and Phosphorus elements, of which the addition of Phosphorus element can lower the weight of slag by about 50%; but the anti-oxidizing effect will gradually invalidate with the extension of time and consuming of trace elements. Therefore the anti-oxidation reductants appear!

　　3>Research on tin slag separation device

　　Research on tin slag separation device is the so- called tin reduction machine, its principle is to put the tin slag into the rolling drum, heat the tin slag to 400℃ and compress it to separate the usable tin from the unusable, the usable solder are collected into the hot stove and then be formed for further use, the unusable tin slag SnO2 (tin grey) are accumulated for removal and stored in containers for recycling.

　　This equipment belongs to the off-line separation that uses physical method. It is impossible to reduce Sn from the oxidized slag SnO2. The so-called tin recovered is the pure tins entrained in the slag. Most manufacturers are using Phosphorus element to improve its anti-oxidation properties, the antioxidant within solder alloy is used up after high-temperature separation; The gradual increment of oxygen progressively deteriorated the solder. Therefore the solder recovered oxidizes very quickly;

　　When using the tin slag separation device,i.e process the tin slag with tin slag reduction machine, it occupies space, needs special operation, power consumption, high noise etc; its processes of removal, transportation, storage and reduction etc are complex, increasing the management cost. Directly exchanging tin bars from manufacturers is acceptable when the reduction rate of tin is not high per se after deductions of expenses such as space rentals of the equipment and warehouses etc, salary of staff members, electricity fees and investment in equipment etc! The feasibility of using such equipment that can generate secondary pollution and also consume electricity, will be questioned when the supply of the electric power is tight！

　　4> Use of antioxidant solder

　　Japanese scholars Tadashi Takemoto <3> et al. had carried out studies by adding Phosphorus and Ge elements into the solder such as SnAg and SnAgCu, its chemical compositions are listed in Table 3. The equipment is a little wave peak tin stove than can hold 15 Kg tin and the test temperature is 250 °C. Test results show that: the weight of tin oxide slag increases with time linearly; the weight of the oxide slag can be effectively reduced after adding small amount of the Ge and Phosphorus elements, of which the addition of Phosphorus element can lower the weight of slag at about 50%; Chemical analysis of oxide slag show that Ge accounts 2-9% of the trace elements added and the P is 4.5 times more. The main content of oxide slag is SnO, 90% of the oxide slag are of metal when the oxygen content is about 5%.

　　but the anti-oxidizing effect will gradually invalidate with the extension of time and consuming of trace elements. Therefore the anti-oxidation reductants appear!

　　research on the tin slag reductant (powder) and its application

　　The antioxidant microelements within lead-free solder are used up rapidly and the solder quickly loses its efficacy due to the microelements being inclined to agglomerate on the surface of the solder and can preferentially absorb the oxygen in the air that Sn element; therefore the most practical and effective measures are to add a type of anti-oxidation-reductant so as to immediately reduce the oxide slag formed and prevent its agglomeration as well as hold further formation of oxide slag; domestic and abroad manufacturers released anti-oxidation-reductants (powders) for tin slag (slag of soldering, SnO2, i.e. called tin slag industrially) one after another.

　　So-called REDOX reaction is the elements contained in the reactants gaining and losing electrons in the reduction-oxidation reaction; elements that gain electrons are oxidant, elements that lose electrons are reductant.

　　Conditions that anti-oxidation reductants shall possess：

　　a>. Must conform to the requirements of environmental protection, do not affect working environment of the production site and do not affect the composition of solder alloy.

　　b>. Reacted residues shall be non-sticky or not fly away, shall not contaminate the surface of PCBA and existing production equipment (such as wave soldering machine etc).

　　c>. Be non-flammable, noncorrosive, it shall not change the existing production processes, shall not affect the existing equipment routine maintenance.

　　d>. The amount of the consumption shall be less, the reduction rate shall be high, easy to handle the residues reacted which shall be biodegradable; it can substantially save energy and reduce consumption for enterprises from the point of view of environmental protection.

　　ICHIMURA-- JR07 tin slag antioxidant developed by Shenzhen Kunqixinhua Technology Co., Ltd is an organic   polymer compound made of many surfactants, wetting agents, dispersing agents etc via scientific methods; it can   separate the tin wrapped by oxide slag and reduce tin (Sn) for further use from tin oxides (SnO2); and the effective compositions preferentially combine with the oxygen in the air the Sn element, which obviously lower the O2 content inside the solder, prevent further oxidation of the molten solder, enhance the flow-ability of the molten solder and effectively improve the PCBA soldering.

　　Its reduction process of the oxides can be roughly expressed as: O2 + R = OR; PbOx + R = Pb + OR ; SnOy + R = Sn + OR : where PbOx represents for the lead oxide, R for liquid reductant, and Pb for lead reduced, SnOy for tin oxides, Sn for tin reduced, OR for substances newly formed that can carbonize to generate sparks under long term high temperature and no wetting of reductants.

　　Advantages of the product:

　　The saving of the solder reaches 38% after deducting the cost of the anti-oxidation reductant, ICHIMURA --JR07 as per assessment report by the FLEXTRONICS Company. Certain high appraisals on the anti-oxidation-reductant, ICHIMURA --JR07 were given by the companies such as FOXCONN, HASEE, SOLECTRON, PRIMAX,GBM and HUNTKEY etc after relevant evaluations on the serial products.

　　Evaluation method of ICHIMURA --JR07 tin slag anti-oxidation reductant

　　lInterpretations on terms

　　Customer status: name and address of the customer to be evaluated；

　　Equipment status: types and line No. of the equipment to be evaluated;

　　Product status: name and model of the products being manufactured on the equipment;

　　Tin level: parameter measured when the wave peak is static;

　　The amount of tin adhered onto the single board: the weight difference of a single board before and after entering the stove, i.e. the actual amount of tin adhered on each board；

　　The tin consumed of single board: is the actual amount of tin consumed by each board, i.e. the sum of the amount of tin adhered on the board and the amount of tin slag formed；

　　Total tin slag formed: tin slag removed before and after the using of the reductant;

　　Exchange rate of tin slag: is the ratio based when exchanging the tin slag with tin bars suppliers, e.g. Calculate the ratio = the selling price of tin slag  ÷ the buying price of tin bars,  based on the selling price of tin slag and the buying price of tin bars if the tin slag is sold or exchanged；

　　Formation rate of tin slag is the ratio of amount of the tin slag removal to the amount of tin bar input under same tin level = amount of tin slag ÷ the amount of tin bar input;

　　Utilization rate of tin bars is the ratio of the amount of tin adhered onto the single board to the amount of tin consumed by each board= the amount of tin adhered onto the single board ÷the amount of tin consumed by each board, principally the formation rate of tin slag + Utilization rate of tin bars=100%;

　　Reduction rate of reductant: the tin utilization rate of tin bars after using the reductant;

　　Duration of test: it begins from the date of beginning to the end of the evaluation ；

　　lData collection

　　lCalculation method of cost savings

　　Most electronic manufacturers adopt the mode of using tin slag to exchange tin bars or using a reduction machine to separate the tin entrained inside the slag and then recycle to use. Tin waste exists in each step of the utilization. The waste rate of each step is the ratio between the total sum of waste and the amount of tin bars added in each step, 1minus waste rate equals utilization rate; the collection of data is onerous and also exists with big errors in different step of the recycling, therefore the amount of the tin adhered onto single board is adopted in the evaluation.

　　Formation rate of tin slag is the ratio of amount of the tin slag removal to the amount of tin bar input= amount of tin slag ÷ the amount of tin bar input;

　　Utilization rate of tin bars is the ratio of the amount of tin adhered onto the single board to the amount of tin consumed by each board = the amount of tin adhered onto the single board ÷ the amount of tin consumed by each board, principally the formation rate of tin slag + Utilization rate of tin bars=100%;

　　Continuously observe and confirm the level of molten tin solder within the melting chamber is consistent before and after use of the JR07 reductant in a week; provided the same type of PCBA is being manufactured (i.e. the amount of tin adhered onto the board is consistent before and after use of the JR07 reductant );

　　The amount of tin adhered onto the single board before using the reductant(1)= the amount of tin bars input B - (the amount of tin slag produced C x  the exchange rate of tin slag D)/ packs produced each week;

　　The amount of tin adhered onto the single board after using the reductant(2)= the amount of tin bars input B/ packs produced each week (the residues removed is regarded as valueless);

　　The cost of the reductant (3) = the amount of reductant input x unit price of the reductant;

　　Packs of PCBA produced each week (4);

　　Then, the expenses saved by single set of wave soldering equipment each week = [The amount of tin adhered onto the single board before using the reductant(1) - The amount of tin adhered onto the single board after using the reductant(2) ]X Packs of PCBA produced each week (4) X The cost of the reductant (3).

　　Using method of ICHIMURA --JR07 tin slag anti-oxidation reductant：cont’d

　　lSteps:

　　STEP 1:  Designate one side of the tin stove that facilitates operation as the reduction zone. The area of reduction includes the length (i.e. the width of the tin stove ) and width of 20- 30 cm, the width (as of the diameter of stirrer) shall be 80-100 cm if the stirrer is installed.

　　STEP 2: Partially remove the tin slag from the molten solder when the wave is normal, leaving 2-3 kg tin slag in the reduction area.

　　STEP3: Pour JR07 onto the surface of tin slag left previously in the reduction zone and evenly stir it (first time): pour 100-200g JR07 for tin-lead solder and 150-300 g JR07 for lead-free solder.

　　STEP4: Slightly stir and mix it after every 4-6 hours, and pour JR07 again and slightly mix it into muddy paste.

　　STEP 5: Remove the over-agglomerated tin slag from other parts to the reduction area.

　　STEP 6：Pour the JR07 onto the tin slag and evenly mix it as per STEP 3, repeat the steps and operation after about 4 hours.

　　lCalculation methods of the consumption of reductant

　　30-50 kg of tin slag can be reduced by one kg of ICHIMURA --JR07 tin slag anti-oxidation reductant;   the reductant consumed by each set of wave soldering equipment in 24 hours is 15KG÷30=0.5KG reductant, if the amount of tin slag formed in 24 hours in each soldering equipment is 15kg; the adding rate each time is 0.5KG÷6=0.083KG, if six times of adding is needed in 24 hours on the basis of 4 hours once;

　　lMain technical points

　　a. Before cleaning out, the residues covering the reduction surface shall be evenly mixed into muddy substances and then removed, add JR07 reductant as per the requirement;

　　b. JR07 reductant shall be timely added onto the tin slag just removed from other parts, mix it evenly;

　　c. Only remove the muddy substances on the top that contain no tin in every cleanout, it is absolutely forbidden to insert the tools into the tin stove removing slag from its bottom to top, otherwise it will entrain lots of usable tin causing unnecessary losses;

　　d. The tin content in residues removed every times shall be within 7-13%;

　　e. It is recommended to reduce the tin slag floating on the surface of tin cylinder from the jet nozzles when the equipment is in maintenance, add the reductant as per calculation methods on the basis of the pre-estimated amount of the tin slag;

　　f．Keep the residues wet enough, i.e. muddy paste by adding of the JR07 reductant, otherwise the residues harden, turn dry and carbonize to lead to possible fire disaster.

　　g．The tin content can be calculated out based on the amount of residues after washing by water, because the residues removed out  can dissolve into water; tin content calculated is regarded as the determining standard of whether it complies the standard (usually it ranges from 7-13%) .

　　lPrecautions:

　　a.The residues of JR07 after reaction can harden, dry and carbonize, leading to fire disasters under long term high temperature, although it has no flash point and is non-flammable. Therefore a sufficient amount of the reductant shall be guaranteed when using it.

　　b.Keep the exhausting fan of tin stove working normally when using the reductant although no harmful smoke or gases are detected in the reductant.

　　c. Only the usable tin within the slag can be reduced and other compositions such as dust that contain no tin can’t be reduced into usable tin by the reductant.

　　d. The product shall be kept from rain, heat and direct sunlight etc during transportation.

　　e. Prevent eye contact or swallowing the product when using; flush with fresh water or see a doctor if careless contact or swallowing happens.

　　f．It may become viscose or precipitate when stored under 10℃, but it will not lose its effects, sway evenly before use;

　　g．Store it in sealed containers in cool and ventilated places, its validation period is 1 years.