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Lead-free Manufacturing
Analysis of solder paste using the Frequently Asked Questions

     Solder reflow soldering is used in SMT assembly process of the main board-level interconnect method, this method of combining the need for welding the welding characteristics of an excellent combination, these features include easy processing, design of a broad variety of SMT compatibility, high reliability and low cost of welding; However, in the reflow soldering is used as the most important of the SMT component and board-level interconnect methods, it has also been asked to further improve the welding performance challenges, the fact that on the ability to withstand reflow soldering technology, this challenge will determine whether to continue as a primary paste SMT soldering materials, especially in the ultra-fine pitch technology continues to progress in the circumstances. Next, we will explore the impact of improved reflow performance of several key issues for the hair to stimulate the industrial sector developed new methods to address this issue, we were briefed on each issue are as follows:

Underside of the mounting

Double-sided reflow soldering has been used for many years, in this first printing the first side wiring, installation of components and soft-melting, and then turned over to the other side of the circuit board for processing, in order to save more purposes, eliminating the need for some process on the first surface of the soft melting, but at the same time soft-top and bottom melting, the typical example is the bottom surface of circuit board containing only small components, such as chip capacitors and chip resistors, printed circuit board because of (PCB) design of the more to more complex components installed in the bottom surface of the increasingly large, the results of the soft melting off when the components become an important issue. Clearly, the component is due to the soft melting off when the molten solder to the component perpendicular to a fixed power enough, strong enough to be attributed to the vertical fixed component to increase the weight of components, poor solderability, solder flux wetting or insufficient amount of . Among them, the first factor is the most fundamental reason. If the right behind the three factors should be improved there are still elements off phenomenon exists, it must use the SMT adhesives. Clearly, the use of soft melting binder will enable self-aligning effect when the component variation.

Man is not welding

Is not full of welding wire in the adjacent bridge formed between the solder. In general, all the factors that can cause solder paste slump will lead solder is not full, these factors include: 1, heating rate is too fast; 2, Solder paste thixotropy can be bad, or the viscosity in the shear recovery too slow; 3, the metal loading or solids content is too low; 4, powder particle size distribution is too wide; 5; flux surface tension is too small. However, the slump is not necessarily caused by welding is not full, in soft melting, the molten solder does not solder over the surface tension driven off under the possibility of loss of the phenomenon of solder will not weld over the problem even more serious. In this case, due to the loss of solder gathered in a particular region of the excessive amount of solder will make too much molten solder is not easy to become disconnected.

In addition to the factors that suits the slump caused by paste, the following factors also caused by welding under the common reasons: 1, relative to the space between the solder joint, the solder paste deposition too; 2, the heating temperature is too high ; 3, Solder heat faster than the circuit board faster; 4 flux wetting speed is too fast; 5, flux vapor pressure is too low; 6; flux of solvent composition is too high; 7, flux resin softening point is too low.

Intermittent wetting

Solder film refers to the intermittent wetting of water on the surface appear smooth (1.4.5.), This is because the solder can adhere to most of the solid metal surface, and in molten solder coating is hidden are certain not wetting the point, therefore, in the initial melting of the solder used to cover the surface, there will be intermittent wetting phenomena. Metastable molten solder coating to a minimum surface energy driving force will occur under the action of contract, soon after the separation of a small gathering into a ball and ridge-like bald. Intermittent wetting can also be from the parts in contact with the molten solder caused when the gas release. Because the thermal decomposition of organic matter or mineral water co-operation with the release of water will produce gas. Those relating to gases, water vapor is the most common elements in the welding temperature, water vapor with a very strong oxidation that can melt the solder oxide film on the surface or some other surface of the interface (a typical example is the molten solder at the junction of metal oxides on the surface). Common situation is the higher soldering temperatures and long residence time will lead to more serious phenomena of intermittent wetting, particularly in base metals, the reaction rate of increase would lead to more violent gas release. At the same time, the longer residence time will be to extend the gas release time. These two aspects will increase the amount of gas released to eliminate the phenomenon of intermittent wetting method is: 1, lower welding temperature; 2, shortening the length of stay of soft melting; 3, using the flow of an inert atmosphere; 4, lower pollution levels.

Low-residue

Without clean-up of the soft-melting process, in order to obtain decorative or functional effect, often require low-residue, the functional requirements, examples include "the test circuit through the flux residues to probe testing as well as the cladding layer inserted between the connector and the cladding layer or insert a welded joint and soft contact hole near the electrical contact between the implementation of "flux residues more often result in the implementation of electrical contact to the metal surface there is too much residual material covered, this will hinder the establishment of electrical connectors in the circuit density, increasing the case, this problem more and more by people's attention.

Obviously, do not clean up low-residue solder paste is to meet this requirement, an ideal solution. However, the related necessary conditions for the soft melting has made this issue even more complicated. To predict the different levels of inert atmosphere, the soft low-residue solder paste melting welding performance, to propose a semi-empirical model, the model indicates that, with the reduction of oxygen content, the welding performance is rapidly improving, and then gradually stabilized The experimental results showed that with the reduction of oxygen concentration, weld strength and the paste will increase the wetting ability, in addition, welding with the flux intensity in the solid content increases. Experimental data, the proposed model is comparable, and a strong proof that the model is effective, can be used to predict solder paste and material properties, therefore, can assert that, in order to successfully use welding process without clean-up low-residue solder, should use the soft melting inert atmosphere.

Gap

Gap refers to the component lead and the circuit board solder joints did not form between the solder joints. In general, this can be attributed to the following four reasons: 1, solder cladding is inadequate; 2, lead coplanarity poor; 3, wetting is not enough; 4, solder loss dates This is determined by pre-tinning of printed circuit boards Slump on the solder paste, lead the core suction (2.3.4), or in the vicinity of the through-hole solder joints caused, lead coplanarity issue is the new lighter 12-mil (m) spacing of the four-wire flat IC (QFP dates Quad flat packs), a special issue of concern, in order to solve this problem, put forward before the assembly to pre-coated with solder joints of the method (9), this method is to expand the local solder joint size, and along the muster of the solder pre-covered area to form a local welding zone can be controlled, and thus to offset the changes in lead coplanarity and the prevention of clearance lead role of the core can slow down the heat absorption rate and to enable the underside than the top surface of the heat more to be resolved, in addition, the use of wetting slower flux, the higher activation temperature or can delay the melting of the solder paste (such as powder and lead powder mixed with tin solder paste) are also able to maximize the reduce the wicking effect. in use tin-lead coating before the finishing circuit board with solder mask to cover the connecting paths can also prevent the hole from the vicinity of the core caused by suction.

Solder balling

Solder into a ball is the most common and most difficult issue, which refers to the soft solder melting processes far away from the main solder bath where coagulation into spherules of varying sizes; most cases, these spherules are by a flour paste in the solder composition, solder ball into people worry that there will be a short circuit, leakage and welding point, insufficient solder problems, with the fine pitch technology and do not clean up the progress of welding method, it is more to the more urgently requires the use of non-solder ball into the phenomenon of SMT process.

Lead solder into a ball (1,2,4,10) reasons: 1, as printed circuit technology resulting from the inappropriate grease; 2, solder paste has been exposed to excessive oxidation of the environment; 3, solder paste too much exposure to wet conditions; 4, inappropriate heating methods; 5, heating too fast; 6, preheat section is too long; 7, solder mask and paste interaction; 8, flux activity is not enough ; 9, solder powder oxides or excessive pollution; 10, dust particles, too; 11, in particular the soft melt processing, the flux inside an inappropriate mixing of volatile chemicals; 12, due to solder paste formulations caused by improper solder Slump; 13, solder paste prior to use did not fully restored to room temperature, opened the package used; 14, printing the thickness of thick lead to "collapse" to form solder balls; 15, solder paste in the low metal content.

Solder beading

Solder beading on the use of solder paste and SMT technology, when the ball into a special phenomenon., Simply put, welding beads are those very large ball, with his stick with (or not) a small solder ball ( 11). They form in the legs with a very low care components such as chip capacitors around. Solder beading exhaust caused by the flux in the warm phase of the role of this exhaust more than the cohesion of solder paste, solder paste at low exhaust contributed to the formation of isolated gap under the granular component in the soft melting, the melting of the isolation paste emerge again from the next element, and coalescence effect.

Welding beading following reasons: one, the thickness of printed circuit is too high; 2, solder joints and components overlap too much; 3, in the component under too much solder paste coated; 4, placed too much pressure components; 5, when the temperature rises too fast preheat; 6, preheating temperature is too high; 7, in the moisture from the components and solder compound release; 8, flux activity is too high; 9, the powder used is too thin; 10, metal load is too low; 11, Solder slump too much; 12, solder powder oxides too; 13, solvent vapor pressure less than. Elimination of solder beading may be the easiest way is to change the pore shape of the template, so that in the low care components and solder joints between the legs folders have fewer solder paste.

Welding Welding lift angle

Raised seam welded angle means the lead after the wave soldering and welding fillet weld spacing from the circuit with a minor group of the four cores flat IC (QFP) of the solder joint is completely lifted, especially in the components near the edges and corners where a possible reason is that the wave before the sampling, when added to the lead on the mechanical stress, or when dealing with the circuit board by mechanical damage (12), in the wave before the sampling time, using a tweezers across the QFP components of the lead, in order to determine whether all of the lead in soft melting when they welded on the baking; its result is not produced at the weld toe, which can be down from the observation that, if the board the following heating in the welding District / angle between the weld interface caused some secondary soft-melting, then lifted his lead from the circuit board and angle can reduce the inherent stress of weld to prevent this problem one way is to wave after (rather than in wave before) to conduct random checks.

Tombstone (Tombstoning)

Tombstone (Tombstoning) refers to the lead-free components (such as chip capacitors or resistors) to leave one end of the substrate, or even the whole components are supported on one end of it.



Tombstoning also known as the Manhattan effect, Drawbridging effects or Stonehenge effect, it is uneven at both ends by the soft-melting components, wetting caused; so uneven molten solder surface tension of tension exerted on the components on both ends, the following the SMT the progress of miniaturization of electronic components on this issue have become increasingly sensitive.

The formation of the reasons for this situation: 1, uneven heating; 2, component issues: differences in shape, weight, too light, weldability differences; 3, poor thermal conductivity of the substrate material, substrate thickness uniformity of the poor; 4, the heat capacity difference between pad larger pad solderability difference is relatively large; 5, solder paste flux uniformity in poor or poor activity of the two differences in the thickness of solder paste on the pad large thick paste, printing accuracy, dislocation serious; 6, preheating temperature is too low; 7, placement accuracy, component offset serious.

Ball Grid Array (BGA) into a ball bad

BGA balling is not often encounter, such as welding full, ball alignment, bump the amount of leakage, as well as insufficient solder defects, this is usually due to the soft ball when melting under a fixed force or self-energy arising from the insufficiency. Fixed power shortage may be caused by a low viscosity, high-barrier thickness or the speed resulting from high-level radioactive gas; while since anyone, under generally weak or solder flux activity caused by low volume.

BGA balling effect can be used alone or to the solder ball and solder paste and solder ball and flux paste used together to achieve; the correct alternative would be to the whole pre-forming and the flux or solder paste used together. The most common method seems to solder ball and solder paste is used in conjunction with the use of tin-63 tin-62 or the ball the ball into the welding process produced excellent results. The use of flux to carry 62 or tin-tin solder balls 63 of the cases, the defect rate with the flux viscosity, solvent volatility and a decline in pitch size increase, but also with the deposition flux thickness, flux activity, as well as solder joint diameter increased, carried out using high-temperature melting solder paste ball welding system, there is no leakage was observed with ball phenomenon, and its alignment accuracy with the solder paste deposition thickness and volatile solvents, flux activity, solder joint size and weldability, and the metal load increases in the use of solder paste 63, the solder paste viscosity, spacing, and soft melting cross-section of high-melting temperature into a ball rate is almost no effect on . Asking the conventional printing process of the release date, the easy-to-the release of solder paste on the solder paste into a separate ball is essential. Pre-formed into a ball the whole process is also the development of a very bright future. Links to reduce the thickness and width of solder right into the ball to improve the success rate is also very important.

The formation of pores

The formation of pores is usually a welded joint related problems. In particular, the application SMT welding techniques to the soft paste, when, in the use of leadless ceramic chip cases, most of the large pores ( "0.0005 inches / 0.01 mm) is in a LCCC solder and printed circuit board solder joints between the At the same time, in the LCCC tilting the castle near the fillet weld, only a small amount of small pores, the presence of pores will affect the mechanical properties of welded joints, and will damage joints strength, ductility and fatigue life, which is because the pore growth of convergence can be extended to form and lead to fatigue cracks, porosity also cause stress and covariant solder increase, which is another cause of damage causes. In addition, the solder will occur in the solidification shrinkage, weld layered plated through holes such as the exhaust, as well as entrainment flux also contributed to the pore causes.

In the welding process, the formation of pores, armed system is more complicated, in general, porosity is a sandwich-like structure when the soft melting of the solder flux in the entrainment of the exhaust caused by the (2,13) mainly by the formation of pores metallization area weldability decision, and with the flux activity in the reduction of the metal powder to increase the load as well as the wire connector to increase the area of coverage under the change, reducing the size of the solder particles only sales increase in porosity Xu. In addition, the porosity of the formation of solder powder coalescence and the elimination of a fixed allocation of time between the metal oxides on. Paste the earlier coalescence to form a pore is also more. Typically, the proportion of large pores with the total pore volume increased. And the total pore volume as shown in the results compared to those caused by porosity of the formation of stimulating factors, the reliability of welded joints will have a greater impact on to control the pore formation methods include: 1, improved components / shirt at the end of weldability; 2, using a high activity to help solder flux; 3 to reduce the solder powder oxides; 4, using an inert atmosphere, heating .5, slow down the process of soft-melting prior to preheating. Compared with the above, in the BGA assembly in the formation of pores in accordance with a slightly different pattern (14). in general. 63 the introduction of tin solder BGA assembly block pores are mainly is generated at the board level assembly stage. In the pre-tin printed circuit board, BGA joints with the pore volume of the volatile solvents, metal components and soft melting temperature increased, but also with the reduction in powder size increased; This flux emitted by the decision of the rate of viscosity to be explained. According to this model, in the soft melting temperature have a higher viscosity of the flux medium would hinder flux emitted from the molten solder, therefore, increase the number of entrainment flux will be increase the possibility of deflation, resulting in the BGA assembly have a greater porosity. without regard to a fixed area of metal weldability of circumstances, the flux activity and soft-melting atmosphere on the pore-generated effects seem to be negligible. the proportion of large pores with the total pore volume increases, which shows results of the analysis and the total pore volume compared to the situation shown in the BGA has caused pore generating factors on the reliability of welded joints a greater impact, and this was the gap in health in the SMT process similar to the city.

Summary

Solder reflow soldering SMT assembly process is the main board-level interconnect methods, the main issues affecting reflow soldering are: underside components fixed, not welding, intermittent wetting, low-residue, clearance, solder balling , solder beading, solder fillet lifting, TombstoningBGA into the ball bad, forming pores, etc. The problem is not limited to this, not mentioned in this article, there are also leaching the role of intermetallic compounds, non-wetting, distortion twist, lead-free soldering and so on. Only by solving these problems, reflow soldering of SMT assembly as an important way to the era of ultra-fine pitch continue to successfully maintained.
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