Wednesday, January 5, 2011

【 Weak current College 】 common circuit repair basics "eight" 】

BGA solder balls reset process
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1, introduction
BGA as a bulk package of SMD has promoted the development of SMT, producers and manufacturers are aware: in bulk-pin BGA packaging has strong vitality and competitiveness, but the price of a single BGA devices, for there is often many times while researching products, test is often required to BGA remove from the substrate and want to reuse the device. Because after it removed the BGA solder balls are destroyed, you cannot directly be welded in the substrate and must be reset ball, how to solder for regeneration of the technology challenge facing our technology staff. In Indium Corporation can purchase dedicated to BGA solder, solder BGA each individually to repair process is clearly undesirable, this article describes a SolderQuick of preformed worse for the BGA solder regeneration technology.
2, equipment, tools and materials
Preformed bad fixture flux deionized water cleaning plate cleaning brush 6 inch flat pliers acid brush reflow furnace and hot air system microscope means a set of (some tools can be selected depending on the circumstances)
3. processes and considerations
3.1 ready
Confirm the BGA fixture is clean. The reflow furnace heated to the temperature curve of the desired temperature.
3.2 process steps and considerations
3.2.1 the preformed bad into the fixture
The preformed bad into the fixture, the labeled side facing the following SolderQuik on fixtures. Ensure the preformed worse and fixture is loose. If you need a preformed worse to mount fixtures for bending, you cannot enter the operation after the procedure. Preformed worse does not fit into the fixture is mainly due to the fixture on the dirt or improper adjustment of flexible fixture.
3.2.2 in BGA rework flux coated with just the right amount of
Use with flux injection syringes in need to rework the BGA welding surface coating of some flux. Note: confirmation in flux used BGA solderingsurface is clean.
3.2.3 the flux is coated evenly with acid and brush the flux evenly brush in BGA packaging of the entire welding surface to ensure each pads are covered with a thin layer of flux. Ensure that each pad has flux. Thin flux welding results than thick.
3.2.4 the need to rework the BGA into the fixture, need to rework the BGA into the fixture, coated with flux of one face of preformed worse.
3.2.5 flat BAG, gently bump at BGA, preformed worse and navigate into the fixture BGA, to confirm the BGA flat on the preformed worse.
3.2.6 reflow
The fixture into the hot-air convection furnace or hot air reflow station and begin returning heating process. All use of the station curve must be set to have developed BGA solder balls regeneration process specific curve.
3.2.7 cooling
Use tweezers to fixture from the stove or reflow station and placed in the thermal plate, cool 2 minutes.
3.2.8 remove
When the BGA cooling after it is removed from the fixture's welding spherical up on clean-up.
3.2.9 immersion
With deionized water immersion BGA, over 30 seconds until the paper carrier soaked before proceeding to next action.
3.2.10 stripped welded ball carrier
Use a dedicated set of pliers to BGA solder from remove. Split method it is best to start from the corner split. Peel off the paper should be complete. If you split process paper tore then immediately stop, and then add some deionized water, wait 15 to 30 seconds before continuing.
3.2.11 removal of litter on BGA, stripped carrier, occasionally leave a small amount of paper, use tweezers to paper clips. When using tweezers with confetti, tweezers in welded ball to gently move between. Caution: the head is sharp tweezers, if you are not careful you'll put fragile solder mask scratched.
3.2.12 cleaning
Remove the paper carrier immediately after the BGA in deionized water to clean. A lot of deionized water wash and brush hard brush BGA.
Caution: when using a brush to prop BGA to avoid mechanical stress.
Note: to get the best cleaning, scrubbing in one direction, and then turn 90 degrees and then brushing along one direction, and then turn 90 degrees to the direction along the same brushing until turn 360 degrees.
3.2.13 rinsing
In rinse in deionized water, this will remove the BGA the residual amount of flux and steps in the previous cleaning residues of confetti. Then air dry, not dry paper towel to dry.
3.2.14 check package
Using microscopy package if there is no pollution, solder and flux on purchase. If you need to clean the repeat 3.2.11-3.2.13.
Note: because this technique uses the flux is not a no-clean flux, so careful cleaning prevents corrosion and prevent long-term reliability failure is required.
Determine if the enclosure is clean of the best method is to use ionizing dose-effect devices on map or Ionic contamination test. All the process of test results to meet the pollution is lower than the standard 0.75mgNaaCI/cm2. Another, 3.2.9-3.2.13 cleaning steps can use gutter cleaning or spray cleaning process instead.
4. conclusions
Because the devices are expensive BGA, BGA rework becomes necessary, where critical solder regeneration is a technical difficulty. This process of practical, reliable, you only need to purchase a preformed worse and fixture for BGA welding regeneration, the process of resolving a BGA rework key technical challenges in

Solder paste analysis using the frequently asked questions
★ Focus
Solder reflow soldering be used for SMT assembly process of the main board level interconnect the welding methods to the required welding characteristics very well together, these features include ease of processing, design of various SMT wide compatibility, highWeld reliability and low cost; however, in reflow soldering be used as the most important SMT component level and Board-level interconnection method, it also calls for further improvement of welding performance challenges, in fact, reflow soldering technology can stand up to this challenge will determine whether to continue as a solder paste Prime SMT soldering material, especially in the Super fine pitch technology continues to progress. Next, we will explore the impact of the improved reflow soldering performance several major issues, stimulate the industry research for solutions to this issue of the new method, we separately on each issue brief.
Bottom components of fixed
Double-sided reflow soldering has been used for years in this, the first printed wiring, installing components and reflow, and then turn it over to the other side of the Board for processing, for greater savings, certain technology eliminates the first melt, but at the same time soft melted top and bottom, a typical example is the circuit board on the underside with only small components such as IC chip resistors, capacitors and due to the printed circuit board (PCB) design is becoming more and more complex, the symbol on the underside are increasing, the result is cohesive components come off as an important issue. It is clear that the phenomenon is due to the component shedding reflow when molten solder the components of vertical fixed force shortage, insufficient vertical fixed force can be attributed to the component weight increase, symbol of poor solderability, flux wetting or solder less. Where the first factor is the most fundamental reason. If you are on the following three factors be improved after a component shedding phenomena exist, you must use the SMT adhesives. Obviously, the use of binders will reflow time component auto-alignment of variation.

No welding full
No welding is a leader in the adjacent bridge between forming welding. Typically, all can cause solder paste slump of the factors will lead to no welding full, these include: 1, heating up too fast; 2, paste the thixotropic properties of poor or solder paste viscosity at shear recovery slow; 3, metal load or low solid content; 4, powder particle size distribution is too broad; 5; flux surface tension is too small. However, is not necessarily caused slump is not welded full, soft melting, melting of the solder welded full under the impetus of the surface tension has broken may, solder will drain is not welded full problem becomes more serious. In this case, the loss due to solder and assembled in an area of excessive solder will make molten solder becomes too much and not easily broken.
In addition to cause solder paste slump of factors, the following factors can also cause less common causes of welding: 1, relative to the space between the solder, solder paste deposition too much; 2, the heating temperature is too high; 3, solder heat faster speeds than circuit board; 4, flux wetting too fast; 5, flux vapor pressure is too low; 6; fluxes solvent composition is too high; 7, flux resin for softening point is too low.

Intermittent wetting
Solder wettability of intermittent refers to the water appears in smooth surface (1.4.5.), this is because the solder can be glued in most of the solid metal surface and molten solder cover hidden under some points have not been wet, so the initial melting of the solder used to cover the surface, there will be intermittent wetting phenomena occur. Metastable of molten solder overburden at the minimum surface capable of driving force of contraction will happen, soon after they gathered into separate small ball and Ridge-like bald onwards. Intermittent wetting can consist of parts and the melting of the solder contact, emit gases. Since the thermal decomposition of organic matter or inorganic compounds of hydration and release moisture will produce gases. Water vapor is the most common of the gas components, welding temperature, water vapour with strong oxidation, capable of oxidize molten solder coating surface or subsurface interface (a typical example is the junction with the molten solder of metal oxide on the surface). Commonly higher soldering temperatures and long residence time will lead to more severe intermittent wetting phenomena, in particular in the base metal, reaction speed increase will lead to more severe gas release. At the same time, the longer the stay time gas release time. These two areas will increase the amount of gas released, eliminate intermittent wetting phenomena: 1, reduced welding temperature; 2, shortening the length of stay of soft melt; 3, using the flow of inert atmosphere; 4, lower pollution levels.

Low residue
On having clean reflow process, in order to obtain a decorative or functional effect, often require low residues, on the functional requirements of examples including the "tested through the circuit of the flux residues to probe test surfacing as well as in the Insert header and surfacing between or in the Insert header and reflow soldering points nearby introduced through-hole electrical contact between the" greater flux residues often lead to implementation of electrical contact on the metal surface for excessive residue cover, which could hamper the establishment of electrical connections, increasing density in the circuit, this issue even more people's attention.
Obviously, no cleanup of low residue solder paste was satisfy this request for an ideal solution. However, the conditions necessary for it to reflow the problem becomes more complicated. In order to forecast at different levels of the atmosphere of inert reflow solder-low residues of welding performance, raise a semi-empirical model, the model predicts, as the oxygen content of decreasing, welding performance is rapidly improving, then gradually became more stable, the experimental results show that, as the oxygen concentration in the lower weld strength and solder paste wetting capacity will be increased, in addition, the welding strength with the flux of solid content increases. The experimental data model is comparable, and strong evidence that the model is effective, can beused to predict solder paste and materials for welding performance, therefore, to assert that to the welding process successfully used do not clean up of low residue solder, should use inert atmosphere of reflow.
Gap refers to the component leads and circuit board solder joints are not forming welding points. In General, this is attributable to the following four reasons: 1, solder cladding; 2, the leader coplanar poor; 3, wetting is not enough; 4, solder loss date this is pre-Tin of printed circuit board solder paste slump, leader of the wick effect (2.3.4) or through holes near the spot, the leader of in-plane is a new lightweight 12 mils (μ m) distance of four core wire flat IC (QFP jujube Quadflatpacks) of a particular concern, in order to resolve this issue, made before Assembly to solder precoat solder joint method (9), this Act is to extend the size of the local spot and follow up of solder pre coverage form a controllable local welding area, and thus to satisfaction leader coplanarity change and prevent the gap, the leader of the wick effect can slow down the heating speed and let the bottom than the top heated more to be addressed, in addition, the use of moist slow flux, high temperature or to delay the activation of the molten solder paste (such as flour and mixed with tin-lead solder of) can minimize wicking effect in using tin-lead cover before finishing circuit board, with solder mask to override connection path also prevents from nearby hole wicking effect caused.

Solder ball
Solder ball is the most common and most difficult issue, this refers to the soft solder melting processes within the main solder bath not far solidified into size of pellets; the majority of cases, these pellets is determined by the solder paste in solder powder, solder ball make people worried there is short circuit, leakage and the welding point solder shortage occurs, with the subtle spacing technique and do not have to clean up the progress of welding methods, people are increasingly urgently require no solder ball of SMT technology.
Lead solder ball (1, 2, 4, 10) causes include: 1, since the circuit printed process oils caused by improper; 2, solder paste too much exposure to have the effect of the environment; 3, solder paste too much exposed to humid environments; 4, improper heating method; 5, heating too fast; 6, preheating section is too long; 7, solder mask and solder paste interaction; 8, flux activity is not enough; 9, welding powder oxides or excessive pollution; 10, dust particles too much; 11, in particular in the reflow process, flux, mixed with an inappropriate volatiles; 12, due to improper solder paste formula cause solder slump; 13, solder paste before use, not fully restored to use the open packaging at room temperature; 14, printing thickness too thick lead to "collapse" formation of Tin solder ball; 15, low metal content.

Solder knot beads
Solder knot beads are in the process of using SMT solder and solder ball when a particular phenomenon., in brief, weld beads are those very large welded ball, stick with it (or not) small solder balls (11). they formed with extremely low supporting feet component such as a chip capacitor. Solder knot beads is caused by the flux exhaust, in preheating phase this exhaust function exceeds the solder paste of cohesion, exhaust contributed to solder in low clearance symbol under isolated agglomeration, in soft melt, melt the isolated components down again from the solder paste up and coalescence.
Welding knot beads causes include: 1 the thickness of the printed circuit is too high; 2, solder joint and component overlapping too much; 3, the symbol of coloured too many paste; 4, placement of components under pressure; 5, preheating temperature rise too fast; 6, preheating temperature is too high; 7, moisture from the components and solder material released; 8, flux activity is too high; 9, the powder is too thin; 10, metal load is too low; 11, solder paste too many slump; 12, welding powder oxides too much; 13, solvent vapor pressure is low. Eliminating solder junction Pearl of the most easy way might be to change the template pore shape, so that in low supporting feet between the components and solder joint with fewer solder paste.

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