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Lead-Free Solder Joint Reliability: to be concerned about the hot spot

At present, the electronics manufacturing is in from leaded to lead-free solder the special stage of transition, lead-free materials, PCB, components, testing, reliability, etc. are not standard, because of lead and lead-free mix, especially when When the lead-free solder-side components using leaded solder and lead process when serious reliability problems.

Leaded solder to lead-free soldering transition

Lead-free process on the challenge of the first component is the high temperature. Need to consider the impact of high temperature on packaged parts. Because traditional surface mount components of the packaging material resistant to 240 as long as they will be able to satisfy the high-temperature soldering temperatures of lead solder, while the lead-free soldering, the soldering temperature for complex products of up to 260 , so components can be packaged No high temperature must be taken into account.

Addition, in consideration of high temperature on the internal connection of the device. IC's internal connection methods are gold ball bonding, ultrasonic bonding, as well as flip-chip and other methods, especially in BGA, CSP and modular composite components, modules, and so a new type of components, such as flip-chip BGA, CSP Internal packaging chip bump with Sn-3.5Ag solder paste is welding, melting point 221 , if such devices are used for lead-free solder, then the device solder joints within the assembly of solder joints and surface almost simultaneously re-melting, solidification time , which for the reliability of the device is very harmful. Therefore, the internal connection materials, lead-free components have to meet lead-free soldering requirements.

Leaded solder-side components of the vast majority of Sn / Pb plating, while the lead-free solder-side components coating the surface of many types. Exactly what kind of coating the best, there is no conclusion, it remains to be lead-free components of the perfect standard.

Lead-free process on the requirements of flux, first of all fluxes and alloys due to chemical reaction between the surface and therefore a different alloy composition To select a different flux. Lead-free flux to be specially formulated. As the lead-free process, in-depth, due to the efforts of solder manufacturers, lead-free solder paste quality has greatly improved. The current lead-free solder joints from the appearance point of view has been improved over the previous few years.

The transition period should be concerned about the reliability issues

Lead-free solder reliability issues to manufacturers and users are very concerned about the issue. In particular, the current process of shifting from leaded to lead-free solder the special stage of transition, lead-free materials, printed boards, components, etc. are not standard, even in the reliability of testing methods there is no standard case, the reliability is very worrying. At this stage lead-free processes, especially in the country is still in the chaotic phase. Because of lead and lead-free mix, especially when the use of lead-free solder-side components are lead solder and leaded process when serious reliability problems, these issues are not only the current transitional phase should pay attention to lead-free soldering, and for the transition phase of the leaded solder is also paying particular attention to issues.

Because lead is relatively soft, easily deformed, so the hardness of lead-free solder joints than in Sn / Pb high, the intensity of lead-free solder joints than Sn / Pb high, the deformation of lead-free solder joints than Sn / Pb solder joints is small, However, this does not mean that lead-free reliability. As the lead-free solder wettability is poor, so empty, displacement, and other welding defects are more erected and another due to the high melting point, if the flux activation temperature can not meet the high melting point, due to flux invasion zone temperatures are high, a long time will make welding surface at high temperature oxidation can not occur under the re-infiltration and proliferation, can not form a good interface alloy layers, which resulted in solder joint interface strength (tensile strength) for the poor to reduce reliability.

According to the United States Flextronics, Agilent and other companies of the reliability tests, such as thrust test, bending test, vibration test, drop test, after hot flashes, such as high and low temperature cycle reliability of test results, in general have a relatively similar conclusions: large Most civil and telecommunications fields, due to use of the environment is not much stress, the mechanical strength of lead-free solder joints or even higher than those with lead, but the use of high-stress areas, such as the military, high and low temperature, low air pressure and other bad environment, due to lead-free creep big, so the reliability of lead in lead-free than to have a lot of difference.

With regard to lead-free solder joint reliability (including test methods) are still the early research stage.

Is now in the lead-free and leaded solder transitional period of change, most lead-free technology is lead-free solder with a lead pin of the component mix. In the "lead-free" solder joints of lead content of solder may be derived from the components side of the ball pin, or BGA.

Leaded solder and lead-free solder-side mix, there are lead solder melt first, while the lead-free solder-side (the ball) can not be completely melted, so that components can not generate the metal side of the interface between the alloy layers, BGA, CSP side of the original structure the destruction caused by failure, it is lead solder with lead-free solder-side when the quality of the worst mix. BGA, CSP lead-free solder balls can not use leaded Process.

Have an adverse effect of high temperature components. Ceramic resistors and capacitors specific temperature curve slope (rate of temperature change) is very sensitive, due to ceramic body and the PCB thermal expansion coefficient of the CTE difference between large and likely to cause cooling in the solder component body and solder joints crack, component cracking phenomenon the difference with the CTE, temperature, is proportional to the size of components. 0201,0402,0603 rarely cracking a small component, but more than 1206 major components is more opportunity for cracking failure.

Aluminum electrolytic capacitors is extremely sensitive to temperature. Connectors and other plastic packaging components (such as QFP, PBGA) failure at high temperatures increased significantly. Rough statistics, the temperature increase for every 10 , moisture-sensitive components (MSL) the reliability of a drop. Solve the measure is to minimize the peak temperature and moisture-sensitive components of the dewetting baking treatment.

Lead-free solder rework quite difficult, mainly due to poor wettability of lead-free solder alloy, high temperatures, a small process window.

Lead-free Rework should be noted: Select the appropriate rework equipment and hardware | tools, the proper use of rework equipment and tools, correct choice of solder paste, flux, solder wire and other materials, welding parameters set up correctly.

In terms of environmental protection, legislation, market competition and product reliability in terms of lead-free imperative. Lead-free soldering is currently still in transition and the initial stage, from theory to application are not yet mature, and the urgent need to accelerate the lead-free soldering technology from theory to applied research. Faced with July 1, 2006 around the corner, we should be prepared, for example, data collection and theoretical learning.


Lead-free solder

The standardization of lead-free solder alloy composition has not yet clearly defined. Lead-free core and the primary task is to lead-free solder. According to statistics, a total worldwide developed solder paste, wire, wave soldering, etc. more than 100 kinds of lead-free solder bar, but the real can be used recognized only a few.

The most likely alternative to Sn / Pb solder alloys are Sn-based alloys non-toxic. With Sn-based, add Ag, Cu, Zn, Bi, In, Sb and other metals to form binary, ternary, or multi-alloys, by adding metal elements to improve the alloy properties, improve solderability, reliability. Although the Sn-based lead-free alloys has been more widely used, but in comparison with the Sn63/Pb37 eutectic solder, lead-free solder alloy is still a high melting point, surface tension of large, poor wetting, the problem of higher prices.

Lead-free plating

Lead-free standard is currently not yet perfect, so lead-free solder-side components coating the surface of many types. United States plating of pure Sn and Sn / Ag / Cu multi-comparison. Japan-side component solder plating types are more different from each company, in addition to plating of pure Sn and Sn / Ag / Cu, there are also plated Sn / Cu, Sn / Bi alloy layer and so on. As the relatively low cost of Sn plating, the Sn plating process are more likely due to surface oxidation of Sn to form a thin oxide layer, add power resulting from pressure on uneven Department will launch the Sn to form Sn to be. Sn have a narrow pitch QFP components, such as Office, will easily lead to short-circuit and affect reliability. For the low-end products, and life requirements of less than five years, components can be plated pure Sn, for high-reliability products, and life requirements of the components is greater than 5 years using the first plating layer thickness of about 1m above Ni, and then re-2m ~ 3m thick Sn.

To promote lead-free solder should be

While the international and domestic are applied in varying degrees, lead-free technology, but is still in transition and the initial stage, from theory to application are not yet mature, there is no uniform standard, lead-free soldering of the solder joint reliability is also no uniform understanding of Therefore, both international and domestic lead-free application technology are all very confusing, most enterprises may be lead-free soldering materials, but still have lead components solder side. Exactly what kind of lead-free solder better, what kind of lead-free solder plating PCB pads better and which parts of the lead-free soldering materials, solder side solder joint reliability is more favorable, what kind of temperature curve for the most reasonable Lead-free solder printing, welding, testing and other equipment requirements, etc. What are not clear-cut explanation. In short, the right lead-free soldering technology, divergent views, with a separate argument, each set of practices. This state of the reliability of lead-free soldering products very bad. Therefore, there is an urgent need to accelerate the lead-free soldering technology from theory to applied research.

Leaded and unleaded mix issues for example,

First of all, there are processes encountered in lead-free parts of lead. Some SMT processing, although they have not started lead-free processes, but also met with lead-free components, especially in BGA / CSP and LLP. Some parts factory is no longer producing leaded devices, and therefore can not be procured leaded devices, such known devices are lead-free procurement of the situation is not terrible, as they can by increasing the welding temperature, generally up to 230 ~ 235 can be. There is also a measure can use lead-free solder and lead-free processes, because the general situation is that the current transitional phase of lead-free solder and lead-solder-side mix, its reliability can still be accepted. But the worst is inadvertently encountered a lead-free components, the production had not found that the production or use of leaded solder and lead technology, the result is very bad, because there are lead solder and lead-free solder-side mix of the least effective.

Secondly, the wave problem. Wave more problems, for example, there are processes encountered in lead-free parts of lead; lead-free process instrumentation hole, hole lead is not on the tin; stratification more serious; bridging, lack of weld defects such as multiple; tin pot surface oxide wait.

Countermeasures can be considered from the following aspects, first of all get the materials ready. Material Preparation should pay attention to whether the material components of lead-free solder side, if it is lead-free components, we must find out what is coating materials, in particular, BGA, CSP and new packaging devices, such as LLP and so on.

Lead-free standard is currently not yet perfect, so lead-free solder-side components coating the surface of many different types of solder-side components such as Japan plated Sn / Bi layer, if the solder contains lead, when the lead content of <4wt%, when, Bi with the lead the formation of the low melting point 93 , affect product reliability, so-plated Sn / Bi components only in the use of lead-free solder.

Followed by materials management. For lead, lead-free co-existence of two kinds of technology companies, be sure to note that the strict formulation of materials management system, there are 10 million can not lead, lead-free solder paste and component confused.

In addition, lead-free printing to improve printing accuracy, improve placement accuracy, strict control of temperature

Curve, to minimize the peak temperature.
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