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1、AbstractThe Compatibility Issues for Lead-free SolderingThe lead-free process and product reliability related issues are hot topics after the EU directive onRestriction of Hazardous Substances (RoHS) enacted. In this paper, we shared some practical experience on the lead-free soldering research and
2、application work. The lead-free PWB surface finishes/component termination finishes and their possible compatible problems during lead-free conversion scenarios (forward/backward compatibility), also the corresponding solutions are discussed. The compatibility of other factors like quality control a
3、nd reworking process, including our initial study onsolderability test method is introduced too. SEM, cross-section and wetting balance methods are used toconduct the related analysis. We believe it will help SMT engineers to understand the importance ofcompatibility of parts and the lead-free solde
4、ring issues well.1. IntroductionThe electronics industry has put much focus on the developing of lead-free soldering due to both competitive market pressures and environment issues. As a result of related research and development activities, some candidate solder alloys have been identified and stud
5、ies on process factors,components, equipment, reliability and other factors have given positive conclusions. But the nearcoming of deadline for the Restriction of Hazardous Substances (RoHS) by EU has given us morepressure: all products must comply by July1, 2006. The situation seems urgent than bef
6、ore. We can notstill stay on the endless discussion and quarrel about if lead-free is feasible or which solder alloy shouldbe adopted any more. We have to make substantial progress toward a smooth transition to lead-free soldering technology now.Some famous companies, especially those in Japan have
7、produced part of lead-free productsrespectively. For example, NEC adopted Sn-Zn solder on the volume production of its laptop first in 1999; the digital video cameral of Sony with Sn-Ag-Bi-Cu solder came into the market in 2000. Our maincompetitor, Nokia, had been used Sn-Ag-Cu on the GMS2110 even i
8、n 1996. Motorola also devote to theresearch and development of lead-free product. CGISS brought out their first Environmentally Preferred Product - Eric 800 Mhz Tetra two way radio recently.At the same time, components suppliers also bring corresponding actions. In Jan of 2004, PhilipsElectronics an
9、nounced the small signal discrete plastic surface mount devices (SMD) in 100% lead-free packaging In these devices, the tin-lead-plating wil be replaced by pure matte tin (100% Sn). The conversion of Philips small signal discretes in plastic SMD will quickly be followed by the conversion ofits entir
10、e product portfolio of glass and ceramic products to lead-free 1 . So, it is time for us to consider more about how to make a full transition to lead-free soldering. During this progress, compatibility issues are critical factor to consider, which include the compatibility of第 1 页lead-free solder wi
11、th design, process, components, PWBs, quality, equipment, business and so on. Here, we will focus on the components, PWBs, quality/reliability and rework compatibility issues.2. Compatibility of lead-free component and solder2.1 Lead-free PWB surface finishesNow, several PWB lead-free surface finish
12、 options have been existed, such as OSP, ENIG (electroless nickel/immersion gold), immersion Sn (I-Sn), and immersion Ag (I-Ag), Ni/Pd. Among them, OSP and ENIG have been available for years. But considering the characters like wetting, storage, planarity, multiple reflow withstanding abilities of t
13、hese finishes are different, engineers have to determine the choices based on the requirement of practice and compatibility. The properties of common lead-free surface finishes are summarized in Table 1, and compatible surface finishes for different solder alloys are listed in Table 2.Table 1. Compa
14、rison of lead-free PWB finishesFinishpropertyI-SnENIGI-AgOSPCharactersThin coating(0.31.3)Long shelf life;High resistance to Thin(0.070.15 );coatingThin coating (0.10.5 );Solderability (forFreshBoard)Solderability (afterStorageHeat/Exposures)ReworkBest BestFastestdegradationPossibledamageStill excel
15、lentNot possibleEasily processableBetterLess degradationPossibleEasily processableBetterLess degradationPossibleCostMedium/lowHighMediumLowConcernsWhiskerBlack Pad issue;formationEnvironmentalunfriendlyprocessOxidation and base Short storage life;metals diffusionDeterioratesaftermultiple reflowsLead
16、-free Alloy Table 2. Compatibility of PCB finishes for kinds of lead-free solder pasteCompatible Surface CoatingsBestSn3.5Ag0.7CuSn3.5AgSn4Ag2Bi0.5CuSn8Bi3ZnSnAgBiInSn, Ag, ENIG, OSPSn, Ag, ENIG, OSPSn, Ag, ENIG, OSPSn, Ag, ENIG, OSPSn, Ag, ENIG, OSPOSPAgOSPAgSn2.2 Lead-free component termination fi
17、nishes The tin/lead had been the most widespread finish metallization due to its compatibility with solderused in the past. The lead-free substitute material must guarantee the same properties with regard to 第 2 页solderability and reliability of the soldering joints. Now, the industry can offer some
18、 components with lead-free termination finishes such as tin, Ni/Au, Ni/Pd or Ni/Pd/Au, SnBi, SnCu and Ag. 2.2.1 Tin Today, Tin plating has come to be the preferred lead-free finish by most suppliers for its good solderability and low cost. But the issue of tin whisker growth for bright tin has gener
19、ated a great deal ofconcerns, especially for fine-pitch packs and long service life products2(see fig.1 and 2). The emergence of matte tin gives a better solution. Matte tin electroplating layer has larger grains size, which provides fewer grain boundaries leading to whiskering. The use of Nickel ba
20、rrier layer (2 m), tin alloyand post-plating annealing are also useful solutions. Fig.1 Whisker on pure Tin plated MLCC after Fig.2 Whisker between pure Tin hook terminalsTemp cycle/shock2.2.2 Sn/Bi Sn/Bi is also a good Sn/Pb substitute. But when other components or solder alloy contains Pb, a low m
21、elting Sn-Pb-Bi phase forms, thus the solder joints becomes weak in mechanical strength. So, Sn/Bi is a surface finish compatible with total lead-free process. Likewise, from the environmental aspect, Bi isnot preferred since it is a byproduct of lead mining 3 .2.2.3 Ni/Pd and Ni/Pd/AuNi/Pd finish i
22、s mainly adopted on ICs and it was first introduced in the late 1980s by TexasInstruments. Till 2000, more than 40 billion Ni/Pd-finished IC packages are in the field. Studies showsNi/Pd finishes achieved equivalent or better reliability results versus Sn/Pb plated component 4 . In 1996, Ni/Pd/Au fi
23、nish was proposed as an alternative to the original Ni/Pd finish. Evaluation onthis finish shows improved wetting performance in solderability test with a wide spectrum of lead-free solder alloys; also it can get and at least equivalent performance in visual appearance of solder joints when compared
24、 to Ni/Pd finishes. Both of these two kinds of finishes have long field history and good performance, and no whiskering concerns are needed. They are compatible with both Sn/Pb paste and lead-free solder paste, so doesntrequire the end user be totally lead-free to use. The structures of two kinds of
25、 finishes are described in Fig 3. 第 3 页Fig.3 The layer structure of Ni/Pd and Ni/Pd/Au finish2.3 The compatible problem during lead-free conversion scenarios Although much work has been done on the lead-free PWB and components, the availability ofcomponents is still one of the key factors influenced
26、 the progress for total lead-free soldering besidessolder alloys and equipment. This doesnt simply means the elimination of Pb from termination finishes,but also the additional requirements for components from the point of view of product engineering mainly result from the higher melting temperature
27、 of lead-free solders, for example, the soldering heat resistance. The latter may be more difficult: for a number of components like BGAs, relays and crystals, since it is noteasy to withstand high temperature for reasons of physics 5 . Certainly, the Moisture Sensitivity Level(MSL) degrades with th
28、e temperature rising and costs incurred by process changes for supplier are also obstacles in change-over to lead-free finishes. Only increased demand from the industry can trigger the lead-free process of suppliers.Thus, it is impractical to develop and convert the huge amount of electronic compone
29、nts in the market at the same time. The conversion to lead-free products is likely to occur in the two scenarios described in Table 3 6 . Table 3: Types of Lead-free Assemblies PossibleDefinitionComponent TerminationSolder PasteBoard Surface FinishForward CompatibilityBackward CompatibilityTotal Lea
30、d-freeContain PbPb-freePb-freePb-freeSn/Pb(Ag)Pb-freeMay contain PbMay contain PbPb-freeThe forward compatibility means the board assembly soldering process has been converted to lead-free technology with a change in solder alloy and the reflow profile to match this change. However, some components
31、still have Pb in their termination finishes (or solder balls) due to the later conversiondate of the component suppliers lead-free roadmap. Hence, during soldering, the Pb from the component finishes will contaminate the lead-free solder paste and bring some negative impactmicrostructure, mechanical
32、 properties and reliability. One big problem of the forward compatibility scenario is the existence of voiding in solder joints. In2003, we performed Pb-free solder paste 1st pilot run project on one phone product. In this work,95.5Sn/3.8Ag/0.7Cu lead-free solder and common components (may contain P
33、b, such as BGA parts)were adopted. It is found the lead-free solder voids for BGA and QFN are larger and more than thetraditional Sn/Pb/Ag alloy 7 . In fact, the voiding problem is not occasional, but the inevitable resultcaused by the interaction between the Sn/Pb/Ag solder ball on the BGA and the
34、no-Pb solder paste used 第 4 页on the board. The melting point of lead-free solder is higher than solder ball. During reflow, the leadsolder ball melts first and encapsulates lead-free solder paste, so entrap a lot of flux and cause theformation of voiding. Fig.4 describes the voiding mechanism. Alloy
35、 AAlloy BFig.4 The voiding mechanism in BGA Fig.5 The big voiding in solder joints of(melting temp: Alloy A Alloy B) leaded BGA/SnAgCu solder Then, the backward compatibility scenario seems not suffer the same voiding problem. But otherproblems emerge when adopting lead-free components, especially w
36、hen the peak flow temperature arelower than the melting point of lead-free solder balls. Severe grain coarsening, poor self-alignment and lack of Ball Collapse are the key issues will be faced. a)b)c)For BGA soldering, the solder balls may not fully molten with reflow profile below theirmelting poin
37、t. Hence severe grain growth occurs and Pb diffusion along grain boundaries,which may worsen mechanical properties of solder joints. Due to the fact that the solder ball alloy may not completely melt, poor or no self-alignmentwill occur. This creates potential possibility for open joints when the co
38、mponent is misaligned to some extent during or after the placement process 6 .Solder balls may not completely collapse after cooling, that is, lack of Ball Collapse aftercooling. This lead to abnormal solder joint shape. Coplanarity limits may also become a critical factor to avoid open joints for l
39、ack of contact between solder paste deposits andsolder ball, especially for fine pitch packages.Fig.6 Solder balls misalignment in backward Fig.7 Duller solder joint appearance of 0805 Compatibility scenario with SnAgCu solder 第 5 页Based on the discussion above, the solder joint reliability of backw
40、ard compatibility may be lower. So, if manufacturers have to experience this transition period for economic or secure concerns, reflow profiles exceeding its lead free melting temperatures are recommended. But improving peaktemperature seems inconsistent with one of the original intentions that appl
41、y the lead component and lead-free solder: to avoid the components and PCB suffering higher thermal stress with a relative lowertemperature process. Now, conscious efforts are making to minimize the lead-free soldering temperature, but highersoldering temperature is still needed to offer a robust pr
42、ocess window for volume production. Butrecently, some opinions 8 tend to believe no need for higher temperature during the implementation of lead-free. The reason is: numerous on-site examinations over years have found the actual temperatureused for Sn/Pb solder can lies between 225 and 238. Hence,
43、with a good reflow profile the viable solder alloys with melting point below 213 should be able to perform under existing process conditions without requiring changes. This opinion seems subjective in some extent. At least in Motorola, the actual soldering temperature would not reach 225 or 238. Ano
44、ther key factor is the solder alloy that accepted by most institutes and corporations (also the only qualified lead-free solder by Motorola till now)is Sn/Ag/Cu solder with a 217 melting point, higher than the 213 limit. That is, for us, hightemperature by lead-free has to be considered. 3. Compatib
45、ility of quality and reliability The existing standards, such as IPC Workmanship Standard are nearly all for leaded products. The conversion to lead-free will certainly require new standards when customers and suppliers make accept and/or reject decisions. For example, whisker growth is a big concer
46、n for obvious reasons. Althoughnumerous independent studies have been made, there is no standard procedure for whisker testing inthe industry. Recently, National Electronics Manufacturing Initiative (NEMI) recommends standard test methods to assess it 9 , but some additional work still be needed. As we know, the appearance of lead-free solder joints a
