00157The Compatibility Issues for Leadfree Soldering

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 頁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 頁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 頁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 頁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 頁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

47、 we know, the appearance of lead-free solder joints after reflow is different with leaded ones, withrough and slightly duller surface due to the formation of pure tin dendrites when cooling; and for osp and ni/au board finishes, solder may not wet the corner of pads for high surface tension(as fig.7

48、 andfig.8 show). so, inspector (and aoi) training is strongly needed. fig.8 poor wetting performance of lead-free fig.9 the wetting curves of au surface with solder on pad corner tsc solder solderability tests and solder reliability testing need standardization too, thus qualification time for 第 6 頁

49、components can be reduced if these tests are universally acceptable. the must ii solderability testerof matc-a is based on the wetting balance theory, and assesses pass/fail data by iec and mil standards. since its oven can reach 300, so it is still compatible with lead-free testing. recently, we pe

50、rform the initial solderability study with the multicore ecosol tsc solder pellets (95.5sn/3.8ag/0.7cu)on au surface. the result shows with 250 or 260 test temperature, the solderability data from must ii is basically reliable. but the original pass/fail standard has to be changed, especially for th

51、e wetting time tb. after all, 0.6s/1s is too rigorous for lead-free solder.at the same time, we should notice that even the types of defects for lead-free solders are the same as for sn-pb, lead-free solder has generally been found to generate more defects of voiding,tombstoning, solder beading, bri

52、dging and misalignment. so, both process and quality engineer may take considerable effort on achieving high quality product.4. compatibility of reworking processreworking is also an important part of the volume manufacturing process for lead-free pcbassembly, especially during the transition period

53、 when every part of the chain goes through the learning curve. however, lead-free solder reworking has been found to be more difficult because the lead-free solder does not wet or wick as easily as the sn/pb solder.most of the reworking equipment for sn/pb can still be used for lead-free process. bu

54、t the soldering parameters must be adjusted to accommodate the higher melting temperature. also, care must be taken to avoid or minimize the potential negative impact of the reworking process on reliability, such as cucracking of the through-holes by cte mismatch, insufficient adhesion between solde

55、r pads and masks,the warpage and delamination of component for higher temperature, also the electromigration anddentritic growth for flux residues 10 .besides, reworking should try to adopt the same lead-free solder alloy as originally used on thesolder joint, different lead-free solder formulations should not be mixed on the same solder joint. if more than one alloy is used in the production process, operators should be trained to use the correct wirecoreor flux gels for each part. conclusion:the compatibil