SAC Solder

在平时和客户们的会议交流中，因为Indium公司的产品线比较广，不同的客户群常常有针对自己独特的应用或是技术、销售服务等各种问题。

刚刚开始正式做一线销售，我有时候在会议前会有点紧张（特别是新的客户），怕自己回答不上客户的各种问题，帮不上忙。但是后来老练了一点，即使有些问题我自己真的不懂或是即时没有答案，但是总是可以“follow up”跟进的。会后积极主动利用各种资源找出答案，及时回复客户就好了。 现在的会议中， 我更会了“审时度势”的提问，从客户口中问出对项目、生意有用的信息。

最近我们在联系一个国际大客户做一些项目。 客户要求我们的科研副总裁李宁成博士(Dr.Ning-Cheng Lee)过来和他们交流，做一个roadmap meeting. 客户还提出相关的内容请李博士来讲。在我们和李博士内部交流后，决定先向客户提问，问清楚他们的需求后，我们才好“对症下药”。 比如说，客户想了解低银合金Low Ag Alloy, 低温合金在波峰焊中的使用Low Temp Alloy for Wave Soldering，微间距印刷0.3mm fine pitch printing等等。我们知道客户已经在使用别的供应商的SAC0307的低银合金了，并且客户的NPI工厂里没用波峰焊，但是为什么客户会叫我们来介绍呢？对现在的SAC0307合金不满意？哪些性能不满意需要改进呢？波峰焊又是怎么一回事呢？等等。 所以，我们也决定向客户提问，问清楚他们为什么有这些“需求”，真正的原因是什么……

有一个销售同事更有意思，当我告诉他我有一个做激光设备的客户（他们在使用我们含铟金属的材料），有一次我被客户的几个“资深工程师”追问关于铟金属的一些不太相关的问题，我根本不知道答案……我的销售同事建议说，以后出现这种情况，当你觉得他们问的东西有点过的时候，你可以反问他们一句“Why you asked this question? Why you want to know?”  这样就可以尝试挖掘出客户问问题的真正目的了。当然，要见机而行!

Cheers!

Pic: Google Image

FLUX:
To solder directly to stainless steel, Indalloy #2 Flux (activation range 100-371°C) must be used to remove the surface oxides, allowing a clean surface for the solder to wet. This flux is recommended for mechanical assembly joining only. Due to the corrosiveness, it is not recommend for electrical applications because, if the post reflow flux residue is not thoroughly removed using warm water with mechanical scrubbing, the joint will be compromised due to the potential for corrosion during its life. An alternate solution would be to nickel plate the stainless steel, so a weaker flux (RA, ROL1) can be used that is less corrosive and can be easily removed with an appropriate solvent.   

Another alternate solution is to use a forming gas consisting of nitrogen and hydrogen. This method of oxide removal is generally used when the soldering temperature can be above 350°C which is ideal for activating the hydrogen to reduce the oxides. With this method, there is no post-reflow flux residue to clean up.

SOLDER:
The solders usually recommended for stainless steel joining applications contain a considerable amount of tin; however, the actual solder choice has to fit the temperature range of the application. Generally, a low-temperature application may require Indalloy #1E (52In,48Sn) - 118°C (eutectic), while Indalloy #182 (80Au,20Sn)- 280°C (also eutectic) is a great solder choice for high temperature. If you are looking for a solder in the moderate range of temperatures, Indalloy #121 (96.5Sn, 3.5Ag); 221°C (eutectic) is an excellent choice as well as any of the SAC alloys in the same temperature range. There are also many other solders to choose from that will work equally as well. Please see our solder alloy physical properties chart or consult our Applications Engineering staff at Indium Corporation.

Folks,

I thought I would post a few short thoughts as the new year begins. Here it goes:

1.    A billion hours ago the stone-age was the future, a billion minutes ago Caesar ruled Rome, a billion seconds ago Jimmy Carter was President, a billion passives ago you took your last break (about 4 hours ago). As exciting as the latest quad core microprocessor is, the largest number of components that we assemble is passives, approaching two trillion per year. That is about 6 billion a day. If you lined up all of the 7 billion people in the world, each year you could give every man, woman and child several hundred passives from all of the passives that are produced. If two trillion passives (assume 0402s) were lined up end to end they would circle the earth 50 times!

2.    Schools in Indiana are no longer required to teach cursive writing. Key board skills are considered more important.  Yikes! I’m all for keyboard skills, but I want my grandkids to be able to write in cursive. If not, how do they write their names? Are we less than a generation away from people writing their names as an “X?”

3.    Thoughts on lead-free solder reliability in long term mission critical environments from a NASA study:

        “Test vehicles assembled with lead-free materials (notably tin-silver-copper) exhibited lower reliability under some test conditions.”

Some people would respond to this statement by saying, “I told you that lead-free solder was no good.” However, another way of stating the results would be, “Lead-free solder performed better in more tests than tin-lead solder did.” The ratio, by my count, was about 5 to 3 in favor of lead-free. However, I agree that lead-free is not ready for mission critical (>20-year) service life. The main reason being that, in some cases, when lead-free solder joints failed in these types of studies, the results were much, much worse than tin-lead solder joints. These failure modes need to be understood and addressed. In addition, tin whiskers and pad cratering are looming problems in these, mission critical, long service life quadrant D applications as discussed in the Navy's Manhattan Project (http://www.navyb2pcoe.org/pdf/LFEMP_book.pdf).

4.    SACM has arrived. SACM is a SAC105 alloy that is doped with manganese. Work performed on SACM by Liu, Lee, et al was reported in a May 2009 ECTC paper, Achieving High Reliability Low Cost Lead-Free SAC Solder Joints Via Mn Or Ce Doping. The thorough testing reported in this paper suggests that SACM has promise as a material candidate for quadrant D applications mentioned in #3. In explaining the superior performance of this material the authors state:

“The mechanism for high drop performance and high thermal cycling reliability can be attributed to a stabilized microstructure, with uniform distribution of fine IMC particles, presumably through the inclusion of Mn or Ce in the IMC.”

We have had to wait awhile for this material to become commercially available as it is a challenge to manufacture doped solders like this in large quantities.  I think this paper should be on you "must read" list.

5.   I had not planned on reading Steve Job’s biography , as I thought I knew quite a bit about him from reading recent articles in Forbes, Fortune and Business Week. But I went ahead and downloaded it to my Kindle anyway. This work by Walter Isaacson is a masterpiece. To share one tidbit from it that relates to those of us in electronic assembly: 

"In almost all cases electrical engineers first design the circuits that perform the functions of some device like a mobile phone or tablet. Mechanical Engineers are then left to fit the circuits into the “box.” (Hence MEs are often called “box stuffers” by EEs). Jobs completely changed this approach. He told the engineering team how he wanted the product to look and function first, then they had to determine how to make it work that way. I’m convinced that only through this approach are the revolutionary design concepts that Jobs and Apple came up with possible."

The book also points out his many flaws (e.g. Jobs would regularly park in handicap spots, the author reports several times that Jobs just didn’t think the rules applied to him, etc.). Another interesting thought (read it and see if you agree with me) that if Steve was not Paul Jobs' adopted son, Apple would have never happened.

Cheers,

Dr. Ron

Tombstoning

• the transition to Pb-Free (higher reflow temperatures, and related flux issues)
• miniaturization (0201s and 01005s)

• The reflow oven operator can slow down the ramp rate. A slower ramp rate allows for more uniform warming of the PCBA.
• Another technique is to employ a "soak" just below the melting temperature (solidus) of the alloy. For example, for a SAC305 profile (217°C solidus), one may implement a "soak" at 205 to 210°C for 30 to 120 seconds. This allows for the cooler parts of the PCBA to "catch up" to the warmer parts. After thermal equilibrium has been achieved, one can spike the temperature up to the appropriate peak temperature (i.e. 245°C). This technique (depicted in the reflow profile shown at the right) allows for all of the solder paste deposits to melt and wet the component terminations at roughly the same time; thereby, mitigating tombstoning.

Phone: +1.315.853.4900
E-mail: ebastow@indium.com

上周是感恩节。根据美国的习惯，我们拷了一只火鸡Turkey来庆祝。

平时向客户们介绍我们公司的preform预成型焊片这一大产品系列时，我们经常会用这个有趣的故事：你们知道这个细条小圆柱形的焊片用在哪里吗？ 感恩节的火鸡里！其实是pop-up turkey timer。 在全美销售火鸡和火鸡测温器的厂家，都可以大量地使用我们这种低温合金的焊片。每年感恩节烤火鸡，大家会把timer插在火鸡里面，等火鸡里面也达到一定温度（熟了），在timer里面的低熔点合金(含有铋金属Bi/Bismuth)就会融化膨胀，使其里面的弹簧自动跳起来。

Bi58Sn42是一种很常用的无铅低温合金。它可以用在散热器heat sink/heat pipe的焊接上，也可以用在电路板的印刷焊接上。有些客户板子上的某些元部件有不能承受超过180⁰c 或是190⁰c(Sn63Pb37的熔点是183C,SAC305是217C)，所以就常会用到BiSn锡铋合金。或是分温度层(step soldering)焊接，也会用到这种低温合金。 锡铋这两种金属的价格虽然随着市场大宗商品价格的变动有些变动，但是和贵金属金银等相比，价格还算可以。

铋是一种很脆的金属，所以锡铋BiSn的韧性不够。但是如果加上1%的银，Bi57Sn42Ag1，就可以大大提高合金的柔软性和韧性了。

Cheers!



Pic: Google Image

PS: 上周我给加拿大的客户发邮件，不小心顺手写了祝感恩节快乐的语句。后来一点击发出后，就马上后悔了，因为我突然想起加拿大人感恩节不是在11月，而是在10月份。因为加拿大更加靠北边，他们的粮食收成季节早过美国，所以也提早庆祝感恩节…好在其中一个加拿大客户还挺幽默的，马上回复了我的邮件，祝我们这里感恩节快乐

 Folks,

In gathering information on the status of lead-free soldering, some helpful friends pointed out two great sources of information: NASA and The Navy. NASA sponsored an impressive lead-free reliability investigation: "Lead-Free Solder Testing for High Reliability Project 1." This project is finished and the reports are online. There is a follow-on project: NASA DOD Lead-Free Electronics Project 2 which is currently underway. The Navy sponsored a project with ACI and the summary is here. I am currently studying these documents to help develop the consensus.  Some preliminary info follows:

Regarding -20^°C to +80^°C thermal cycling, NASA concluded:

“Under the conditions of this test, Sn3.9Ag0.6Cu (SAC) and Sn3.4Ag1.0Cu3.3Bi (SACB) were always more reliable than eutectic SnPb regardless of component type (CLCC, TSOP, BGA or TQFP).

It has been shown that conditions that highly stress the solder joints by maximizing the CTE difference between the PWB and the component will favor SnPb over SAC6. Conversely, conditions that minimize the stress put on the solder joints (e.g., compliant components such as BGA’s and/or a thermal cycle with a small delta T) will favor SAC over SnPb. The current test falls into the latter category and we can say with some confidence that the lead-free alloys tested will outperform eutectic SnPb under field conditions that are even less stressful than the -20 to +80^°C thermal cycle test conditions.”

For -55^°C to +125^°C thermal cycling, the conclusions were more cautious, likely because the data were mixed:

“The feasibility of using Pbfree solder alloys in place of SnPb solder alloys for new product designs was demonstrated under thermal cycle test conditions. Additional investigation and characterization of Pbfree solder alloys will be required as a segment of a Pbfree solder alloy implementation plan. The application/introduction of Pb-free soldering processes for legacy product designs is not recommended without extensive materials characterization and product design review.”

These results seem to be consistent with what others report, lead-free assembly produces good thermal cycle results for commercial-type thermal cycling, but the results are mixed for harsh environment thermal cycling.

前段时间有个客户说Indium3.2的印刷性能很不好，锡膏在印刷时的滚动不行，而且还堵塞网孔。Indium3.2是一款水洗性的无铅锡膏。

我们详细问了客户一系列问题后，发现客户用的这款产品SAC305, Type4 powder，金属比重居然有89.5%。根据我们对产品的了解和时间，89.5%的金属比重远远高于我们公司推荐的。这应该就是问题所在。于是我们马上建议了新的金属比重的Indium3.2.

焊锡膏中一般有50/50体积比的金属和助焊剂，重量比大约为90/10。 各个公司不同产品的金属含量比重都会有点差别。但正是这细微的差别，很可能就是产品性能最佳表现决定的关键。比如说3号金属粉和4号金属粉的差别，因为4号粉的表面积比3号粉多，需要更多的助焊剂来“清洗”金属球表面的氧化物，所以4号粉的金属比一般会比3号粉低一点点。金属比的不一样，也会使锡膏的viscosity黏度不同，这会影响印刷时的表现。

Cheers!

Pic:Indium Corporation

PS: 上周看了US Today报纸中的一篇报道“’Made in China’ benefits U.S.”这还是我第一次看见老美自己写出报道，公开说认识到了中国制造其实是在多方面对自己国家有利的；而不是以前老说的中国制造削弱了美国的制造业，抢走的美国的就业机会，等等。如果你看到报道上说的数据，每1美元中国制造的产品，有45美分是给了中国（产品的物料成本，中国人民的劳动力，土地，各种资源等），还是55美分是进美国人自己的口袋（营销，渠道，和售后服务）！！郎咸平是这方面的专家，好久没有看他的书了，最近要温故一下了。 

• High thermal conductivity (33W/mK)
• Higher melting point than SAC alloys (221C)
• Low tensile stress, so suitable for large die (5800psi)
• Excellent thermal cycling properties (-55 to 125C)

1.  Preform (a specially-shaped solder piece) with TACflux® used to hold the preform and die in place
2.  Solder paste, which holds the die in place with no extra materials added 
3.  Soft solder die-attach wire, a fluxless type of solder wire, which is melted onto the substrate metallization under an inert cover gas, and the die directly mounted onto the molten solder pool, then allowed to cool.

Folks,

I have often pointed out that SAC solder's poor wetting is both a curse and Godsend.  It is a curse when trying to fill a through-hole in wave soldering, and a Godsend when assembling close lead spacings as shown in the image (below). Indium Corporation colleague and friend, Mike Fenner (image below), pointed out that, when I say that, "SAC solder doesn't wet well", I should be saying, "it doesn't spread well". His explanation follows:

SAC is different from SN63, and I think it is helpful to explain the difference by making a subtle differentiation between wetting and spreading.

The way that solders spread and wet to a surface is a balance of competing forces. We have surface tension acting to make the molten solder shrink into a ball, and wetting forces trying to make it spread across the surface. Wetting is also the action of the solder dissolving into the surface to form an intermetallic. This intermetallic is essence of the solder joint. The balance changes with different alloys, surfaces, and processes.

Most people are very familiar with the way that tin lead solders behave - and that governs their expectations. The different balance in SAC means the solder tends to spread less for the same wetting and, therefore, can give the impression of a lower quality joint. This lack of spread is usually expressed as 'poor wetting'.

I would explain this by saying the “active ingredient” in both solder families is tin. SAC alloys have a ~50% higher concentration of tin than the Sn63 solder alloy. This gives them a higher surface tension which increases the balling (coalescing) force. At the same time, the less dilute tin, in SAC solders, dissolves into a surface faster. So the final SAC joint can have a well formed intermetallic, but not high spread. These relationships will vary with surface finish and, of course, flux chemistry and process conditions come into play, but that’s for another day. Meanwhile I hope this simplified explanation helps.

Thanks Mike!

Cheers,

Dr Ron

The solder image is courtesy of Vahid Goudarzi of Motorola.

上来扫尘了！前段时间生了个大胖小子，所以从怀孕到破腹产，见识了很多美国先进的医疗器械，联系到平时销售工作中了解到的客户成品，有些感慨“原来成品长这个摸样, 是这样使用的”！幸运也好，或是有些“不幸”吧，成品用在了我身上。

在电子制造加工行业(Electronic Assembling Industry)，亚洲很多工厂都是在做high volume, low mix 的产品，特别是computers, consumer devices & communication发面。 美国和西欧的工厂，更多是做high mix, low volume, 比如说医疗器械。

医疗器械的产品，直接涉及到生命安全，而且大部分都是long life span的，所以对可靠性(reliability)的要求十分高。 这也使很多加工医疗器械产品的工厂对焊接材料的选择和测试要求很严格，特别是对焊锡膏(solder paste)。

大件的没有和人体直接接触的医疗器械板子，很多都还是在用有铅SnPb焊接材料，比如说我这次生产前使用的超声波测试仪，监视宝宝心跳和我宫缩的测试仪，都应该还是有铅板子。不过很多医疗器械的客户们都在密切关注着2014年RoHS2对这方面的要求；客户们也在密切做准备，评估无铅材料，来应对届时的变化需求。

和人体有直接接触的医疗器械，很多都已经是无铅了。 前段时间有个客户生产一种一次性心跳读数器heart rate reading device，板子的形状像一个很小的婴儿鞋子，只有成人手掌的1/4大小左右，而且十分薄。当时就使用了我们的SAC305 焊锡膏和solder preforms (预成型焊片)。据说这种携带型小仪器可以扣在衣服上读病人的心跳，读出的数据直接通过wi-fi传送到医院的数据中心进入系统；医生和医院就可以随时随地远程收集病人的数据……

世界人口在不断地增多，也在不断地老龄化(aging population)，这使对医疗器械的需要在增加。这应该是一个有前景的大行业，也希望大家都能享受到这些科技进步带来的便利，益寿连年。

Cheers!



Pic: Google Image

PS: 还是那句我喜欢的话“生命在于运动（一下）”。和身边很多孕妇朋友相比，我才发现原来自己从小坚持和热爱运动在怀孕和生产过程中帮了那么大忙，生之前那一天我妈妈还在感慨说我走路一小时都还是健步如飞的，哈哈：-）

Example:

Figure 1: Example shown Indium8.9 flux with SAC lead-free alloy

The reason for approaching this subject is that often there has been some confusion in regards to the difference between max slope (a category reported on most profiling software) and the ramp rate listed on a data sheet.
























Figure 2: Max Slope

The max slope is very often attained in the first zone as the PCB moves from ambient temperature into the oven. In most cases the oven zone setting for the first zone is 100°C or better. The change in temperature between ambient and the first zone then is a minimum of 75°C (assuming 25°C as ambient) and so it’s easy to see that the greatest change in temperature (max slope) in most cases is typically found in the first zone

The focus of max slope is more from a component view point, to avoid thermal shock, usually 3°C/s is recommended as the upper limit

Figure 3: Ramp or Average Rate


The ramp rate may be better described as the rate (change in temperature over time) from ambient (room temperature) to peak. And is more practically used in a ramp to spike type profile

From the view point of the solder paste, the lower the ramp rate the better, usually 1-2°C/s. This is to drive off volatiles and help minimize solder defects such as solder balling, solder beading, and tombstoning. This rate becomes even more important as the solder paste deposit continually decreases in size, as we move to 0201’s and smaller discrete components and from 0.5mm pitch area array packages to 0.4mm and smaller. Due to this miniaturization, the observance of graping and head-in-pillow have become more common. The reflow process window is becoming very narrow and this attribute (ramp rate) has become as important as time above liquidus and peak temperature.

I'd love to discuss this with you, if this topic is affecting your SMT process. If you'd like, feel free to contact me.

There seems to be a growing trend to use a low-Ag or Ag-free solder alloy for Surface Mount Technology (SMT) electronics assembly, similar to what is commonly offered for bar solder, used in wave and selective soldering.

For through-hole performance, the strength and stability come from the entire barrel of solder, whereas it is usually the foot and heel fillets that give SMT solder joints their strength.

Lets talk about the other issue with using a eutectic solder alloy in SMT: tombstoning.  One of the benefits of using the SAC (tin-silver-copper) alloy for SMT and solder paste, is that it has a built-in plastic range, similar to that of Sn62 (62Sn 36Pb 2Ag).  It is this plastic range that prevents tombstoning, and takes into account the inconsistent heating of the solder across the part (which is the sole cause of tombstoning).  Switching to a eutectic alloy eliminates the plastic range and opens the door for tombstoning.

Any powder manufacturing issues, such as the inconsistent distribution of dopants throughout the alloy and powder matrix, takes a back seat to the surface mount reliability concerns. 

There are other alternatives, such as SAC0307 (99Sn 0.3Ag 0.7Cu)… But, with the price of Ag finally coming down, and a long history of SAC usage, we don’t think it’s going to be a major player.

Next time, we'll talk about the manufacturing and costs associated with low-Ag and Ag-free alloys. 

I hope this helps. Contact me with any questions.

下周在上海将会有SMT行业中一年一度的大盛事，Nepcon上海展会。 

Indium 公司将在5月12号周四上午11点，在展位2E06举行一个关于SAC105+Mn的记者招待会。

Indium公司几年前就研发了关于SAC105+Mn的合金，并申请了专利。这里有相关的文章“Achieving High Reliability Low Cost Lead-Free SAC Solder Joints Via MN or CE Doping
”与您分享，由Indium公司的Dr.Weiping Liu 和 Dr.Ning-Cheng Lee李宁成博士等共同著作。  

这些年来随着金属成本的上升，中国市场对低银的可靠性高的焊接材料的需求，Indium公司也对SAC105+Mn的焊锡膏等焊接材料做了一系列的研究。实验数据表明，用SAC105+Mn合金做出来
的焊锡膏，跌落测试(drop test)比SAC305好很多，热循环测试(thermal test)与SAC305媲美。目前已经有相当一部分中国客户在使用这款焊锡膏了，工厂的产率与原来用SAC305时的相差无几。
更多的信息，我们将于您在记者招待会上分享。热切期待届时与您相见。

Cheers!




Pic: Indium Corporation

PS；我以前在博客中也与大家分享过几篇关于SAC105+Mn的小文，请看这里1,2。谢谢您持续的关注！今年宝宝快出生了，不能与您在Nepcon上海中相聚，盼来年再见！

Folks,

When the industry was preparing to transition to lead-free solders almost ten years ago (can it have been that long), tin-bismuth solders were serious candidates. Their low melting point, of about 138C, made these solders interesting candidates to replace tin-lead solder. However, if contaminated with lead, tin-bismuth solders can produce a eutectic phase that melts at 96C. In such situations the resulting solder joint exhibits poor performance in thermal cycle testing. Since early in the transition to lead-free solders it was expected that there would be numerous components and PWBs with lead-based surface finishes, this property made tin-bismuth solders unacceptable.

Another aspect of tin-bismuth solders is that they expand on cooling. This phenomenon can result in fillet lift in through-hole solder joints.

However, as we are now well into 2011, almost no components or PWBs have lead-containing finishes and many portable electronic devices have no through-hole components, so it may be time to reconsider tin-bismuth for some applications.

Some years ago, Hewlett Packard (HP) had performed work to show that adding 1% silver to tin-bismuth solder enabled this alloy to outperform eutectic tin-lead solder in 0 to 100C thermal cycle testing. Even at these low reflow temperatures, HP demonstrated solder joint strength with SAC BGA solder balls that was 65% that of tin-lead solder. Expanding on this work, Indium Corporation's Ed Briggs and Brook Sandy performed stencil printing and reflow experiments consistent with the requirements of current miniaturized components using this 57Bi-42Sn-1Ag solder. All of their results were promising. Ed presented a paper at SMTA Toronto,summarized the Hewlett Packard work, and reviewed the results of this new work.

So for applications consistent with 0-100C thermal cycling, 57Bi-42Sn-1Ag solder may be something to consider if the high temperature of SAC solder paste is an issue to components or PWBs in a product

Cheers,

Dr. Ron 

PS: Read my follow-on posting about bismuth.

从去年年中到现在，由于国际原油价格的连续上涨，带动了大宗商品包括金属价格的飙升。近期的利比亚动乱，使情况进一步“恶化”。

我们常有的锡铅SnPb63/37焊锡膏，和无铅锡银铜SAC305焊膏，都含有锡，SAC305还含有3%的银。根据下面的图片，不难看出锡的价格比去年年中涨了近两倍，银的价格更是涨了两倍多。这都使焊锡膏Solder Paste的金属成本上升了。随着原油和大宗商品的价格上升带来各国一定的“通货膨胀(inflation)”, 大家也应该发现，生活中各种商品的价格也有攀升。同理，焊接材料(solder materials)的运输成本，包装成本，劳动力成本，企业运营经费等都跟着上涨。





虽然Indium Corporation的主营焊接材料现在面临一定的成本挑战，但是我们还是采取积极的方法, 在保证品质和服务不变的情况下，给客户们提供物美价优的有竞争力的产品！

Cheers!

Pic:
1. www.lme.com
2&3. www.kitco.com

PS:国内的亲友们都“抱怨”物价的狂涨，美国这里又何尝不是呢。

在这里给大家拜年了：祝愿大家身体健康，万事如意，财源滚滚，阖家幸福！

有人告诉我2011年是金兔年(Golden Rabbit Year)。我上网查了一下，还是得不到考证。但是却让我想到了，Indium公司和金gold (Au) 这种金属，还是有一定相关性的。

金gold (Au)自身的熔点是⁰C，但是如果和锡tin（Sn）在一起，做成80%Au20%Sn的共晶合金，熔点就只有280 ⁰C了。金锡共晶金属有很高的焊点强度(joint strength)，抗腐蚀能力，导热性能好(thermal conductivity)，能够与各种贵金属兼容，还符合无铅的要求；可靠性很高。

金锡共晶金属在SMT里或是电子焊接中的用途很广泛。在SMT中，如果需要分温度层焊接(step soldering)，金锡的280度正好作为第一梯度的焊接；第二梯度的焊接合金可以选择锡银铜SAC或是锡铅SnPb；如果有第三梯度，可以选择锡铋SnBi。 在IGBT，automotive， 和 Radio Frequency (Power Amplifier) 的第一层焊接中，金锡常常是首选。 在电子焊接中，金锡的用途就更广泛了，特别是在医疗器械、仪器中。比如说在catheter导管的应用中，就可以用金锡做成微细的像小弹簧形状的物体，放入心导管中，帮助心肌梗塞的病人…随着全球人群的老龄化，在医疗器械、仪器方面的金锡应用，应该会越来越广泛。(当然，我可不希望有一天自己要靠它来救命哦。)  

当然，金的价格不菲，特别是这几年的疯长。所以选用这种合金，出于经济的考虑因素，也是要很谨慎的。 如果几年前我也买了金存着，现在也是一笔增值可观的小财富了：-）

Cheers!

PS: 我们家的张“小兔”宝宝五月底就要出生了，他现在已经在我肚子了老是练功夫了，踢来滚去的，肯定是我怀孕以来突然多看了功夫片和武侠小说的缘故。如果今年真的是金兔年(Golden Rabbit Year)，只希望小兔以后能够健康和财运亨通（lots of gold!），并且有一颗金子般的心(a golden heart).

Pic:
1. Baidu image
2. www.indium.com Indium Corporation

Folks,

Back in October, I posted comments on lead-free reliability.   In this post, I mentioned that I chaired a session at SMTAI on “Alternate Alloys”. At this session, Greg Henshall presented a paper on the  Low Silver BGA Sphere Metallurgy Project. This paper was a collaborative effort of six companies.  In addition, Richard Coyle presented an overview of the work of three companies titled “The Effect of Silver Content on the Solder Joint Reliability of a Pb-free PBGA Package.” Both projects evaluated lead-free thermal cycle reliability as a function of silver content and compared the results to SnPb reliability.

Both papers concluded that, as far as 0^oC to 100 ^oC thermal cycle reliability is concerned in their experiments, SnPb < SAC105 < SAC305 < SAC405

Coyle’s presentation summed it up best:

“Each of the SAC alloys outperformed the SnPb eutectic alloy in every test, including the long, 60 min. dwell time test. This tends to diminish the argument that SAC is less reliable than SnPb.”

To be clear, it was two papers by two different groups coming to the same conclusion. It would probably be a stretch to say that the conclusions of either group were “almost unique".

Denny Fritz responded to this blog post with this point:

“No one I know will dispute your ranking of SAC better than SnPb solder using the commercial temperature cycle Henshall uses – 0C to 100C. But, harsh environment electronics have to perform to either -40C or -55C, and most use a top end cycling temperature of 125C. IT IS IN THAT WIDE THERMAL CYCLE TESTING THAT SnPb outperforms SAC solders.”

Denny’s point is well taken. I believe it can be said that SAC alloys have demonstrated acceptable reliability in commercial, non harsh environments (i.e. mobile phones, PCs, consumer electronics, etc.) However, it cannot be said that acceptable reliability for SAC has been established for military (RoHS exempt) and harsh (i.e. automobile engine compartment) environments.

A short time ago, Werner Engelmaier wrote an article on this topic (Global SMT Vol 11, No. 1, Jan 2011, pp 38-40.), which among other things he said:

“Of course, ‘Dr. Ron’ selectively picks data agreeing with the point of view he held from the inception of the Pb-ban under RoHS on a plot with an expanded x-axis overemphasizing the differences and supporting a solder joint reliability ranking of SnPb < SAC105 < SAC305 < SAC405.”

Ouch! My motives were not quite so nefarious, I chaired a session and wanted to share the conclusions.

However, Werner makes good points in his article, data exist disagreeing with this reliability ranking and he suggests some good points on how to conduct reliability tests so that comparisons can be made between data sets.

In reading some of his other articles, I was delighted to find that we actually agree on the state of lead-free reliability in thermal cycle testing. Here is a statement of his circa 2008 (Global SMT, Vol 8., No. 8, Aug 2008 pp 46-48.):fting, and n

“It has been 2 years since the infamous ban of Pb-solders under RoHS. What have we learned? For solder joints, no dramatic differences in reliability are apparent. The data bases for LF-solders have grown, the favored LF-solders might be shifting, and no reliability model exists as of yet. Nevertheless, progress has been made.”

Cheers,

Dr. Ron