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Implementation in manufacturing

Successful introduction of lead-free soldering (part V)
Implementation in manufacturing

Part V of this six-fold article series on the transition to successful lead-free soldering deals with a reliably performed process implementation on the shop floor. This helps in moving to an environmental friendly Pb-free procedure. Part I (introduction) was published in EPP Europe issue # 1/2, part II in # 3/4 (selection of material and equipment), part III (Taguchi experiments) in issue 5/6, and part IV (design of a robust process) can be found in issue # 7/8.

Gerjan Diepstraten, Vitronics Soltec

Before we can run a lead-free process, there are a number of considerations that must be addressed, and conditions that must be established. The equipment options available to improve the procedure must be installed before kicking off lead-free soldering. For example, in wave soldering, the solder must be exchanged. Beginning lead-free processing requires a good plan, and the answer to key questions such as ”what soldering defects can we expect, what are the criteria for acceptance or rejection, and what levels of contaminations are allowed in solder?”
Oven considerations
In the lead-free reflow process, higher temperatures direct to other profiles. Due to the temperatures and different paste chemistry, different residues will evaporate during the soldering process. In order to keep the machine clean with low maintenance downtime, an accurately working flux management system will be needed. This must be installed and tested before the reflow process starts.
A controlled cooling system is recommended for several reasons, because time above liquidus, grain structure and board exit temperatures are all defined once the oven has proper cooling capability. This naturally involves more than simply ambient air fans; rather, one needs a direct air, fully integrated heat rejecting system, designed to achieve a good cooling performance with low nitrogen consumption. Cooling is achieved in such a system by recirculating a mixture of distilled water and propylene glycol. This environmentally friendly mixture requires infrequent replacement.
Due to higher temperatures in the oven, boards will have a tendency to warp. In order to prevent this, a board support has to be installed. This will keep the boards flatter, resulting in fewer defects.
Lead-free soldering requires different flux chemistry than leaded solders, due to higher process temperatures. The flux type will determine which preheat configuration is most suitable for that process. In case water is used as the flux solvent, a quartz rod heater in the first zone is recommended to bring up the board rapidly. In the second and third zones, a forced convection-heating module must evaporate all water from the flux before the assembly enters the wave. Select a wave soldering machine with the flexibility in the platform to change configurations quickly. This way we can easily swap preheat modules and find the optimum for each individual process.
Exchange of solder
It is a common misunderstanding to think that replacing the tin-lead with a lead-free alloy is only a matter of draining and refilling the solderpot. Lead residues will contaminate the lead-free alloy so much that it will be out of its specifications. Taken into consideration that the maximum allowed amount of lead is less then 0.2%, the change of alloys needs to be done with care.
This procedure must be followed strictly: first, all tin/lead solder in the pot must be drained. Bins can be used to collect the solder. It is necessary to remove all of the solder. Since most solderpots are designed with special chambers to help maintain a stable solder wave, the job is quite difficult and labor-intensive. Once the solderpot is empty, it must be refilled with pure tin. Additionally, the solderpot, including the interior parts and surfaces, must be rinsed with tin. Then, the pure tin must be drained. Finally, the lead-free alloy can be melted. An adjustment to the system’s control software is needed to prevent damage to the impellers (enable temperatures of the impellers to increase, otherwise they may attempt to run when the solder is not fully melted). This procedure is not only labor-intensive, but also requires a lot of handling of molten solder. The alternative is to replace the solderpot with a new one. This action provides the advantage involved in ”swapping” the solderpot, in that one may swap back to the tin-lead solderpot as part of the out-of-control action plan. Moreover, the tin-lead solderpot is available for special product runs that are not lead-free.
Material compatibility
There has already been an abundance of information on lead-free soldering, but not much regarding practical experiences, and the issues concerning compatibility of materials with lead-free alloys, which are sometimes more aggressive. What we have learned thus far is that due to the high tin content of the alloys, materials like stainless steel 304, a material that is often used for solderpot parts, is not applicable anymore since it will be damaged after a few months. We found it necessary to use a more corrosive resistant stainless steel 316. This material is reliable enough for those parts in the solderpot where solder velocity is low.
For the other components, we have applied a corrosive-resistant coating on the SS 316, as we did on the impellers. Stainless steel, not titanium, is used for these extreme-condition parts, partly due to the high cost of titanium, and because making the pieces from this metal would require very high skills. The anti-corrosive layer is preferred above a ceramic coating, due to its superior hardness, i.e., ±2000 Vickers. Unlike ceramic, this coating cannot be damaged by occasional careless maintenance. An additional advantage of such a layer is that due to its smooth surface, solder does not stick to the metal, which make these parts very easy to clean. For the solderpot itself, steel is used, due to its superior heat conductivity. A heat resistant coating prevents iron from dissolving into the solder. We noticed that the dissolution rate of iron in the lead-free alloy is dependent upon the following: materials in general, type of lea-free alloy, temperature and velocity of solder.
Acceptable contamination levels
As with traditional Pb-soldering, many metals will dissolve also relatively quickly in lead-free alloys. The rate of this dissolution depends on the base materials, solder composition, temperature and the flow velocity of the solder. The rate of dissolution of a specific metal is lower if this is already present in the lead-free solder.
Regarding contamination of lead-free solders, there are three major concerns:
Lead – During the next few years, we will encounter situations where solder is lead-free, but is being used with components or boards with lead-finish. As a result, any lead-free alloy containing bismuth will exhibit a low melting ternary eutectic phase of SnBiPb (melt point 96ºC), the composition of which is determined to have 15.5% Sn, 51.5% Bi and 33% Pb.
Copper – High Sn pick up copper more rapidly than low Sn alloys. Also, the amount of copper in a lead-free material determines how much copper will be dissolved. The main question then is what copper maximum is allowed? From the SnPb process we know that contents of 0.2% and higher result in problems such as increased bridging. The maximum copper contamination in SnPb is usually specified as 0.3%, also maximum for some SnAg alloys. The high tin content (96.5%) in SnAg results in a rapid increase in the copper level, particularly with boards that have numerous copper pads. Some processes will run out of spec after 4 to 5 months. In SnPb wave soldering, we have the option of separating the copper from the SnPb and removing it. In lead-free soldering, we must exchange the solder.
For SnAgCu, the picture is somewhat different. First, this alloy already contains copper; consequently, the scavenging effect is slower. Second, results from experiments have shown that the SnAgCu alloys that contain 1% or more copper do not scavenge copper. The copper level stabilizes at this 1% level.
Iron – The rate of dissolution of iron in SnPb is low. With lead-free alloys, we have learned that the numbers are about a factor of 10 higher. For example, the contamination of a solder bath of SBA+ was 0.002% Fe in one year.
In general, we see two kinds of lead-free wave soldering processes involving contamination. First, companies are running lead-free production and frequently checking the composition of the alloy. Then after one year, the contamination more or less stabilizes. If the levels are within their specification, the control interval will increase. Second, companies are having real concerns with solder contamination. A number of them are soldering with SnAg. This alloy is very sensitive to scavenging copper. Continuously out-of-spec conditions cause these users to look for alternatives.
Nevertheless, we’re still left with the question of what level of contamination is allowed? Because of the change of the alloy, the melting point will shift and the melting range will increase. Sometimes, new melting points of the different alloys inside the solder will be introduced of SnAg contaminated with lead.
In lead-free soldering, we can find some defects such as fillet lifting and tin whiskers, but also some other defects seem to occur more so than in the tin/lead process, such as voids in the solder joints. To date, there are no international standards fordefects in lead-free solder joints, which makes it more complicated to define what is acceptable, and to what limits.
Fillet lifting
Fillet lifting is a separation of the solder fillet from the copper pad around the PTH during the cool-down stage. The primary reason for this is the of thermal coefficient mismatch (different expansions) of alloy, copper pad and the board. We see fillet lifting occur with bismuth-bearing alloys in combination with lead contamination, but also with other alloys like SnCu. Although we might expect reduced reliability of the solder joints, thermal cycle testing still shows good numbers in most cases. SBA+ joints are very strong and have shown good thermal cycle data. Therefore, and due to a lack of standards for lead-free assembly, some companies accept fillet lifting.
Voids
We see an increase in the number of voids in the lead-free process, especially when water-based VOC-free fluxes are used. The diameters of voids range from 10µm to 1mm. In general, porosity does not affect the reliability of the solder joint. Large voids, however, may reduce fracture resistance strength. Voids can decrease electrical and thermal conductivity of the interconnection path and cause thermal failures. There are many possible causes of formation. Voids can be the result of solder shrinkage behavior during solidification. Outgassing in the plated through holes during soldering may produce holes in the solder. Alternately, voids can be the result of poor wetting of the solder joint.
Tin whiskers
Pure tin surfaces are vulnerable to spontaneous crystal growth. These crystals can have diameters of 0.1 to 5µm, and can grow to several millimeters in length. Such tin whiskers can start growing after plating, or after a couple of years. Due to their dimensions and different shapes, e.g., straight, kinked, or hooked, they may result in circuit shortages. Whisker growth is dependent upon temperature and humidity. Critical temperatures are about 50ºC with a humidity of 50%. In order to avoid tin whiskers, thermal stress introduced in the soldering processes should be as low as possible (another reason for a straight ramp reflow profile). The Sn content is important; the higher the pure tin level, the greater the opportunity for tin whiskers to form.
Before any data can be analyzed, it obviously must be measured. These procedures are important for machine characterization and calibration. Collecting data is necessary in order to obtain use-ful analysis. We have to dif-ferentiate between variables (measured data in units) and attributes (which have to be counted).
Variables in reflow
Variables in the reflow process include machine and datalog parameters. Machine parameters include conveyor speed, temperature of heating zones (MV), temperature of the cooling unit (MV), and water temperature zones. Datalog parameters include time to reference, time above reference, mean temperature, min. slope, max. slope, average slope, peak difference, min. temperature, max. temperature and time to reach max. temperature. Attributes are soldering defects such as voids, skips, solderballs, bridges and tombstones.
Increased yield and minimized machine downtime are goals that must be achieved with the introduction of a lead-free process. After beginning, we should try to establish a repeatable measuring process, and methods for proper calculation of Cp values and to perform with this data machine calibration.
For most companies, 6s (Sigma) is a magic word. 6-Sigma is equal to a Cp of 2 (defects of 0.002ppm). Statistically, a process is capable if Cp=1; this corresponds to a high defect level of 2700ppm. Therefore, others consider Cp=1.33 as the target (defect level 64ppm).
Key to a reliable calibration procedure is the use of an appropriate PCB to run the measurements with. FR4 boards are not preferred (for calibrating measurements), since the Tg values will drop after a number of runs and the board will warp because of the higher temperature. Also, delamination can occur after multiple re-flow cycles. As a result, thermocouples mounted to the calibration board may loosen and measure gas temperature instead of the temperature of the board material. The use of profiling sensor devices designed to run through the process tunnel delivers more reliable results and better data. Remember that all tools used for calibrating must themselves be monitored and calibrated periodically.
Do not use high-temperature tape for connecting thermocouples. Connected with Kapton they have an accuracy of ±5ºC, and are very much dependent uponthe operator’s skill level. High temper-ature solder or epoxy deliver far moreconsistent results and have an accuracyof ±1K.
Wave machine calibration
In wave soldering, we make an important distinction between machine and datalog parameters. Machine parameters include conveyor speed, preheater temperatures and solder temperature. Datalog parameters include temperature slope (max and min), average slope, max temperatures, Delta-T at the wave, dwell time (for chip and main wave), contact length, parallelism to the wave, and immersion depth in the solder wave.
For verifying the flux quantity, a procedure must be defined. Attributes include soldering defects such as voids, skips, hole filling, fillet lifting, solder balls, bridges, tombstoning and solder excesses.
The same statistical approach used in reflow can be applied in wave soldering. Calibration must be done with equipment designed for wave soldering. Commerci-al tools are available that can measurecontact times and preheat profiles. Inaddition, flux performance can be checked with water sensitive paper or a glass plate. Some users prefer a precise method of verification, and measure the amount of dry flux applied on a very accurate balance.
EPP 163
Zusammenfassung
Zwar hat die EU ihre Direktive zur endgültigen Einführung der bleifreien Elektronikmontage in einem Akt blinder Hörigkeit gegenüber kurzsichtigen Einflüsterungen von 2004 auf 2006 verschoben, doch kommt der Druck für einen schnellen Übergang klarerweise aus Südostasien. Europäische Provinz-Lobbyisten sind hier nicht gefragt. Will man in der europäischen Elektronikmontage nicht – ähnlich wie in der Halbleiterfertigung – auf hintere Plätze verwiesen werden, muß man jetzt handeln. Damit von den bisher eher akademischen Erörterungen des Themas eine direkte Übertragung zu den praktischen Fragen direkt in Fertigungslinien erfolgt, werden wir in einer Reihe von exklusiven Beiträgen hier für weitere Klarheit sorgen.
Résumé
L’UE a repoussé de 2004 à 2006 sa directive relative à l’introduction définitive du montage électronique sans plomb dans un acte de sujétion aveugle à des considérations à court terme, mais la pression pour un changement rapide vient clairement d’Asie du Sud-Est. Les lobbies provinciaux européens ne sont ici pas à leur place. Si l’on veut éviter que l’assemblage électronique européen soit relégué aux dernières places comme c’est le cas de la fabrication de semi-conducteurs, il est impératif d’agir maintenant. Afin que les débats plutôt académiques laissent la place aux questions concrètes et à des répercussions pratiques sur les chaînes de fabrication, une série d’articles exclusifs sera publiée en vue d’assurer plus de clarté en la matière.
Sommario
È vero che la CE ha rimandato dal 2004 al 2006 la sua direttiva per l’uso di impianti di montaggio di componenti elettronici senza uso di piombo, seguendo dubbiosi suggerimenti, ma è chiaro che la necessità di un passaggio il più rapido possibile ha le sue origini nel sud-est asiatico. Il lobbismo di alcune province europee arreca più danno che vantaggio. Se nel settore europeo dei montaggi di componenti elettronici, come già successo nella produzione di semiconduttori, non si desidera retrocedere rispetto alla concorrenza, è necessario agire ora. Per fare in modo di poter passare da discussioni accademiche all’applicazione pratica delle tematiche nelle linee di produzione provvederemo a chiarire tale argomento in una serie di esclusivi articoli.
Nomenclature
Bi = bismuthK = Kelvin
(thermodynamic temperature)
Cp = process capability
Cr = chrome
Mo = molybdenum
MV = measured value
Pb = lead
ppm = parts per million
SBA+ = tin bismuth silver antimony
Sigma = Sigma, standard deviation
Sn = tin
The advent of lead-freetechnology on the shop floor
Lead-free manufacturing on the electronics shop floor is now a very topical issue for practically everyone in this business. The urge for up-to-date information, usable in production lines for the transition to lead-free appears immense. We constantly keep an eye on the developments and trends in this highly relevant scenario, providing our readers with valuable information. Now, in an exclusively publishedseries of articles from oven specialist company Vitronics Soltec, we will bring even more understanding, insight and clarity down to the line, regardless if pure SMT (reflow) or wavesoldering is used. Lead-free technology begins now to emerge from the scientific world into the practical considerations of production managers and operators in their daily work life. A professional magazine like us has to be as helpful as possible in this transition.
Useful lead-free web addresses
References
Data Analysis for Machine Characterization & Calibration. M. Cieslinski, Panasonic Factory Automation, Franklin Park, APEX 2001
Soldering in Electronics, R.J. Klein Wassink, Second Edition 1994
Contamination of lead-free solders with copper and lead. Alan Gickler and Craig Willi, Johnson Manufacturing and Michael Loomans, Northwestern University, Evanston
Assembly Process Characteristics for Pb-Free Through-Hole Soldering, H. Hartono, M. Yunus, M. Meilunas, K. Srihari, UIC, Binghampton, New York
http://nepp.nasa.gov/whisker
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