Tuesday, 8 May 2012
Monday, 7 May 2012
Role of fluxes in Electronics Soldering and more ..
We have lots of more detailed information on fluxes, but we try to keep it as simple as possible to be able to the reader to understand the topic without having to refer constantly to a dictionary.
Pierre RICHARD
Role of fluxes are mainly in getting rid of oxides on the surface of metals to be soldered, so that those metals can unite creating a proper wetting.
Wetting is only possible when metals are in direct contact, without any other agents (oxides, grease, or other contaminants) in between.
What fluxes do is to break the bond between oxides and metals. It then carry them away in a suspension form. So the fluxes do not neutralize oxides, but just displace them.
The next step will be to get rid of those oxides in suspension and the flux residues.
Fluxes also create a state of optimal spreading of the solder on the conductor cleaned of its oxides and other contaminants.
The most widely used flux since around 5000 years is the pine sap resin. The resin that sweats from those trees is called colophony.
This liquid resin passes through a semi-liquid state before turning into solid crystal in contact with the air.
A solvent is used to transform these crystals into a solution which is then distilled to be used adequately in the soldering processes.
This distillate of the original pine sap is called Rosin. It is normally dissolved in an alcohol base to form a Rosin flux. Resin fluxes are fluxes made-up of natural (Rosin) and synthetic resins.
Elements are sometime added to those fluxes to increase their potential to reduce oxides. Those elements are mostly catalysts which increase the reaction without changing the process involved. Sometimes agents are added to increase the time spend of activity of the flux, thus allowing the flux to be active until the final stage of the soldering process. This is specially the case for the reflow ovens where heat is kept for much longer than in the case of manual touch-up.
Those Rosin fluxes have an organic origin. Their big advantage is that their long organic molecules, once activated by heat of the soldering process, break-up and their residue is totally neutral, which means not at all active and without any risks to unprotected conductors. They are very efficient fluxes with non-corrosive residues. On the other hand, those residues are sticky and should absolutely be cleaned-up to eliminate the risks of absorbing humidity and potentially conductive dusts. Cleaning is done with alcohol base solvents with or without saponifiers (agents capable of transforming residues into soap).
Synthetic Resin Fluxes have been designed to answer to more and more specific demands of the industry. They are mostly made-up of organic long chain molecules which break-down once their soldering temperature reached.
Depending on the need, some halides are sometimes added to increase the duration of action of the flux. These kind of fluxes are sometimes called “No Clean” or “Low Residue” fluxes. The “No Clean” fluxes were designed for applications that don’t need to have the residue be cleaned-off. The “Low Residue” flux can also be left uncleaned on the board depending on the specifications or function of the device. Those “Low Residue” or “No Clean” fluxes contain Rosin solids in smaller quantities than in normal Rosin fluxes such as R (Rosin), RA (Rosin Activated) or RMA (Rosin Mildly Activated). Those fluxes are generally less active than the RA and RMA types. They contain less active agents and they have been chosen mainly because of cleaning cost reduction because it is possible sometimes to leave the residues on the board without being a functional problem on the short and long run. Those residues are non-sticky and non-corrosive.
Depending on the surfaces to remove oxides, it will be necessary to use more or lesser active fluxes. In the case of high activity, we use organic acid type fluxes (OA) with a stronger activity as a reducer as well as a stronger corrosive property. Generally this corrosive factor is still present after soldering, which impedes that those flux residues be cleaned-off thoroughly after the soldering process so that those residues don’t continue to corrode conductors and non-protected metals. The great advantage of those water washable organic fluxes is their great potential of reduction, joined with an easiness of cleaning in deep water.
Not only we have different compositions of fluxes but there are also different forms or states of solvents. We find flux in solid form such as blocks and powder. It is also found in paste or gel form as well as in the liquid state or foam. Powder flux is used in wicking braid. Paste and gel flux are used to elongate their action on a surface, mainly for touch-up wave hand soldering. Liquid flux is mainly used in machine and hand soldering processes. We find gel flux in nearly all solder wire for touch-up. For machine assembly (reflow and wave) we mainly use liquid fluxes which are sometimes foamed-up to be able to control better the quantity of deposition.
It should be known that flux is ESSENTIAL in all soldering processes to guarantee the efficiency of the wetting action. No one can do without it and everyone should know what type to use and how to use it adequately.
Due to adequate measures of protecting the environment, more and more products are banned from the manufacturing sectors. Strict measures have been taken about particular products such as lead, hexavalent chromium, fire retardant products, etc. (RoHS et WEEE). It is also strongly recommended to reduce the volatile organic compounds (VOC) which deplete the ozone layer (Kyoto and others accords). Halides and alcohol used in cleaning PWB are in this category of VOC. To replace all these products, the industry has to be very creative but even with the best ingenuity, nothing will really replace as a drop-in solution most of those banned-to-be products.
As far as fluxes are concerned, the industry is going in the VOC-Free, Halide-Free direction. Will there also be a ban on Rosin in the future?.. What then to replace it with?
Water is becoming the most popular flux solvent, specially in the lead-free technology. For this lead-free technology, organic flux (water-based) is mostly recommended because of its superior reducing and corrosive potential. This, to counteract the problems of reduction of wettability of the new soldering alloys (tin/silver/copper).
Due to the higher temperature and change of alloys in the lead-free processes, fluxes have to be replaced by different ones with specific formulas for those new profiles and specific alloys.
Fluxes are classified with letters such as RA (Rosin), RE (Synthetic Resin), OR (Organic Acid) and IN (Inorganic Acid). Another letter describe the flux activity. “L” represent a low activity flux, “M” a medium activity flux and “H” a high activity flux. Also a third element comes to describe the type of flux. It is a number such as zero or one which describes the content of halides.
“0" (zero) represents a flux containing no halides.
“1" describes a flux containing some halides.
A flux can have then a code such as ROL0, which means a rosin base flux with a low activity and no halides.
ORH0 stands for an organic flux with a strong or high activity without halides.
REM1 will stand for a synthetic resin flux with medium activity and some halides, etc.
There are some other factors to increase the efficiency of fluxes, such as inerting and convection oven profiles.
Inerting consists of using an inert gas such as nitrogen to protect the soldering process from oxydizing with the surrounding oxygen of the air. Inerting also permit a reduction of quantity of flux used in the process and in enhancing the surface appearance of the soldered joint.
Oven profiles for this lead-free technology, should be designed to correspond to the activity of the new fluxes and to temperature or time above liquidus (TAL) or melting point. There should also be an increase of temperature in the pre-heat zone and a decrease in the cooling zone to have acceptable results.
What is a "Rosin flux" with Colophony
It is a glassy mix of abietic acid with some related (isomorphic) compounds and numerous hydrogenated modifications of that acid.
** Note that while rosin is distilled from resin, resin is not rosin.
For more details about fluxes and where to buy Fluxes in Australia, please go to the OKAY Technologies website
Head-in-Pillow BGA Defects
Head-in-pillow (HiP), also known as ball-and-socket, is a solder joint defect where the solder paste deposit wets the pad, but does not fully wet the ball. This results in a solder joint with enough of a connection to have electrical integrity, but lacking sufficient mechanical strength. Due to the lack of solder joint strength, these components may fail with very little mechanical or thermal stress. This potentially costly defect is not usually detected in functional testing, and only shows up as a failure in the field after the assembly has been exposed to some physical or thermal stress.
Head-in-pillow defects have become more prevalent since BGA components have been converted to lead-free alloys. The defect can possibly be attributed to chain reaction of events that begins as the assembly reaches reflow temperatures. Components generally make contact with solder paste during initial placement, and start to flex or warp during heating, which may cause some individual solder spheres to lift. This unprotected solder sphere forms a new oxide layer. As further heating takes place, the package may flatten out, again making contact with the initial solder paste deposit. When the solder reaches the liquidus phase, there isn't sufficient fluxing activity left to break down this new oxide layer, resulting in possible HiP defects. Since component warpage is unpredictable and inconsistent, the focus must turn to the interaction of process variables and those that can be altered to reduce the incidence of HiP defects. These variables include BGA ball alloy, reflow process type, reflow profile, and solder paste chemistry. Each of these variables are studied and discussed below.
With the need for better drop resistance, many lead-free BGAs are being made in alloys other than SAC305. Since SAC305 has significantly lower drop resistance when compared to Sn63/Pb37, component manufacturers have been moving away from this type of alloy and towards alternative lead free alloys such as SAC105, which is composed of tin plus 1% silver and 0.5% copper. There also are many alloys competing for market share that are SAC105 plus a fourth element, often referred to as a dopant, such as antimony (Sb), magnesium (Mg), nickel (Ni), cobalt (Co), or indium (In). These additives create finer grain boundaries and reduce the intermetallic formations of the tin with silver or copper, resulting in a more reproducible grain as well as a more uniform grain formation in the lead-free alloy. These also yield a different oxide and surface condition, depending on the element used and cooling rate during assembly. This different oxide and surface condition can cause some issues with the flux activity which impacts solder wetting and complete joint formation of the BGA.
SAC305 with tin-silver intermetallic and coarse grain structure that leads to fractures during drop Solder Sphere (Ball) Issues:
The following images are analyses of BGAs that are known to have had head-in-pillow problems. The balls were inspected under SEM and it was determined that there are very distinct grain structure variations within the balls. Inspecting these components demonstrates that there are three distinct classifications of balls on the component; these were labeled these as “shiny”, “matte”, and “spotted”.
Further inspection shows grains structure differences and chemical composition differences.
As a point of clarification, the large dimples on the ball surfaces are from test probes which easily penetrated any of the surface irregularities or containments during component testing by the manufacturer.
On this single BGA there exist three different grain structures and surface elements. One theory explains that this is due to variations in cooling rates when the solder ball was initially formed.
AIM developed a test procedure to understand the interaction of these elements with specific paste chemistries. This allowed a classification of reactivity levels of some of these dopants. It was discovered that very low levels of magnesium directly affect standard solder paste flux chemistries in the 30 ppm level, while indium affects them in the 500 ppm range, nickel and cobalt in the 400 ppm level, and antimony in the 1000 ppm level. Although the grain structures all appeared similar, the flux interaction was different. This difference was determined by a viscosity test that was conducted while the paste medium was in contact with the solder alloy doped with the aforementioned elements.
Other factors that appear to influence the head-in-pillow issue include; types of reflow, reflow profiles, and solder paste chemistry. Some data obtained suggests that vapor phase reflow may result in more head-in-pillow defects than does convection reflow. It is not clear whether or not this is truly related, however, as it has only been seen as a trend.
An experiment was performed to measure the impact of reflow profile on head-in-pillow solder joint formation. The experiment utilized two different reflow profiles. The first profile was a standard ramp-soak-spike, as seen below.
The second profile, as shown below, included a hotter soak zone and longer dwell time at liquidus.
There was not any perceived difference in the defect rate depending upon the profile utilized; each resulted in random cases of head-in-pillow depending upon the component tested.
The next factor tested to determine its impact on head-in-pillow was solder paste chemistry. During this experiment, it was found that solder paste chemistry appears to have the single greatest effect on the head-in-pillow defect. When changing from an older lead-free solder paste to a new higher-temperature activation paste, the defect, in many cases, was eliminated. In other cases, it was more difficult to remove. However, in the experiment run, the solder paste chemistry appears to have the largest impact on head-in-pillow.
An experiment was conducted utilizing various solder paste chemistries to measure their effect on head-in-pillow incidents. It was determined that irrespective of the reflow profile used, simply by changing to the AIM NC257 solder paste, head-in-pillow was completely eliminated. Although this solder paste is halide-free, a solder paste containing >0.5% halide also was used in this experiment, and the defect was once again eliminated.
This indicates that solder pastes (such as NC257) with an activation system able to provide sustainable high-temperature fluxing activity are capable of creating a homogenous connection beyond the ball and the paste alloy interface. Below left is a picture of a head-in-pillow that was formed using a lower activation temperature activation system. To the right of it is a joint formed with a high-temperature activation system with no evidence of head-in-pillow.
Based on the above experiments, the following chart was generated to show the relative impact of variable(s) that contribute to the head-in-pillow issue, rated on a scale of 1 to 10, with 10 as the most likely to eliminate head-in-pillow.
Based on this preliminary study, it appears that the two most significant factors are solder paste flux chemistry and wetting of the BGA alloy ball. Frosty, non-uniform structures appear to perform the worst for BGA head-in-pillow. This is logical, as these are intermetallic regions on the surface of the solder ball. The intermetallic connection of Ag/Sn and Cu/Sn possess much higher melting temperatures than the alloy themselves.
They are also crystalline in structure and can repel wetting. Although additional studies are necessary to corroborate these results, there is a strong indication that this surface structure is one of the leading causes of the head-in-pillow defect that is so costly to board assemblers.
Karl Seelig - AIM
Cranston, Rhode Island, USA
AIM - More details and graphs from the above article and more articles can be found here
http://aimsolder.com.au/index.php/technical-articles
Technical Articles
Download pdf's from www.aimsolder.com.au website
Halogen-Free Assemblies Testing
The increased interest in halogen-free assemblies is a result of Non-Government Organizations (NGOs) exerting pressure on electronic equipment manufacturers to eliminate halogens. The NGOs primary focus is on resolving global environmental issues and concerns. As a result of an increase in the enormous .... (download file below)
 Halogen-Free Assemblies Testing.pdf |
[ ] | 93 Kb |
Head-in-Pillow BGA Defects
Head-in-Pillow (HiP) solder joints have become a more prevalent defect as the industry continues to migrate to more lead-free assemblies. This defect occurs when an electronic and mechanical bond is not made between BGA solder spheres and the solder paste during reflow. Perhaps a larger problem may be that this defect can make it through both x-ray inspection and .... (download file below)
| Head-in-pillow BGA defects.pdf |
[ ] | 402 Kb |
Specialty Solders: A Study of Indium/Lead
More complicated electronics will require the increased use of specialty solders. Specialty solders, such as indium alloys, offer advantages for gold soldering, step soldering and fatigue resisatnce. (download file below)
| A Study of Indium - Lead article.pdf |
[ ] | 610 Kb |
A Practical Guide to Achieving Lead-Free Electronics Assembly
To successfully achieve lead-free electronics assembly, each participant in the manufacturing process, from purchasing to engineering to maintenance to Quality/Inspection, must have a solid understanding of the changes required of them. This pertains to considerations regarding design, components, PWBs, solder alloys, fluxe s, printing, reflow, wave soldering, rework, cleaning, equipment wear & tear and inspection. (download file below)
| A Practical Guide to Achieving Lead-Free Electronics Assembly.pdf | [ ] | 252 Kb |
A Study of Lead-Free Wave Soldering
This brief study of lead-free wave soldering focuses upon copper dissolution and solder maintenance issues. Unfortunately, it is determined that waste and changeover costs can dramatically increase with lead-free wave soldering. (download file below)
| A Study of Lead-Free Wave Soldering.pdf |
[ ] | 87 Kb |
Reflow Profiling: Time Above Liquidus
To ensure the long-term reliability of an electronic assembly, particular attention needs to be paid to the reflow profile. One area of the profile often questioned is the time above liquidus. If the time above liquidus is too brief, there is the risk that marginal or non-wetting will occur. At the other extreme, if the time above liquidus is too long ..(download file below)
| Time Above Liquidus.pdf |
[ ] | 469 Kb |
Controlling Copper Build Up In Automatic Soldering Equipment
Since "nothing solders like solder", HAL (Hot Air Leveling) will continue to hold a significant place in the surface finishing industry. Furthermore, the wave soldering process will continue to be a viable means of electronics assembly. However, as automatic soldering processes using lead-free alloys have become increasingly prevalent, questions have arisen about copper dissolution into these alloys.
(download file below)
| Controlling Copper Build Up In Automatic Soldering Equipment.pdf |
[ ] | 50 Kb |
A Comparison of Tin-Silver-Copper Lead-Free Alloys
Tin-Silver-Copper alloys are the leading candidate lead-free substitute. However, as there are several different Tin-Silver-Copper alloys, background information is necessary to determine which alloy is best suited for the broadest range of applications. The Sn96.5/Ag3.0/Cu0.5, Sn95.5/Ag3.8/Cu0.7, and Sn95.5/Ag4.0/Cu0.5 alloys are ..(download file below)
| Lead-Free Comparison of Sn Ag Cu Alloys.pdf |
[ ] | 1500 Kb |
Considerations for Printing Lead-Free Solder Pastes
SMT printing will require reexamination and process adjustment when lead-free soldering is implemented. If a high quality solder paste is used and standard rules for SMT printing are followed, consistent stencil life, aperture release, print definition, high-speed print capabilities and print repeatability may be expected.
(download file below)
| Considerations for Printing Lead-Free Solder Pastes |
[ ] | 53 Kb |
SN100C Lead Free Assembly Products
AIM Australia SN100C Lead Free Assembly Products.
For your Lead Free Soldering, AIM has lead-free no clean, water soluble and rosin solder pastes, lead-free bar solders and compatible fluxes, and lead-free cored wire, solid wire, performs and spheres.
The call for Halogen-Free Electronic Assemblies.. but exactly what is a "halogen free" solder paste
The increased interest in halogen-free assemblies is a result of Non-Government Organizations (NGOs) exerting pressure on electronic equipment manufacturers to eliminate halogens. The NGOs primary focus is on resolving global environmental issues and concerns. As a result of an increase in the enormous “e-waste” dump sites that have begun showing up around the world, NGOs are pushing consumer electronic manufacturers to ban halogen- containing material in order to produce “green” products. Not only are these sites enormous, but the recycling methods are archaic and sometimes even illegal. This stockpiling and dumping has created growing political and environmental issues. In order to deal with this issue, the question of why halogens are a focal point must be addressed.
As a safety measure halogens are added to organic materials as a fire retardant. The common halogens used only give off bromides with elevated temperatures, decomposing and releasing bromine in order to extinguish fires. These are toxic as well as corrosive when decomposing. However, they are benign when at ambient temperatures. The jury is still out on the replacement products for these brominated organics, with uncertainties about long term health exposure and environmental impact.
Rather than replace halogens with potentially equally harmful substitutes, it would be more logical and effective to address this issue at the e- waste dump sites. If modern recycling processes were applied to these sites, this environmental problem could be minimized. Methods such as shredding followed by fluid bed separators could yield higher value returns for the recyclers and mitigate the impact of the halogens.
Electronic equipment suppliers are feeling the pressure from NGOs and are moving towards halogen-free electronic assemblies. That in turn trickles down to the suppliers of electronic assembly materials, board materials and components. The general trend is that every component sub-assembly on the board must be halogen-free. Other committees refer to these component sub-assemblies as substances, articles and/or preparations. Solder paste and flux products are included in these restrictions. Some electronic manufacturers require solder pastes to be tested, some want the flux tested, yet others want combinations tested.
Solder paste that creates the electrical/mechanical connection on a circuit board now constitutes the article. The rules of sample preparation regarding dilution should apply as they are consumed in one unit. So what is halogen-free? There are many committees, consortiums, and organizations working on this issue. Some organizations have started publishing maximum limits to be the determining factor. These can be 900 ppm of either Br or Cl and a combined total of 1500 ppm. Others have set 1000 ppm of either Br or Cl. There is an attempt to have a defined limit on halogen content.
How does one actually determine if a solder paste is halogen-free? The most popular test method is known as “oxygen bomb”, which is a combustion test immediately followed by ion chromatography. This is an environmental test procedure and may be performed in a variety of methods. The most popular seems to be BS EN 14582:2007, although thereare othertestmethods available,including EPA SW-846 5050/9056 or JPCA ES-01-2003.
AIM, a global supplier of solder paste, wire, bar, flux and epoxy, decided to run some round robin testing to determine repeatability of halogen-free testing results to EN14582:2007
AIM manufactured a batch of solder paste medium (“NC-A”) that was intentionally doped with 13,000 ppm Br, as well as a completely halogen-free version (“NC-B”). All samples were chlorine- and fluorine-free. Solder paste was manufactured with these two mediums and the medium itself for “NC- A” was also sent for testing. The solder paste was composed of 89% metal with the 11% balance being the medium. AIM prepared these three sample types and sent them to six independent testing laboratories around the world.
Based on the testing that was performed by six different laboratories, all of whom used the same method, it is readily apparent that the precision of this testing method is poor.
It is also observed that there is an interaction between the metal alloy and the flux medium in a solder paste. The graphical view in Figure 4 of the “NC-A” flux medium made intentionally with 13,000 ppm bromide added and the “NC-A” SAC305 solder paste of the same medium shows a notable difference. Since solder paste is 11% by weight medium and 89% by volume metal (in this case), one might expect a 11% drop in reported bromide or 1430 ppm, which is not the case. Additionally, there is no consistency in the net difference reported between laboratories further suggesting lack of repeatability in the test method. This also points out that testing of the medium alone for compliance is not recommended since the bromide levels are reduced or consumed when mixed with the alloy in the solder paste.
When these mediums are tested to IPC J-STD 004, no halides are found. However, the current IPC test does not detect halogens. In order to fail the IPC test, the halogen has to be ionic and soluble in water/alcohol. Based on the test results following this IPC standard, these pastes and mediums are halogen-free.
Further testing would need to be completed to determine if a reflowed residue contains halogens, as all of the above were run on un-reflowed paste. This adds further complications to the current test methods because of the extra step to extract the reflowed residue and the potential of compromising sample integrating. If further reduction or consumption of the halogen content takes place, then soldering fluxes and paste would have virtually no impact on a circuit board, component or sub- assembly relating to halogen content of the final product.
SIR results for the halogen-free verses the halogen- containing products in this study indicate that the halogen-containing product has slightly better SIR values than the halogen-free equivalent. This is due to the relatively low amount of organic activators required to interact with the halogen to get acceptable soldering results whereas the halogen- free products require a much higher concentration of organic activators that have a slightly negative effect on SIR values.
Conclusions:
The electronics assembly industry is being pushed in the direction of halogen-free materials. Already, many manufacturers around the world are pushing their suppliers to provide them with materials to meet the criteria of this latest industry buzz word. However, many questions concerning this issue remain: If we do want halogen-free what are the gauge R and Rs of the test procedures? How critical is 1500 ppm versus 900 ppm? Is this a test that represents the real impact of the halogenated fire retardants? Should the halogen content only be tested on the final assembly? The brominated fire retardants were introduced to eliminate the more toxic antimony oxide that was used previously. What is the impact and the danger of the replacements? How pertinent is this to electronic assembly when the real issue is illegal dumping of e-waste? The results of the testing regarding this topic create more questions than answers. Until these questions are answered, the attempt of electronic manufacturers and their suppliers to meet the requirements of NGOs to provide halogen-free products is confusing, unduly expensive, non- beneficial, and potentially more harmful than supplying halogen-containing materials.
For all the graphs and charts, please go to http://aimsolder.com.au/index.php/technical-articles
Subscribe to:
Posts (Atom)