Processes and their parameters: when updating procedures and developing processes, rely on "real world" data, not lab tests.

Ed.: For the full article, please visit circuitsassembly.com/cms/content/view/2580/

The Pb-free conversion process consists of two interlinked aspects. On one hand is the physical reality; i.e., everything that occurs around the production line. We have to add to this what is normally called "know-how": technical information and experience related to materials, equipment, process parameters and their effects on the final product. But these elements would hang in air without ordering, delivery, storage and organization of components, PCBs and material such as solders, fluxes, cleaners, solder iron tips, spare parts, nitrogen and everthing else that makes production hum. This area is known as logistics.

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Companies have to make sure workers remain in tune with technical requirements. When switching from Pb-bearing to Pb-free solders, keeping personnel informed is of utmost importance. The latest information has to be shared. The system of communication must be flexible to cope with the glut of information stemming from the introduction of several solder alloys.

Processes. Processes can be visualized in many different ways. A fishbone (Ishikawa) diagram depicts the steps to be performed and routes taken to ensure the production line operates properly. A good idea, perhaps, would be to take such brain-storming sessions farther, as many cost factors that impact production derive from such points as design and layout or purchasing. These sources of problems may become more marked as Pb-free processes are introduced. Although information is just coming out about what land patterns will optimize manufacturability and reliability, certain measures must be taken: e.g., it is highly recommended that assemblies exhibit better thermal equilibrium between the different parts.

When purchasing components it will no longer suffice to define their values and perhaps their tolerances and dimensions. The metalization will have to be described. The thermal loading during processing may have to be specified. At the least, production has to be aware of those details.

As standard Pb-free compatible laminate properties are still being developed, the laminate must be described clearly, lest the vendor ship material that is not compatible with the process. PWB cleanliness upon delivery should be defined, too. Post-soldering residue is expected to increase; thus the cleanliness of incoming material will be valuable. Only then will a no-clean process be attainable (we propose at least 50% of the former MIL-spec value.)

Do not hesitate to extend the fishbone diagram into areas such as purchasing, design and layout.

Procedures. Start by describing--with the production line as the backdrop--all relevant parameters used in the present production environment. As a second step, include incoming materials. Finally, in a general sweep, encompass purchasing and engineers and layout personnel.

Once the collection of parameters in use have been described, it should become clear why they are in use (e.g., conveyor speed for optimal throughput, or requested by customer or because of sensitive components, etc.). At the same time, it will be apparent where significant variance occurs from the present settings and values.

Update documentation to reflect the present situation. Check whether ISO 9000 documents from the last certification still show the values as displayed on the equipment. Sometimes the machine operator changes the settings to improve the process and its results.

After identifying all soldering processes in the factory, it is simple to identify all sets of parameters in use. For some companies it will be one set of values; for others each product will have its own. Sometimes products are collected into "families" and then sets of parameters are identified with products in these families. In most cases these values can be found in the storage medium or stored in the actual machine's control system. Add to the values all necessary comments and information that will aid in decision-making later. Ask design, layout and purchasing if they can add anything to these data.

Quality assurance. Next, engage the quality assurance department. Much needed information should be available there. In particular, you will find out what happens to goods received. Ideally, the understanding of quality resides at the vendor. Regular meetings, discussions and quality audits at the vendor's place can ensure a high level of product quality.

What does QA really test? Is it simply the value of components and their dimensions, or does is include important process aspects such as solderability, component humidity absorption, thermal properties or even actual tests of the surface finish of boards and components? If not all requirements are already met, a lot of work may be in store; for high reliability and low levels of process defects, production will need to know much of the aforementioned information.

Purchasing. It is a common misunderstanding that purchasing has been reduced to only mercantile thinking: everything as cheap as possible. This "cost center" idea, operating on the principle that competition is good, can in the long run cost a company a lot of money. While reasonable to try to keep individual costs down, it is more important to keep the cost of the end-product as low as possible. Low quality product purchased cheaply may result in expensive end-product due to necessary repair processes. Saving a fraction of a cent for each component purchased is smart only if it does not result in large repair costs later.

Soldering Processes

We distinguish between two alloys: Bi-based solders with a melting point around 138[degrees]C of its eutectic with tin, and high-melting alloys that melt between 217[degrees] and 230[degrees]C.

Wave soldering. Table 1 shows typical parameters for wave soldering with different solder alloys. Bismuth solders may be usable for 40 to 60% of products, and they have a number of attractive features. These include a low melting point and a tin content that will not rise substantially, obviating the need to purchase new equipment. Neither the traditional material used for the pot nor the pump will be attacked at these temperatures and levels of tin. And since the difference between the specific density of SnPb and that of BiSn is small, changing the conveyor angle will hardly be required.

However, three clear preconditions have to be met:

* A strict Pb-free policy has to be introduced. No lead on components or the PCB can be permitted. No other process step that can possibly contaminate the joint with lead can be tolerated. The pot and pump may have to be exchanged if they cannot be cleaned sufficiently to get rid of all Pb residues.

* The maximum temperature at which the resulting product is used must be below 100[degrees]C.

* Soldering must take place under inert conditions.

As the solder joints are somewhat less malleable, mechanical shock of joints should be avoided.

Eutectic BiSn-solder can be modified by adding small percentages of other metals.

High-melt solders can be seen in three classifications: SnCu+X; SnAg+X and SnAgCu+X--where X stands for Sb, Bi or others. Their melting points lie between 217[degrees] and 227[degrees]C, about 40-60K above the melting point of SnPb eutectic.

Some use these solders at pot temperatures of 255[degrees]C. This may be marginally acceptable for product that is not thermally demanding. However, the literature shows such temperatures are unrealistic for complicated or heavy assemblies. Experiments of low temperatures sidestep problems that can arise due to high application temperatures. When capping the temperature at 255[degrees]C, leaching of 316 or even 305 steels is much less and the amount of dross generated will also be less. The preheat temperature will not need to be raised to the same degree as when one has to enter into a 285[degrees]C solder flow. Indeed, many things are possible in a laboratory environment that will never work in production situations. It is not surprising that a number of experiments have taken place and reported that the results with 255[degrees]C solders are acceptable or even "good." In production, we cannot expect that such "fiddling" will be successful and that acceptable defect rates and reliability would be attainable for demanding products. In our opinion, a high level of touch-up and repair will automatically put an end to such day-dreaming or intentional whitewashing.

Finally, with Pb-free some parameters such as cooling rate will achieve greater prominence.

Reflow processes. Table 2 shows typical parameters for reflow soldering, again adapted to the different solder alloys. High-melt pastes will be narrowed to two families: SnAg+X and SnAgCu+X, as the melting point of SnCu+X will be too high. Their thermal process profiles will shift to higher temperatures (Figure 1).

[FIGURE 1 OMITTED]

The peak temperature range is 245[degrees] to 260[degrees]C, or the melting point of the alloy plus 30-40K. The literature typically calls for a peak temperature of 235[degrees]C, which may be feasible under excellent nitrogen inertion. Due to solderability problems on components and boards and a general lack of good wetting properties by Pb-free alloys, we anticipate a slow adjustment of the peak temperatures to the aforementioned range.

Choosing the best peak temperature--as close as possible to the melting point of the solder (approximately 220[degrees]C) yet not so high that the activators of the paste will react prematurely--is one of the most difficult conditions to meet. The activators in the paste (Di-carbon series of acids) disassociate at about 160[degrees]C--substantially lower than the optimal 185[degrees] to 200[degrees]C. Profiles without a plateau will become more general, especially when either a better thermal design of the assembly or better equipment prevent the development of a major dT. On the other hand, we do not yet have reliable information about the effect of an increased thickness of the intermetallic layer on the reliability of the Pb-free joint. Thus, we do not even know whether a large dT will have a negative impact on reliability as in the case of SnPb. Perhaps we will not have to react to large dTs (except, perhaps, for components) as before and might be able to use the pyramidical profile without paying the dT price.

Hand Soldering. Hand soldering is used mostly for touch-up, repair and selective soldering processes. In each case it is important that the alloy--and, if possible, the flux--match the solder in the main processes. We have to distinguish even more clearly the material used for secondary treatments in Pb-free solder processes.

Setting the temperature of the solder iron tip depends on the melting point of the solder:

Solder m.p. + "superheat" (for the required heat transfer and flow of the molten metal) + ~70K (for temperature loss experienced during contact of the tip with the joint).

If one wanted to work at about the same temperatures as used during flow soldering, this yields the following calculation for eutectic SnPb solder:

183[degrees]C + 70K superheat + 70K temperature loss [approximately equal to] 340[degrees] to 350[degrees]C (with a certain margin of safety), not 450[degrees]C (as far as one can turn the knob) as regularly observed in practice.

Transferred to the Pb-free model:

Low-melt solder: 138[degrees]C + 70 K + 70 K [approximately equal to] 270[degrees]C

High-melt solder: 220[degrees]C + 70 K + 70 K [approximately equal to] 360[degrees] to 380[degrees]C

Other solder processes harken back to the related process as discussed.

Although many important aspects required to establish the "ultimate" profile for wave or reflow soldering have not yet been investigated, those presented can be a starting point. Do not forget the requirements of components, printed wiring boards, equipment and other process materials.

Ed.: This is the last in a four-part series. Previous articles on wave, reflow and selective soldering equipment can be found at circuitsassembly.com.

Table 1. Typical Parameters for Wave Soldering

Paramater      SnPb-alloys      Bi-alloys            Sn-alloys
Melting Point  (183[degrees]C)  (138[degrees]C)      (217-230[degrees]C)

Flux           low-solids or    low-solids or        low-solids or
               no-clean         no-clean, specially  no-clean with
                                formulated, e.g.     higher
                                with benzolsulfonic  rosin content
                                acid

Conveyor       90-120 cm/min.   90 cm/min.           60 cm/min.
Speed

Conveyor       7[degrees]       7[degrees]           8-9[degrees] or
Angle                                                modified
                                                     solder nozzle

T-Gradient     2 K/sec.         2 K/sec.             2 K/sec.
Preheating

T of the       110[degrees]C    110[degrees]C        solder temp.
laminate                                             minus 120K (i.e.
material                                             150-165[degrees]C)
prior to
entering
the solder

Solder         250[degrees]C    200[degrees]C        melting point + 50
Temperature                     [+ or -] 20 K        K (i.e.
                                                     270-285[degrees]C)

Dwell Time     3-4 sec.         3-4 sec.             4 sec.
in the
Solder

Cooling        approx. 3        approx. 3 K/sec.     3 K/sec.
               K/sec.

Nitrogen       - dross          - improved wetting   - improved wetting
                 reduction      - dross reduction    - dross reduction
               - reduction of   - reduction of       - reduction of
                 maintenance      maintenance          maintenance
                 requirements     requirements         requirements

Table 2. Typical Parameters for Reflow Soldering

Parameter Melting       SnPb-alloys          Bi-alloys
Point                   (183[degrees]C)      (138[degrees]C)

Conveyor Speed     90 cm/min.              90 cm/min.

Peak Temperature        215-225[degrees]C    190-200[degrees]C

Time above         40-60 sec.              40-60 sec.
Liquidus

Time above glass        < 120 sec.           < 80-100 sec.
transition
temperature Tg of
the laminate

Max. Temperature   0.5-2.0 K/sec.          0.5-2.0 K/sec.
gradient

Plateau                 160[degrees]C *      120[degrees]C *
Temperature

Cooling Rate       approx. 3 K/sec.        approx. 3 K/sec.

Nitrogen                for best possible    for best possible
                        wetting              wetting

Parameter Melting  Sn-alloys
Point              (217-230[degrees]C)

Conveyor Speed         70-80 cm/min.

Peak Temperature   melting point +approx.
                   30-40 K (i.e. 245-260[degrees]C)

Time above             60-80 sec.
Liquidus

Time above glass   < 80-100 sec. as new laminates
transition         will perform a higher Tg (e.g.
temperature Tg of  HiTemp FR4 Tg = 170[degrees]C)
the laminate

Max. Temperature       0.5-2.0 K/sec.
gradient

Plateau            recommendable:
Temperature        245-260[degrees]C minus 60 K =
                   185-200[degrees]C.
                   realistic: 160[degrees]C **

Cooling Rate           approx. 3 K/sec.

Nitrogen           for best possible wetting

* as close as possible to the melting point of the solder, however not
so high that the activators of the paste will react prematurely.
** the activators in the paste (di-carbon series of acids) disassociate
at about 160[degrees]C and thus substantially lower than we would wish,
i.e. at temperatures of approx. 185-200[degrees]C. Profiles without
plateau will become more general especially wherever either a better
thermal design of the assembly or better equipment prevent the
development of a major DT.

Markus Walter is technical director of Seho GmbH (seho.de); [e-mail].