Gateway Design - Soldering Technical Notes

Technical Notes - Soldering Technology:

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Soldering Definition

Soldering and welding are metallurgical joining techniques. The terms denote a material connection that is based upon molecular connective forces. When welding, the parts to be connected are melted (welded) into another or connected to each other while in a pliable, doe-like condition by the use of force.

Electronics have pervaded practically all areas of our daily lives and have thus become a supporting column and an essential technology for technical progress. The creation of conducting connections
between components or subassemblies is a central task in the manufacturing of electronic devices. In this regard, soldering acquires exceeding significance. It is therefore natural that trained personnel
that are active in electronic development or production are continually confronted with the topic of connecting technology. In can be noted from experience, however, that the discipline of soldering, in theory and in practice, is somewhat neglected in the highly qualified training of electrical engineers. So it’s no surprise that companies often experience a deficit on know-how when it comes to specific questions about soldering or soldering automation. Especially in the field of automatic spot soldering, incomplete knowledge leads to unused rationalizing potential, and expense-reducing resources
remain untapped. But questions on quality and the securing of continuous production processes also demand detailed knowledge and expert know-how.

This document should contribute to helping the production specialists acquire the needed specific knowledge on the subject of automatic spot soldering, along with the set-up and operation of the ecoSolder – Soldering system. At the same time, unnecessary theoretical ballast should be discarded and the main concentration should be upon practical questions regarding the production technology itself and the operation of the soldering system.

Spot welding procedures that can be automated:
Automatic iron soldering	HF induction soldering
Mini wave	Radiation soldering
Solder bath soldering	Hot-gas soldering
Micro flame soldering	Resistance soldering
Thermo compression soldering	Stamp soldering

Soldering Basics

The metallurgical soldering joining technology follows strict physical laws. Just as a body only swims when it displaces more water than its own body weighs, so can a good soldering joint only come to be when the physical requirements are fulfilled. These prerequisites can be subdivided into:

• metallurgical fundamentals
• thermodynamic fundamentals
• chemical fundamentals
• geometrical fundamentals

In addition, the requirements of the soldering tools and/or processes as well as of the automation technology have to be considered. Looked at this way, soldering is quite a complicated matter. There are influences and specifications from all fields to be considered and, at least partially, to be coordinated in order to achieve the targeted result of a dependable connection. As a rule, a soldering point must fulfill two requirements: good electrical conductivity and sufficient, lasting mechanical durability under the application conditions. The process itself must ensure that the parts to be joined and the environment are not damaged, and also no harmful consequences result.

Soldering Points - Metallurgy

Solderability Table:
Category 1- solderable	Category 2 - conditionally solderable	Category 3 - poorly solderable	Category 4 - not solderable
Gold	Zinc	Aluminium	Chrome
Tin	Soft steel	Alu/Cu	Titan
Tin/Lead	Monel	Stainless steel	Silicon
Silver	Copper/Ni	Chrom/Ni	Cobalt
Copper	Copper/Be	Magnesium	Wolfram
Brass	Spring Steel	Cast iron
Copper/tin	Platinum
Nickel
Cadmium
Category 1:		Solderable by use of halogen-free flux
Category 2:		Conditionally solderable by use of aggressive flux material e.g, zinc chloride solution
Category 3:		Very difficult to solder only after pre-treatment and with special flux material
Category 4:		not solderable

Fig 1: Solderability table

Basically, the parts to be soldered should be pretinned. If no pretinning has been done, the solderability has to be tested by attempting to wet a trial piece. The thicker the tinning, the better. You should try to achieve a thickness of at least 0.2mm. Silvering also functions very well as an alternative to tinning. Especially for higher temperatures caused by the
heating medium (e.g., in the case of induction and flame soldering), silvering the surfaces is worth recommending. Gold plated soldering points, on the other hand, have very low wetting characteristics because the very slight quantity of gold diffuses (dissolves) and the nickel underneath has only medium wetting capabilities. Copper surfaces oxidize very quickly. Therefore, these parts should be treated within a short period and stored appropriately (reaction can take place with other materials such as sulfur).
The general solderability of the individual materials can be read from the above table (Fig.1)

When contemplating general metallurgical issues, naturally the question of the solder has to be considered. Although legislature will be passed, for reasons of environmental safety, to introduce “non-leaded electronics” from 2008 onwards, today tin-lead solder is still used almost exclusively (Sn/Pb solder in accord with DIN EN 29453). Lead-free solders are already on the market, or are in the process of development. Up till now, the production experience with lead-free solder has been very limited. According to well-known solder manufacturers, the percentage of lead-free solders used in Germany is at this time less than 5% of the quantity produced. For this reason, that which follows refers to the still customarily used Sn/Pb solders. The solder must always have a significantly lower melting point
than the material of the parts to be joined. For this reason, only alloyed solders are used for soft soldering, whose percentage of Sn or Pb are different, however, and often compounded with alloying additives. The Sn/Pn solders have definite advantages that come from the melting behaviour, the melting temperature range and the viscosity

• Melting temperature Pb 327.4° C
• Melting temperature Sn 231.8° C

Fig 2: Phase Diagram

The melting temperature of the Sn/Pb solder alloy is always lower than that of the higher melting Pb component and reacts in relationship to the alloy ratio. The phase diagram (Fig. 2) gives information about the melting behavior of the Sn/Pb solder alloy. Sn/Pn solders reach the so-called eutectic point at a ratio of 63% Sn and 37% Pb. In this state, solid us and liquid us occur together, i.e., the change from solid to liquid happens without transition at a
temperature of 183°C. Other alloy ratios pass through a “doey” condition in which one of the alloy components is fluid and the other is solid (crystalline). Eutectic solder is advantageous to work with. The solder temperature is lower than non-eutectic solder and has good flow
characteristics. It runs thinner, flows faster and fills fine empty spaces (capillaries). The processing speed is higher and the texture is finely crystalline and homogenous, which enhances the durability. Eutectic Sn/Pb solder can withstand a continuous temperature of
approx. 80° C. An increase of the Pb percentage attains a higher degree of temperature durability but is less strong. With a percentage of Pb over 50%, the solder becomes a lead solder or high temperature solder whose processing temperature at 95% Pb rises to over 300°. The temperature for continuous use can be set at 130 - 150° C. A considerably poorer processing behavior, however, must be taken into account. Once the soldering cycle is completed, this is the earliest moment that the soldering point may be placed under stress mechanically or electrically (electrical test or work piece movement).
The soldering processes using contactless heat transfer or that use internal heat generation with, however, an injected primary energy (radiation, induction or resistor heating) generally follow this principle as well. It is of great economical concern to keep the time necessary for the soldering cycle down to a minimum. Thus, the question is often raised concerning what possibilities there are to optimize the time. The temperature/time diagram of a soldering point can, within certain limits, be varied, influenced and thereby be optimized from an economical point of view. The following table (Fig. 3) gives information about various measures that offer advantages and disadvantages with regard to shortening the cycle time. The limits must be carefully selected to avoid that a serious negative factor cancels the advantage you are trying to attain. The optimal ratios are determined through experiments made prior to the automation-planning phase. Only the empirical determination of data and parameters with contemporary result control leads you to the optimal production ratios. Every spot soldering point has its own characteristics and very often its own set of parameters. The chemical effects of the flux during the thermodynamic processes are also to be taken into consideration.

Thermodynamic fundamentals

Any metallurgical junction using soft soldering is always in conjunction with the supplying and dissipation of heat. A soldering point, therefore, always passes through a temperature cycle that is determined, on the one hand, by the melting behavior of the solder and, on the other hand, by the physics of the running process. Heat is thermal energy that manifests itself in solid objects in the form of atomic oscillations. The higher the temperature, the more intense are the movements between atoms. Heat can be transferred in three ways: by heat conduction, when objects of differing temperature come into contact; by heat flow, when the media (fluids or gases) transport heat, and by thermal radiation, when heat is transferred via
electromagnetic radiation (e.g., infrared radiation). All three kinds of transportation play a part in soldering technology. Individual methods distinguish themselves by the kind of heat transfer used – but this will be considered later. The behavior of every soft soldering typically follows a temperature / time diagram. (Fig. 3)
As well known, heat always flows from higher to lower temperature levels. The quantity of heat transported depends upon the time and the differences in temperature. The temperature change follows strictly an e-function with a decreasing rate of change. The diagram shows a typical temperature / time curve of a soldering created by the contact with a soldering tool (e.g., soldering iron) and supplying solder. At this point, the temperature regulated on the soldering tip is considerably higher than the required melting temperature of the solder. In
practice, a soldering tip temperature is selected that lies above the melting temperature of the solder by a factor of 1.5 - 1.8. This is necessary in order to take the dynamic conditions into account as well as to keep the time needed for the soldering cycle as low as possible.

Fig 3: Temperature/ Time Diagram

Let’s observe the course of the diagram:

The soldering tip touches the parts to be joined – we are now a point A. The large difference in temperature between the “hot soldering tip” and the “cold parts” cause the temperature
of the parts to increase rapidly up to point B that lies above the melting temperature of the solder by 20 - 30%. At point B, the solder begins to be supplied. The supplying, depositing
and in-flow of the solder require further heat energy that not only flows into the solder from the soldering tip, but also from the parts. For this reason we have a temperature decrease at
point C. Still, it is also important that C remains significantly above the melting temperature. By means of additional heat supply from the soldering point, the temperature rises again
and reaches point D in which the heat supply is interrupted , i.e., the heat transfer medium is retracted. The cooling-off phase thereby begins. At point E, the soldering point temperature falls below the solidus point. The solder changes its condition from liquid to solid. Now the actual physical soldering cycle is completed, and this is the earliest possible moment that the soldering point may be placed under stress mechanically or electrically (electrical test or work piece movement).
The soldering processes using contactless heat transfer or that use internal heat generation with, however, an injected primary energy (radiation, induction or resistor heating) generally follow this principle as well. It is of great economical concern to keep the time necessary for the soldering cycle down to a minimum. Thus, the question is often raised concerning what possibilities there are to optimize the time. The temperature/time diagram of a soldering point can, within certain limits, be varied, influenced and thereby be optimized from an economical point of view. The following table (Fig. 4) gives information about various measures that offer advantages and disadvantages with regard to shortening the cycle time. The limits must be carefully selected to avoid that a serious negative factor cancels the advantage you are trying to attain. The optimal ratios are determined through experiments made prior to the automation-planning phase. Only the empirical determination of data and parameters with contemporary result control leads you to the optimal production ratios. Every spot soldering point has its own characteristics and very often its own set of parameters. The chemical effects of the flux during the thermodynamic processes are also to be taken into consideration.

Measure	Advantages	Disadvantages
Raising of the temperature of the heat transfer medium, e.g., the soldering tip	• Shortening of pre- and after heating times by means of a higher temperature difference	available for working with the flux is shortened • Diffusion depth is lessened • Heat transfer medium wears out faster Flux splashes
Increasing the mass of the heat transfer medium	• Shortening of pre- and after heating times due to a larger available heat volume	• Accessibility to the soldering point is restricted • Higher loss of solder
Preheating of the parts	• Pre-heating time decreases	• Larger technical investment • Higher energy need • Additional heat stress placed upon the parts
Use of very aggressive types of flux	• After-heating time decreases	• Possibly leaves corrosive residues
Cooling the soldering point during the cooling-off phase	• The solidus point is reached faster	• Diffusion depth is lessened • Liquid solder can be deformed
Improving heat transfer by enlarging the cross section, e.g., by pre-tinning the soldering tip.	• Pre-heating time decreases	• Larger technical investment
Advantages Soldering within a deoxidized atmosphere (nitrogen)	• Flowing behavior is improved • Scale forming is reduced	• High operating costs • Larger technical investment

Fig 4: Measures for shortening cycle time

Chemical fundamentals

The chemical processes during soldering concern mainly the effect of the flux that, with few exceptions, are used for every soldering application. The use of flux is the means by which the solderability – or, more exactly, the capacity of the surface to be wetted – is brought about.
A metallic surface is never “clean”. Most metals react with the oxygen in the air – they oxidize. Even precious metals, which react little with oxygen or not at all, are layered with materials
from the atmosphere which contain a large number of impurities. We only have to think about air pollution caused by combustible gases, containing exorbitant amounts of chemical substances and causing problems especially in industrialized areas. The binding forces of the atoms and molecules (covalent bonds) that react on the surface, form a coating very quickly. The internal connection between the parent metal and the solder is inhibited thereby.
A soldering that qualifies as an inter-metallic joint between solder and parent metal can therefore only take place when the oxidation and/or surface coating is removed. Here we generally talk about deoxidization and for this reason we make use of the so-called fluxes. These are applied either previously on the piece to be soldered or added to the solder (cored solder with flux core). The effective phase of the flux always lies immediately prior to the
soldering phase. The flux covers the surface before the solder flows. The flowing solder repels the flux, thereby wetting the “cleaned” surface, and can thus be joined with the parent metal. A standard flux is conceived to unfold its chemical effectiveness within the curve of the soldering temperature. Its effectiveness is of no significance in a normal temperature range, whereas its full effectiveness unfolds at about 150 °C and, at about 250 °C, the effectiveness dwindles on account of thermal stress.

Flux should fulfill an entire series of different requirements:

• Remove the oxide and superficial coatings as quickly as possible
• Form no residues after soldering
• Be effective in an appropriate temperature range
• Not reveal any effects damaging to health
• Make the appropriate handling possible, e.g., use in cored solder
• Be applicable for a large number of parent metals (alloys)
• Be resistant to aging and permit a long storage time
• Not have a tendency to splashing und the influence of temperature
• Not damage various substrate surfaces such as solder-stop varnish

To take these requirements into account, the manufactures use various formulas. Principally, it always involves a single carrier substance, loaded with so-called activators. Flux agents used for soft soldering are summarized in the DIN 8511 – part 2. The following excerpt shows the flux types most commonly used in electronics, which mostly deviate from modern-day formulas. Rosin (colophony) is only used in single cases because of its forming residues that are hygroscopic or that create subsequent splints.

Type group	ISO-KZ	Main components	Used for	Measure for removal of flux residues
F-SW 24	2.1.1 2.1.3 2.2.3	Amines, diamines	Fine soldering, electrical engineering (intended for non-residual flame soldering's)	Check if case arises
F-SW 26	1.1.2	Natural resins (colophony) or modified natural resins with additional organic activators containing halogens (e.g., glutamic acid hydrochloride)	Electrical engineering, electronics, electrical appliances, metallic merchandise	Generally not required
F-SW 32	1.1.3	Natural or modified natural resins (colophony) with organic halogen-free activating additives (e.g., stearic/salicylic/adipic acid), but excluding amines, diamines or urea
F-SW 34	2.2.3	Halogen-free organic acids with natural resins (colophony), but excluding amines, diamines or urea.	Electronics, miniature technologies, printed circuits	Generally not required

Excerpt from DIN 8511 part 2 flux agents

Explanation:

F-SW 24 means: Flux for heavy metals soft welding The higher the type group number, the “milder” the effect of the flux agent. The first job of the flux therefore consists in the chemical
activity of the deoxidization. It breaks up the oxygen bonding of the metallic surface, thereby removing the impurities and coatings on the surface, and thus enables the wetting with the
solder. At the same time, it protects the surface from becoming oxidized again during the soldering process and absorbs the broken off particles. The effectiveness of the flux is adjusted to a certain temperature range. It should be most effective in the soldering temperature range, hence between approx. 200 and 300° C in standard situations. Thus, the application of high temperature solders, for example, requires a different flux than the low temperature solders. Within the range of the soldering temperature, the flux must be significantly less viscous (easier flowing) than the solder in order to be repelled by the solder and not tend to encase the solder.

Fig 5: Schematic representation of the wetting process

The schematic represented wetting process (Fig. 5) shows the interaction of flux and solder as it occurs in the case of cored solder with integrated flux. The behavior is similar to when the flux was applied separately, or deposited as a reservoir. A reservoir is when pre-tinned parts have received a flux coating through immersion in flux and then laid to dry.
Modern flux formulas are made in such a way that a high percentage of the flux substances, especially the activators, decompose chemically throughout the temperature cycle and the
carrier substance itself evaporates partially or completely. Such flux – also called “no clean flux” – forms only minimal residues around the soldering point into what is called a “halo” that can only be seen with a microscope. The cleaning formerly done after the soldering is only performed today in exceptional cases. The selection of the flux quantity is also very important. Thus, for example, cored solders with a flux proportion of 1 to 3.5 percentage of their weight are most common, making experimentation the only way to determine the least necessary
dosage empirically. Since flux must, on the one hand, prevent re-oxidation during the flowing phase of the solder, it should, on the other hand, decompose during the temperature cycle, it is necessary to coordinate two parameters apposed to each other. The feeding-in of nitrogen or another inert gas can be very effective in this regard. The soldering's of the flux are bettered
and the soldering cycle is accelerated. A flux formula can be selected with a fast decomposing characteristic that also shortens the cooling-off phase. The concept of wetting also plays a role with the flux. This has to do with a purely physical behavior that in practice, however, is very strongly connected with the geometrical conditions of a soldering point. For this reason, an entire chapter has been reserved for this topic.

Geometrical fundamentals

Fig 7: Solder geometrical fundamentals

In general, reproducible situations must be ensured. The tolerances consider all subassemblies like: Indexing, work piece carrier, work pieces, system of coordinate and used soldering tools. These sub-tolerances often sum up considerably. Aci-ecotec soldering systems can process tolerances of +/- 0.3mm, depending on the soldering point. Reproducible soldering points are not possible with larger tolerances. The soldering pads should have a minimum width of 0.3mm. The larger the contact surface of the soldering tip, the better the heating of the soldering point and consequently the soldering result.
Sharp edges or corners have an negative influence on the flowing behavior of the solder.
The best for the soldering process is a so-called 90 degrees corner, in which the soldering tip can be inserted and pressed against the soldering point. In general, one can proceed that the total tolerance must not exceed +/- 0,3. (Fig. 6 and 7) Do not forget: The total tolerance
is not made up soley by the work piece tolerances, but also by the tolerances of the work piece fixtures and the transport system and applies for the three dimensions. A closer investigation of the tolerance often shows that this requirement would result in unacceptable single tolerances. What is to do now, what are the possibilities to bring the positioning of the soldering point in relation to the soldering tool within the required tolerance field?
Soldering connection between punched parts and bent parts

Automatic iron soldering

The soldering tool

An automatic iron soldering first requires a suitable heat transfer medium. This heat transfer medium normally consists of one heating element in conjunction with a soldering tip and one or more temperature sensors. In addition to the heat transfer medium, the soldering tool
principally has a solder feeder. The solder, often cored solder with integrated flux core has to be fed to the soldering point at an exact moment with a certain velocity and in exactly reproducible quantity. The activation of the solder feeding therefore must be freely programmable within the running process. It is a matter of course, that on a soldering tool which is automation-capable, different cored solder diameters and the standard supply units
(rolls) can be mounted.

The thermodynamic behavior

The contacting heat transfer during the iron soldering follows the law of thermodynamics. A heat transfer from a body with higher temperature (soldering tip) to a body of lower temperature (soldering point) is concerned here. The classic spot soldering by using eutectic solder is done within a temperature range of 200 to 300 °C. The heat transfer medium is normally adjusted to 320 to 400 °C. Decisive for the correct functioning of a soldering of good
quality is the thermodynamic process. This process, however, is not only determined by the temperature of the heat transfer medium. Rather, a number of factors play an important role.
Beside the difference in the temperature of soldering tip and soldering point, the mass ratio of soldering tip and soldering point, the transitional section respectively the appearance of the interfacial surface, the heat required for deposition of the solder and the heat that flows from the soldering point into the surrounding, determine the temperature curve in the soldering point. The following diagram (Fig. 8) shows the temperature/time course on a miniature soldering point with eutectic solder. The melting point of this solder is at 183 °C. The topmost line shows the temperature curve on the soldering tip.

Fig 8: Temperature / time

Selection of soldering tips

The temperature curve shown in the diagram is an example of a miniature soldering point, e.g. for a component-wire joint on a PCB. But each soldering point requires a specific thermal
efficiency. This is the ratio of the supplied heat to the achieved temperature rise within a certain time. In the iron soldering process the energy supply from the heat medium to the soldering tip shows a certain inertia. Therefore the soldering tip must be capable to store a certain amount of heat. The thermal capacity of the soldering tip is in proportion to its mass and temperature. The temperature range specifies the solder to be used. The size of the soldering tip is a parameter for the temperature course. If the soldering tip is too small in relation to the heat requirement of the soldering point, the soldering will freeze in while the solder is deposited. This results in the cored solder broken off or upset, mostly with reaction on the feed mechanism and an insufficient soldering result. If a too large soldering tip is chosen, the temperature of the soldering point keeps on rising while the solder is deposited.
This minimizes the efficiency of the flux as it has its best efficiency between 200 and 300°C. Besides a bulky soldering tip is often disadvantageous for the welding accessibility.

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