microTip^TM Library

RETRIEVE PAST MICROTIPS^TM

microTips™ are short, two page, highly technical whitepapers intended to help design and manufacturing engineers address universal problems associated with miniature and micro-miniature laser and resistance welding.  Click on the microTip title to access the abstract and then click on the abstract title to download the .pdf microTip. 

NOTE:  microJoining Solutions microTips^TM may be reprinted by the user for personal use only.

Appearances are Deceiving-1

Most manufacturing companies utilizing resistance welding processes to join metal parts use visual criteria to pass or fail welded parts.  Regardless of who does the inspection, the operator or a trained quality assurance inspector, the visual inspection process can not predict weld quality in terms of weld peel or pull strength.  Relying on visual inspection virtually guarantees unnecessary product scrap and welded products that will fail in the field.

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Appearances are Deceiving-2

Many manufacturing companies employing resistance welding processes to join metal parts use visual criteria to pass or fail welded parts.  Regardless of who does the inspection, the operator or a trained quality assurance inspector, the visual inspection process cannot predict weld quality in terms of weld peel or pull strength.  Relying on visual inspection virtually guarantees unnecessary product scrap and welded products that will fail in the field.

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Bond Types

Metal bonding involves the transfer of electrons between one or more metal atoms.  The metal atoms can be identical or different in atomic structure.  Metal atoms tend to lose electrons from the outer shells.  The “electron gas” produced by this separation holds the remaining positive ions together.  The resulting combination of metal atoms is typically called an “alloy”.  The physical properties of the new alloy are determined both by each metal’s atomic structure and how their atoms are aligned to each other.  This “alignment” structure is referred to as the crystalline structure of the alloy.  Alloys are typically harder, less electrically conductive, stronger, and more brittle compared to the individual physical properties of each atom forming the alloy.

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Clean Room Welding

A clean room is a controlled environment where products are manufactured.  These specially designed rooms control the concentration of airborne particles within specified limits.  Sub-micron particles generated by people, processes, facilities, and equipment must be continuously removed from the air.  The only way to control particle contamination is to control the total environment, which includes controlling air flow rates and direction, pressurization, humidity, and the cleaning processes used to maintain the clean room.

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DoE - Laser Welding

The words, “Design of Experiment” generate instant fear in the minds of most manufacturing personnel responsible for any type of welding or joining process.  Complex mathematics and software put off most people from even trying.  This primer seeks to take the fear out of the laser welding DoE process and provide a starting point for conducting your own DoE on your next welding project.

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DoE - Resistance Welding

The words, “Design of Experiment” generate instant fear in the minds of most manufacturing personnel responsible for any type of welding or joining process.  Complex mathematics and software put off most people from even trying.  This primer seeks to take the fear out of the resistance welding DoE process and provide a starting point for conducting your own DoE on your next welding project.

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High Current Connections

Electro-mechanical assemblies such as terrestrial solar cell panels and thermoelectric generators typically use tin or solder-plated copper connecting straps to connect the current generating devices.  Copper connecting straps provide low electrical resistance and thus minimize the energy losses.  In high temperature applications, brazing materials replace the tin or solder as the joining material.  Heat to reflow the tin, solder, or brazing alloys can be generated by multiple heating methods which include:  induction, flame, oven, resistance, and thermode.  This microTip is limited to inductive, resistance and thermode heating methods..

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Magnet Wire Bonding

Magnet wire is solid copper wire, ranging in diameter from less than 25 microns (.001 inches) to over 1.5 mm (.060 inches).  Magnet wire is typically coated with polyurethane, polyester, or polyimide insulation, depending on the required operating voltage and temperature.  Nylon is often added to polyurethane to decrease insulation cracking and improve lubricity when the wire is wound around a tight bobbin or special form.  The combination of polyurethane-nylon is the most commonly used insulation.  It melts at 155°C, making it easily removable by hand soldering, solder pot dipping, and automated solder wire feed using a constant temperature soldering iron or diode laser for the heat source.  Applying heat causes the insulation to shrink back from bonding area, allowing the solder to flow over the newly exposed copper wire.

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Minimizing Weld Splash

Weld splash is the unwanted creation of small metal particles that are expelled from the welding area during the welding process.  These small metal particles can be airborne in the form of “hot sparks” or can solidify as small “balls” or “filaments” that remain loosely attached to the welding area.  Figure 1 shows two flat plates being welded together with weld splash occurring in the form of hot sparks (a) and attached filaments (b).

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Plating Issues

Metal plating over a base metal provides corrosion protection and enhances product appearance.  Plating can also serve as a diffusion barrier to prevent other metals from mixing and as an intermediary layer to accept solders or brazing alloys.  Gold and silver plating enhance surface conductivity.  Electrolytic plating requires the use of an electric current to transfer the plating material to the base metal.  Electroless plating is strictly a chemical process.  Military/Federal (MIL) Standards and derivative standards cover the most commonly used plating process in the United States.  ASTM and ISO standards cover testing issues such as plating thickness and porosity as well as other attributes.

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Process Validation-2

“Process Validation” is the act of confirming by objective evidence that the product produced by the laser or resistance welding system meets its intended use.  The terms “verification” and “validation” are often used interchangeably, but have very different meanings.  “Verification” ensures that the product was made right.  “Validation” ensures that the right product was made.

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Resistance Welding - Battery Pack Connections - Resistance Welding

Portable power applications continue to grow at a rapid pace.  Increasing battery life is one way for a manufacturer to differentiate his product from his competition.  Portable power battery packs are typically constructed by laser or resistance welding multiple metal “connections”, “straps” or “battery tabs” between each individual cell.  This microTip will cover parallel gap resistance welding of 0.25 mm (0.010 in) thick battery tabs.

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Resistance Welding - Brazing Basics

Brazing is the process of joining one metal to another metal using a low temperature interface metal, called a brazing alloy, without melting the two primary metals.  Brazing permits the joining of metals with very disparate melting temperatures, thermal loads, and crystalline structures.  Common brazing examples include relay contacts, machine tool bits, and radiator assemblies.  Brazing alloys typically melt at 400ºC or higher.  Brazing methods include, furnace brazing for mass production, induction heating, block-and-flow, dip, infrared, torch, and resistance brazing.  This microTip will focus on the resistance brazing of small electronic or electro-mechanical components.

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Resistance Welding - Dumet Wire Welding

Resistance welding Dumet wire to brass or bronze terminals has always been a challenge due to the physical composition of Dumet wire.  These challenges include:

• Severe wire sticking to the electrode surface

• Unexpected wire blow-out or “sparking”

• Heavy electrode grooving

• Electrode sticking and contamination from the Dumet plating

This microTip discusses several ways to mitigate the above challenges using two different heat profiles and a simple electrode tip geometry.

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Resistance Welding - Electrode Design

Proper electrode design is critical for achieving consistent weld quality in the world of small and miniature resistance welding.  Each welding application requires the correct electrode material, tip profile, and shank profile to ensure consistent weld quality, minimum electrode sticking, and maximum electrode life.

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Resistance Welding - Electrode Seasoning-1

“Seasoning” is the alteration of new electrode tip surfaces by metallurgical and mechanical forces.  Seasoning occurs at the beginning of the electrode life cycle.  New electrodes initially produce hotter welds with more material expulsion until the electrode tip surfaces are fully seasoned.  Depending on the electrode tip material, part materials, and generated weld heat, electrode seasoning can occur in as few as 5 welds and up to as many as 100 welds or more.

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Resistance Welding - Electrode Seasoning-2

Electrode “Seasoning” is the alteration of a clean electrode tip surface over time by mechanical deformation, oxidation, and part plating and base material build-up.  This alteration changes the weld heat balance between both weld parts and can affect the weld strength.  Newly cleaned electrode tips can produce “hot” or “cold” welds, depending on the welding power supply feedback mode.  “Hot” welds exhibit significant electrode sticking and uncontrolled weld splash.  “Cold” welds have weak weld strength values.  Electrode seasoning typically occurs over a period of 10 to 100 welds before stabilizing.

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Resistance Welding - Electrode Tip Heating Issues

Passing weld current through an electrode produces heat within the electrode body, tip, tip-to-part interface, parts, and part-to-part interface.  With each subsequent weld, the peak electrode tip temperature increases before stabilizing at some average value determined by the welding rate and weld energy.  This residual heat is difficult to dissipate because of the electrode and electrode holder configurations used in small scale resistance welding.  Residual electrode tip heat can be a large problem in automated welding environments where the welding rate can reach one weld per second or faster.  Residual tip heat is generally not an issue with manual welding due to the slow welding rate. 

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Resistance Welding - Impact Weld Force

Weld force is a key variable in the resistance welding process.  Weld force clamps the weld parts together to provide a path for the weld current.  Weld force also affects the heating produced by the electrode-to-part contact resistance and part-to-part contact resistance.  Low weld force produces high contact resistance and increased weld heat.  High weld force produces low contact resistance and decreased weld heat.

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Resistance Welding - Power Supply Feedback Mode Selection

Resistance welding power supply controls come in two flavors, “closed loop” or “open loop”.  Inverter and Linear power supplies offer “closed loop” feedback control over the electrical welding parameters.  Stored Energy or Capacitor Discharge (CD) and most Direct Energy or AC controls are “open loop”, offering no or minimal feedback.  A ‘closed loop” power supply control with multiple feedback modes offers the following advantages over a non-feedback control:

• More process consistency between multiple welding stations.

• Reduced parts scrap.

• Automatic adjustment for variability in part thermal loading and plating.

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Resistance Welding - Weld Monitor Basics-1

The “Holy Grail” of weld monitoring for micro resistance welding is to find one or more real time welding parameters that can positively identify each weld as good or bad in terms of the potential pull or peel strength of the welded parts.  Unfortunately, the present state of the art of micro resistance weld monitoring does not live up to the “Holy Grail” standard for a variety of reasons.  One, the standard deviation of the weld pull (tensile) or peel strength is very large in relation to the range of the weld monitor parameters.  Two, the poor sensitivity, frequency response, and signal-to-noise ratio characteristics of most weld monitor sensors mask potential weld quality information.

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Resistance Welding - Weld Monitor Basics-2

The “Holy Grail” of weld monitoring for micro resistance welding is to find one or more real time welding parameters that strongly correlate with the weld quality in terms of pull or peel strength of the welded parts.  Weld monitoring instrumentation has improved, but the “unknown” weld monitor parameters that closely correlate with the actual weld quality have yet to be discovered.  Weld monitor parameters such as peak weld current, voltage, force, and displacement are useful for identifying macro changes in weld quality, but cannot explain the large variations in weld quality that occur during a normal production run.

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Resistance Welding - Weld Projection Design

Projections are small indentations stamped or machined into one of the parts that will be resistance welded.  While many projection shapes exist for large scale resistance welding, three basic projection shapes can adequately accommodate most small and miniature scale resistance welding applications.

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Resistance Welding - Trouble Shooting

Typically, the production technician or machine operator goes to the process engineer and tells her, “Today, the welding process is not meeting spec” or, “The power supply is blowing up the parts”.  The process engineer responds by telling the operator, “Adjust the power supply”.  The operator adjusts the weld energy and things seem to work again…for a short while before the welding process goes out of limit and the operator is forced to readjust the power supply once again.  Today’s closed loop, high-speed, feedback controlled power supplies produce consistent constant current, voltage, or power weld energy.  The control variance is typically less than 2% of setting.  Therefore, the power supply is rarely the source of unexplained resistance welding problems.

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Solder and Flux

Selective reflow soldering is the localized heating of two solder plated parts to a temperature level that permits the solder plating on each part to melt and flow between each part.  Cooling the solder creates a common electro-mechanical bond between each part.  Selective reflow soldering requires precise control of the heating rate, reflow temperature, time-at-reflow temperature, cooling rate, solder thickness, flux type and thickness, and heat sinking effects.  Hold down force is an additional variable normally associated with hot bar reflow soldering.  Hold down force is generally not necessary when using non-contact heating sources such as hot gas or diode laser selective reflow soldering.

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Thermal Loading

Thermal loading describes how two metal parts react to the heat generated within the parts for the purpose of creating a weld or solder joint.  Thermal loading directly affects the success of all resistance and laser welding and selective soldering processes.  Heat balancing is the art and science of ensuring that the heat generated to melt or reflow solder the parts is greater than the heat dissipated by the parts.

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Weld Joint Testing Basics

Achieving six-sigma production weld joint quality requires weld joint testing and a sampling plan.  Developing a laser or resistance weld schedule also requires a weld joint testing method.  Post weld, non-destructive test methods for both laser and resistance welding include: hermeticity, visual inspection, weld geometry measurements, ultrasound, and X-Ray.  Dynamic weld test methods include: material surface temperature, and for resistance welding only, weld current, voltage, displacement, and force.

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 Weld Quality Validation - Sample Size Selection

Weld quality validation involves the need to select lot sizes and sample sizes when performing the Welding Process Qualification (PQ) and Welding Process Validation (PV) steps within the overall validation process.  Typically, lot size is based on:

• "We have always done it this way before"

• "A lot size of 30 sounds like a statistically significant number”.

• "Inspect 2 samples at the beginning and end of each lot run".

Converting your Cpk requirement to AQL and then using Minitab® statistical software offers an easy method to set up a statistical sound variable data sampling plan.

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Weldability Issues-1

Why do some metals weld more easily than other metals?  The answer can be found by examining the bulk material and surface properties associated with each pure metal or alloy.  Part I of this microTip covers bulk material issues while Part II reviews the effect of surface properties on weldability.  The chemistry, crystalline structures, and microstructures determine the bulk material properties of each metal or alloy, hence their weldability and bond type.  Properties controlled by the chemistry include melting temperature, thermal conductivity, and electrical conductivity.  Properties determined by the crystalline and microstructures include hardness and brittleness.

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Weldability Issues-2

Why do some metals weld more easily than other metals?  The answer can be found by examining the bulk material and surface properties associated with each pure metal or alloy.  Part I of this microTip covered bulk material issues of the weld parts.  Part II of this microTip reviews the effect of material surface properties on weldability.  Material surface properties include surface texture, natural oxide layers that depend on the chemical properties of each pure metal or alloy, contaminants due to processing and handling such as oil and dirt, and applied coatings such as plating.  Each surface property affects the weldability in a different way.

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