ECET 360- wave soldering notes

Wave Soldering

Materials:
- Flux
- Solder
- Oxidation

Purpose of Flux
- Main role is to promote the flow of solder
- Removes light oxidation

Flux Safety
- Flux or its vehicle are often flammable
- Wash hands after handling flux
- Follow all safety precautions outlined in the MSDS sheets

Solder
- Alloy used for joining metals.
- Solidifies below 430° C.
- Most common alloy is tin and lead.

Preferred Wetting

Improper Wetting

Intermetallic Layer

Solder bonds, or "wets," to metals in a process in which it is joined by a metallurgical reaction with the base metal. This metallurgical bonding process results in the formation of an intermetallic compound layer between the solder and the base metal. For example, if molten Sn-based solder is placed in contact with clean Cu, the metallurgical reaction between Sn and Cu will result in the formation of a layer. This intermetallic layer is the "glue" that holds the solder joint together. In normal soldering, the thickness of the intermetallic layer is on the order of 0.5 to 1.0 micrometer. It will rarely grow thicker than this in a liquid-solid reaction. However, solid-state metallurgical reactions can progress after solidification of the solder joint, especially at elevated temperatures.

Dross

Dross is the oxide formed on the solder surface. The rate of dross generated depends on the temperature and agitation. Much of what appears to be dross is, in fact, small globules of solder contained by a thin film of oxide. The more turbulent the solder surface, the more dross is produced. The oxide on the surface protects against further oxidation. Therefore, do not remove the dross more frequently than necessary. Once a day is usually satisfactory. Always use a mask when clearing dross, and place in a closed container.

Dross Blankets
Minimize dross build-up
– Oil can be used by pouring or injection. The oil must be changed relatively often so it can be messy and disposing of it can be an environmental problem.
– Wax works much the same way except that is is placed on the solder as solid chips that melt and form the blanket. Application is not as messy as oil. Disposal is still an environmental issue.
– Marbles are sometimes floated on top of the solder to reduce the amount of surface area that comes in contact with the solder. Marbles don’t reduce the dross build up as much as oil or wax, but they can be reused over and over and do not pose an environmental hazard.
– Nitrogen (controlled atmospheres) has been used to exclude oxygen from the solder pot area. These systems are sometimes referred to as "inert" or "nitrogen blanket" systems.
Dross blankets are materials that are placed on top of the molten solder that minimize the amount of contact between the solder and the air. The less the solder is in contact with the air the less oxidation occurs. Different dross blankets include:

Wave Solder Machine Elements Definitions
- Fluxer
- Preheaters
- Solder Wave
- Conveyor
- Cooling Stage

Foam Fluxer

As the assembly is conveyed over the foam fluxer chimney, the bottom of the board touches the foam head and becomes coated with flux. A short distance after the exit end of the fluxer, an air knife is usually used, made from a tube with small diameter holes drilled in a row along its length. Low pressure air is directed upward at a slight angle toward the entry end of the machine to remove excess flux and drippings without disturbing the components. If the flux is not adequately removed, it can affect preheating, solder joint quality (too much flux can inhibit soldering), and cleanability (what is placed on the PWA typically has to be removed later on). When an air knife is used at this point, it is typically not heated (due to the flammability of the flux solvents) and is often set to hit the PWA surface at angles of approximately 45° to 60°. This is done to both "squeegee" the excess flux away and drive some material up the PTH to aid in solder flow to the topside connections.

Wave Fluxer

Wave fluxing can be used when the leads protruding below the bottom of the board exceed 6 mm and the flux contains a high percentage of solids (usually rosin), or when the flux cannot be applied with a foam fluxer or spray unit. A wave fluxer is more expensive than a foam fluxer, but less expensive than a spray unit. An air knife is available after the exit of the wave fluxer to remove excess flux and drippings from the bottom of the assembly.

Spray Fluxer

Spray fluxing is widely used for the application of low solids fluxes because of the ability to better control the flux deposit for no-clean applications. Low solids fluxes with a solids content in the range of 1-3% often contain no rosin. These are more expensive than foam or wave fluxers.

Advantages vs. Disadvantages
- Foam - easy to set up, repeatable results, can’t let stone dry out
- Wave - good coverage deeper wave, may be messy
- Spray - good coverage, overspray or programming a problem

Preheat
- Evaporate the solvents from the flux
- Activate the flux
- Prevent thermal cracking of components
- Reduces thermal shock
- Allows proper solder flow onto the board
Plate Preheaters
- Infrared Preheater
- Emit Infrared radiation across entire spectrum
- Heats entire assembly evenly

Calrod Preheaters
like an electric oven

Quartz Preheaters
Passes current through metal core with quartz covering & emits IR near visible light spectrum

Forced Convection
- Modification of quartz preheaters
- Heater is encased in structure similar to radiator
- Air forced through the holes of casing
- Easy to set up and use
- Least material selective

Solder Wave

The solder only wets to, or forms joints on, solderable metallic surfaces. Consequently, no soldering takes place on the board surface, which is non-metallic. Poor soldering can occur on any metallic surfaces that are contaminated or have poor solderability

Dual Wave

For wave soldering of surface mount assemblies where, in addition to the usual leaded components, small chip components are glued to the bottom of the board, two solder waves are sometimes used. The first solder wave is usually a high, rather narrow wave made turbulent by some mechanical means. This is achieved by pumping the solder through rows of small fixed or moving holes at the outlet of the nozzle or by means of a unidirectional hollow jet wave. The jet wave's flow trajectory is usually aimed in the same direction as the board's travel direction. This first turbulent wave is followed by an asymmetrical laminar wave. The turbulent action of the first wave causes the solder to move in and around all the chip components to help ensure that all solder joints get soldered.

Conveyor

Conveyor Fingers

If a finger conveyor is too tight, the boards may have a tendency to bow as they are going over the wave solder pot. This phenomena occurs when the finger conveyor is too tight and, as the board goes up through the system, it wants to expand. Since there is not enough room for this expansion, the board bows down into the wave solder pot, creating a change in the contact area with the wave, which also changes the dwell time in the solder, impacting the soldering of the board. Defects typically found due to this situation are indicators something is not correct in the conveyor operation.

These defects include:
· Icicles on the bottom side.
· Solder source side shorts.
· Solder balls on the solder source side.
· Shallow hole fills on outer edges of the board.
· Missing solder on outer edges of the board.
· Excess solder on the trailing edge of the topside of the board

Pallet Fixtures

Conveyor Angle
The assembly is conveyed, usually up a 4° to 12° slope, until its bottom surface contacts the crest of the solder wave, where the pads, protruding leads, plated holes, and bottom side surface mounted components are soldered.

Controlled Atmosphere Soldering
- Used to exclued oxygen from the solder pot area.
- Called Nitrogen systems; sometimes referred to as “inert” or “nitrogen blanket” systems
- Nitrogen

Prevents circuit boards, components, wave, and joints from oxidizing
Less flux is needed on the board
Increased wetting performance
Decreased dross generation

Thermal Profile
- Used to plan the best settings of the machine that will optimize the process
- Critical when using no-clean solder
- Changing conveyor speed will change the profile
- Thermal limits of board and components must not be exceeded

WaveRIDER

Daily Maintenance
- Remove dross from the pot if needed
- Clean up solder spills; wipe down conveyors; wipe down machine and clean fingers
- Foam fluxer - put stone into alcohol or keep submerged in flux
- Spray fluxer - wipe down spray nozzle
- Traversing spray nozzle - wipe down guide rod

Weekly Maintenance
- Pull out solder bath for better dross removal
- Drain fluxer and test for proper activity
- Inspect belts on the pump motor
- Spray fluxer - flush nozzle with alcohol
- Pull out and clean baffles

Preferred Wetting
Improper Wetting
Intermetallic Layer
Solder bonds, or "wets," to metals in a process in which it is joined by a metallurgical reaction with the base metal. This metallurgical bonding process results in the formation of an intermetallic compound layer between the solder and the base metal. For example, if molten Sn-based solder is placed in contact with clean Cu, the metallurgical reaction between Sn and Cu will result in the formation of a layer. This intermetallic layer is the "glue" that holds the solder joint together. In normal soldering, the thickness of the intermetallic layer is on the order of 0.5 to 1.0 micrometer. It will rarely grow thicker than this in a liquid-solid reaction. However, solid-state metallurgical reactions can progress after solidification of the solder joint, especially at elevated temperatures.
Dross

Foam Fluxer
As the assembly is conveyed over the foam fluxer chimney, the bottom of the board touches the foam head and becomes coated with flux. A short distance after the exit end of the fluxer, an air knife is usually used, made from a tube with small diameter holes drilled in a row along its length. Low pressure air is directed upward at a slight angle toward the entry end of the machine to remove excess flux and drippings without disturbing the components. If the flux is not adequately removed, it can affect preheating, solder joint quality (too much flux can inhibit soldering), and cleanability (what is placed on the PWA typically has to be removed later on). When an air knife is used at this point, it is typically not heated (due to the flammability of the flux solvents) and is often set to hit the PWA surface at angles of approximately 45° to 60°. This is done to both "squeegee" the excess flux away and drive some material up the PTH to aid in solder flow to the topside connections.
Wave Fluxer
Wave fluxing can be used when the leads protruding below the bottom of the board exceed 6 mm and the flux contains a high percentage of solids (usually rosin), or when the flux cannot be applied with a foam fluxer or spray unit. A wave fluxer is more expensive than a foam fluxer, but less expensive than a spray unit. An air knife is available after the exit of the wave fluxer to remove excess flux and drippings from the bottom of the assembly.
Spray Fluxer
Spray fluxing is widely used for the application of low solids fluxes because of the ability to better control the flux deposit for no-clean applications. Low solids fluxes with a solids content in the range of 1-3% often contain no rosin. These are more expensive than foam or wave fluxers.

Calrod Preheaters like an electric oven
Quartz Preheaters Passes current through metal core with quartz covering & emits IR near visible light spectrum

Solder Wave
The solder only wets to, or forms joints on, solderable metallic surfaces. Consequently, no soldering takes place on the board surface, which is non-metallic. Poor soldering can occur on any metallic surfaces that are contaminated or have poor solderability
Dual Wave

Conveyor Fingers
If a finger conveyor is too tight, the boards may have a tendency to bow as they are going over the wave solder pot. This phenomena occurs when the finger conveyor is too tight and, as the board goes up through the system, it wants to expand. Since there is not enough room for this expansion, the board bows down into the wave solder pot, creating a change in the contact area with the wave, which also changes the dwell time in the solder, impacting the soldering of the board. Defects typically found due to this situation are indicators something is not correct in the conveyor operation. These defects include: · Icicles on the bottom side. · Solder source side shorts. · Solder balls on the solder source side. · Shallow hole fills on outer edges of the board. · Missing solder on outer edges of the board. · Excess solder on the trailing edge of the topside of the board
Pallet Fixtures