Desoldering THM and SMT

Desoldering is a topic that no one seems to get to excited about until the time comes when people discover that a lot of damage is being done during this operation.

What happens if a solder connection is not done properly and you have to fix it? What do you need to do? If you want to do it correctly, the connection will need to be desoldered then resoldered properly.

Preparing To Desolder
What needs to be done before attempting to desolder? The same criteria for soldering also apply to desoldering; clean surface, use of flux, and proper heat and time, depending on the mass of the joint.

Poor Desoldering
The following three examples show what happens when uninformed individuals try to solve desoldering problems. These represent actual experiences.

  1. A circuit board was brought in because this person "thought he had a problem." On examination, twenty-two areas of damage were found, which included eight lifted runs and fourteen lifted pads. Someone obviously had no idea what had to be done to safely remove the solder from one CPU device. A $2,000 computer board nearly became scrap because someone did not know what to do.
  2. A piece of desoldering equipment was sold to a company. A couple weeks later they brought it a board because the operator was having trouble removing solder from a row of connectors. Using the purchased piece of equipment, a connector was quickly removed by an experienced technician. When the board was taken back to the company by the salesperson, repair people were told to "clean and flux" first. The area where the operator had been trying to desolder proved why the job was not successful: it was extremely dirty. The problem was resolved.
  3. A supervisor from a communications company brought in a small pager board because his people were having problems removing a particular "can." While the supervisor watched, it was desoldered without a problem. The same old problem: Old and dirty equipment and lack of knowledge about what to do.
For a connection that has been made but needs to be redone properly, the use of flux is not usually necessary, but there is nothing wrong in using it. The flux will clean the surface during the desoldering process. Don't forget, though, that flux does not remove greases, grime, or other dirt. Flux is for oxides only.

Desoldering Methods
Different methods of desoldering are required, depending on what has to be removed. Of coarse there are a variety of pieces of equipment to do this job.

Solder Wick or Braid for Through Hole Mount
For desoldering items on single-side boards, one of the most common methods - but certainly not the best - is the use of solder wick. This is a copper braid with dried flux included in the braid. Even with the dried flux already in there, the task is better accomplished if liquid flux is applied before the wick is used. The braid is placed on top of the solder joint, and an iron is applied to the braid. A capillary action starts due to the holes in the braid; the molten solder is drawn into the wick. There are different sizes of braid for the various sizes of solder connections.

One of the problems with using solder wick is the pressure applied on attempting to remove the solder. This is natural, except that people apply more pressure than is necessary. Even the weight of an iron is too much, but operators, almost to a person, add additional weight/pressure to the connection. The excessive weight can result in measling, damage from excessive heat, and/or lifted pads or runs from wherever the solder is being removed. This is because the heat and pressure applied have overheated the adhesive holding the pad or run to the board. Damage has been done that could have been avoided if these facts had been known. If wick is used, cut off the used portion. It is useless and will act as a heat sink, resulting in excess heat and time being required for the next solder removal task.

Solder wick is definitely not a recommended method for double-sided or multilayered boards due to the excessive heat needed and the fact that solder is left in the hole. This small amount of solder strong enough that attempts to remove it meet with a very good chance of removing part of the hole. A simple job becomes a major repair, especially on a multilayered board.

Heat And Pressure Problems
Did you know that when a pad or lead is heated up, the glue holding the copper onto the board loses 80 percent of its strength? Now you know why it is important to use minimal heat and no pressure.

The pressure/weight problem can be controlled by having the board placed in a holder and turned so that it is perpendicular to the bench surface. This eliminates the natural downward weight of a person's hand, wrist, and arm; it is left up to the person doing the job to apply that infinitesimal amount of pressure needed.

Other Desoldering Tools
The squeeze bulb or spring-loaded pullit should not be used for desoldering because of the likelihood that they will affect items sensitive to electrostatic discharge (ESD). The air flowing through the plastic or Teflon tip generates, in most cases, a great deal or static electricity. This is not something you want to use around sensitive boards or devices.

Sweat Joint
A resweat joint will occur if either the squeeze bulb or spring-loaded solder pullit is used. The cold tip and cold air make the joint difficult to remove. Because the solder cannot be kept at solder melt temperature by the tool, it is unable to completely remove all of the solder from the plated through hole. When the tool is removed, the lead will make contact with the side of the hole and reattach itself to the molten solder still around the iron is still there.

Some of these tools have gone from Teflon to ceramic-type tips. Ceramic is a very cold material. Consequently the solder connection has to be overheated to make sure the solder remains molten while it is being removed. You already know what can happen to the board and pad when excessive heat is applied.

Multiple Lead Desoldering
Tools for desoldering all of the leads of a DIP at the same time can be a cause for some concern. All of the leads are not connected to the same mass at every joint. If the tool is applied with sufficient heat to melt all of the leads at once, the heat may be all right where there is only the pad. What about the pad(s) where the voltage/ground plane is attached? It will not be enough for them. So more heat is needed to compensate for their larger mass. What happens now to that one solitary pad sitting there? Overheated adhesive overcures and looses its strength. The result is a damaged lifted pad - and more repairs.

Vacuum Desoldering
A good desoldering tool should be able to remove all of the solder in one operation, leaving the hole free and clean of solder without the leads resweating to the sides of the holes. These are powered vacuum desoldering units. The board is placed in a holder and turned so that is at right angles to the bench top. One reason for turning the board sideways is to prevent the desolderer from applying excess downward pressure just from the normal weight of the hand and arm, that would result if the board was placed flat down on the bench surface. A second reason for placing the board in this position it to allow the person doing the desoldering to see when the solder has melted on a double-sided or multilayered board. This tool is placed over the lead with the proper-sized tip, and movement of the tip starts immediately. When the solder becomes molten all the way through the hole, the operator will see the solder moving on the component side. This is when the vacuum is activated ad the operation completed as quickly as possible.

Desoldering Technique
When the vacuum pump is activated to remove the solder from a plated through hole, the lead over which the desoldering tip was placed is moved in a circular motion for round leads and back and forth for flat leads. The vacuum removes the solder from around the lead and brings in cool air to lower the temperature of the hole and the lead to below solder melt. This prevents the lead from resweating to the side of the hole and allows the component to be removed easily.

The vacuum should be kept on until after the desoldering tip is removed from the lead. This gives the molten solder time to get up to the collection chamber. If that time is not provided, the solder will end up in the heater, clogging it up and aggravating the operator - the one actually responsible for the problem.

Old Connections
If desoldering is required of a connection that has been in service for a long time, the first task is a thorough cleaning of the joint and area to remove the unwanted dirt. The dirt acts as an insulator between the desoldering tip and the solder joint. If the dirt is not removed, additional heat will be needed to melt the solder. This added heat can cause damage to the board.

Consideration should also be given to adding solder to a connection if there is very little solder on the joint or if it is old. The solder connection is what transmits the heat through the hole to the component side. If there is very little solder there, it will be next to impossible to get the heat through to the component side to melt the solder. So add some more solder. As far as the old connection is concerned, the solder joint is not the same as when it was first made. The joint becomes primarily lead rich due to the leaching of the tin into the copper. Adding fresh solder will make it much easier to remove.

The main items to remember are: the proper use of heat for the mass of the joint, the right-sized tip, and lack of pressure when desoldering. This all minimizes the amount of time needed to desolder.

As you can see, the same criteria apply to desoldering as apply to soldering:

Desoldering Stations
There are a variety of powered vacuum desoldering stations on the market. Each has its own peculiarities and design. Some are far easier to use, maintain, and change tips than others. To start with, the handle should be comfortable to hold, with the fingers of your hand in total control of the tool and as close as possible to the desoldering tip. This ensures accuracy and provides greater control to the operator. The closer your fingers get to the tip of the tool, the better the work will be. Try writing your name holding the pencil by its eraser end - the same things applies here.

Station Maintenance
The collection chamber would be better if enclosed to prevent the operator from possible burns. The operators should not have to be concerned about burning themselves, which could happen where the chamber is externally exposed. The chamber has hot solder that has accumulated at a high temperature. A chamber outside the handle does not allow the operator's fingers to get very close to the tip.

Cleaning of the collection chamber should be easy. Keeping it clean ensures that it will operate properly. One suggestion is to put a thin film of pure mineral oil on the inside of the glass tube and on the baffle if it has one. Make sure the same filters are used as were originally supplied with the unit when it was purchased. Cotton batting does not effectively filter out contaminants in the same way as a heavy felt filter. Contaminants getting past this filter end up damaging the motor operating the pump.

Changing Tips
Changing tips, which is a major consideration (one size cannot do all joints), should be done easily - that is, loosen a set screw, pull out the tip, install the proper one, and tighten the set screw. All you need is a simple screwdriver, no wrenches or other tools that are needed for some other systems.

From experience over the years, pistol grip-type desoldering tools do not appear to give the same fine control as other types. A person's wrist does not have the flexibility and control that the fingers have for performing a desoldering task.

Surface Mount
For desoldering surface mount components, the process is a little different from THM. First of all, greater care is a must. Through hole mount components can sometimes be a little forgiving because the plated through hole and the pad on the component side help dissipate excessive heat. Not so with surface mount - there is no forgiveness. Too much heat or time and not only will the led and solder be removed from the board, but the footprint as well. The connection areas are only glued to the board, as noted earlier, that glue loses 80 percent of its strength when heated. The idea with surface mount is the right tool, the right temperature, the right time, and the right technique.

Handpiece Tips for Desoldering SMT
There are a vary wide variety of tips handpieces that can be used for removing SMT items. For example there are bifurcated tips attached to a soldering iron that will remove chip resistors/capacitors and SOTs. A tweezer-type handpiece with the proper tips will also remove these items. For SOICs, there is a tip that looks like a tunnel as well as the tweezer handpiece. For PLCC/SOJ, the preference is the tweezer tool with the appropriate tips. For the large QFP, a different type of handpiece with a tip that will match the outline of the QFP and a vacuum cup in the middle is preferred.

Technique for Removing SMT Items
One special problem that arises with surface mount components/devices concerning removal relates to items that are glued to the surface of the board. Gluing keeps items in place during the manufacturing process. Most of the time you will not have any idea whether there is adhesive involved until you try to lift the item from the board after the solder has melted. So you go back and try again because "this thing has got to come off." This is where damage occurs. The board and heated areas have not has a chance to cool off, and the glue holding the footprint(s) has not regained its strength. Wait until the area has cooled. When all of the solder has completely melted on your next try, give the component/device a very, very gentle tweak. This will break the adhesive bond holding it in place. This applies whether the component is large or small.

Manufacturing Equipment
When it comes to machines used for manufacturing surface mount boards, there are usually three types used.

  1. Vapor phase uses liquid vapors to reflow the solder paste that is applied to the footprints.
  2. Infared, which is somewhat similar to an ordinary oven, is also used to reflow solder paste that has been applied to the board.
  3. Convection heating is another method for reflowing solder paste and is receiving wide acceptance in the industry.
  4. Wave soldering machines have been used for years for THM. When using this machine for SMT, adhesive is used to hold each of the surface mount items is place on the bottom side of the board (solder side). This is where the technique described for desoldering SMT items is an absolute necessity. The operator/technician probably will not know how the board was made, so this is a good habit to get into for surface mount.

For surface mount desoldering tools, a separate catalogue would be needed to show half of them. They are similar in most ways to the soldering tools for SMT. Hot-air hand tools and systems are very popular. Tweezer-type tools for removing nearly every type of component or device are also available. Each person decides how comfortable he/she is with a particular piece of equipment.