Drag soldering: how, when & why
There is no doubt about it: automated machine processes are the most efficient methods for electronics assembly. That does not mean that there are not still occasions when hand soldering of individual components or assemblies is necessary. Hand soldering of electronic components, both through-hole and surface mount, has been going on for years and will continue for the foreseeable future. Some devices, like ball grid arrays (BGAs), defy traditional hand soldering methods, while others like quad flat packs (QFPs) require a great deal of skill and dexterity. High pin counts (greater than 100) and fine pitch (down to 0.015") can present challenges for many experienced solder technicians. So what is the best way to handle these challenges? This article clarifies the differences between methods of hand soldering QFPs and what to consider when selecting that method.
Figure 1. Top view of the corner of a QFP
Just as the introduction of dual in-line packages (DIPs) in 1965 was a significant step in automation of the through-hole soldering process, so was the introduction of QFPs (Figure 1) to the surface mount technology (SMT) automated process.
During the initial SMT manufacturing process, solder paste is printed onto all the SMT lands of the printed circuit board (PCB), then the surface mount devices (SMDs), including QFPs are robotically placed on their corresponding land patterns. At this point, the entire assembly is processed through a reflow oven, resulting in a completely soldered assembly when it exits the oven.
One of the biggest yield detractors of any automated SMT process is the paste printing step. It has the largest amount of variables to control. Problems with the paste printing process can result in misalignment, skips, smear, slump, etc. As the lead count increased and the lead pitch decreased, QFPs became more problematic to process. Of course, this resulted in a greater need to rework QFPs after the initial manufacturing process.
There may be some cases where the device must be removed and replaced, and other cases where the device only requires some portion of the leads to undergo some touch up.
QFP rework—hot air
Figure 2. Hot air rework station.
There are many available hot air rework systems that can perform removal and replacement of QFPs (Figure 2
). The difference in these systems is the level of automation and control. As the level of control on these systems increases, so does the cost. These systems do not have any capability to automate, touch up or rework solder connections; however, they are capable of removing and replacing devices.
At this point, it is important to add a note of caution. Handheld hot air guns (Figure 3) should never be used for QFP rework. Hot air guns have no control at all and completely rely on the technician's skill. While a skilled technician may become very proficient, they can become tired or distracted, which can lead to scrapped devices and assemblies due to thermal damage.
Figure 3: Hot air gun.
Some of these hot air systems cannot only mimic the reflow profile of the original process, but also robotically place and remove the devices. They will use some type of tooling to hold the assembly in place and then controlled hot air will be applied to the area until the solder connections are heated to their liquid state. Once the solder is in its liquid state, the device can be removed automatically by the system or manually by the technician. This is, by far, the safest and most reliable method for device removal and, when available, should be the first choice for device removal. If removal is necessary and no hot air rework system is available, there are different contact tips available for manual device removal. Some of these may have an integral vacuum pick-up.
Once the device is removed, the land pattern has to be prepared for part placement. This preparation step typically is done manually; however, there are some automated and semi-automated scavenging systems that are available. Once the land patterns are prepared, a decision must be made for the method of solder deposit. There are two options for solder deposits for reflow: one is to selectively print or dispense solder paste on the lands, and the other method is to apply solder "bumps." Neither of these methods of solder deposit provides the same level of control that was achieved by the original paste printing process. Now, the device has to be placed and reflowed.
QFP rework—point to point
Point to point soldering is a manual soldering process in which individual solder connections are soldered or reworked one connection at a time (Figure 4). Obviously, when using this method to solder or rework QFP connections, progressively smaller soldering iron tips and wire solder diameters must be used as the lead pitch decreases. Since a technician will be soldering each connection individually, this allows the technician to adjust their heat bridge, dwell time and solder volume to suit the particular connection. This gives the technician significant discretion in regards to the rework process, but also reduces the uniformity and consistency of the solder connections. This process also is the most time consuming.
Figure 4: Soldering of individual leads with the point to point method.
QFP rework—drag soldering
Drag soldering is a manual soldering process that uses a special soldering iron tip that may have a flat surface area or preferably a concave surface to hold molten solder. This "ball" of molten solder is then moved across the "feet" of the QFP leads, allowing the surface tension and natural wetting forces of the solder deposit the correct amount of solder on each lead (Figure 5). This is a similar process to wave soldering only the assembly and the solder source are inverted. Instead of the assembly traversing the top of a wave of molten solder, the assembly is stationary and the solder source is passed across the leads. Thus, the drag soldering tip often is called a "mini wave" tip. Just as in the wave soldering process, drag soldering must be performed with external flux added to the lead/land interface. Because drag soldering relies on the physics of the surface tension of the solder to form the solder connections, it results in very uniform and consistent solder volume and appearance. Typically, the quality of these solder joints will meet the requirements of IPC Class 3 inspection criteria.
Figure 5: Drag soldering with a mini-wave tip.
QFP rework—pros and cons
Some hot air rework systems provide a great deal of control during the rework process. That same degree of control also may result in a somewhat lengthy process. They have almost no use for touch-up, only of QFP solder connections. With the complexity and lengthier times required to prepare, place and reflow the device, it generally is easier and faster to solder the device manually. There is no better method when a QFP has to be removed. Depending on the cleaning process used after rework (no-clean, aqueous cleaning or manual solvent cleaning), the hot air rework method may prove to be necessary because it can more closely duplicate the original soldering process. Hot air removal also is preferred when the device must be recovered for future use because it is the least detrimental to the component body and leads.
Point to point rework and soldering of QFPs may be the best process if there is only a small quantity of leads that require rework (bridges, non-soldered connections and insufficient solder). If there is a large quantity of leads that need to be soldered or the entire device must be placed and soldered, then point to point soldering may take too long. When large quantities of leads are soldered point to point, it will be easier to see the variations in the solder volumes. The use of external liquid flux is highly recommended. This means that this will impact the selected cleaning method.
Figure 6: Cleaning after rework.
Drag soldering can be used for touch-up or for soldering an entire device. It is very fast and results in uniform solder connections. Even with this significant benefit, there remain some potential drawbacks to this method. Because the solder connection depends on the surface tension of the solder to provide consistent solder volumes, the volume typically cannot be adjusted. This may result in a problem trying to meet the IPC-A-610 Class 3 requirements for minimum heel fillet height (solder thickness plus lead thickness) on some lead/land configurations. There is a maximum amount of solder that will be deposited at each connection that is controlled by the geometries of the leads and lands (as well as lead placement and coplanarity). The other drawback is the fact that drag soldering cannot be accomplished without the use of external liquid flux. As a matter of fact, greater quantities of flux typically result in greater ease of the drag soldering process and better results. This means that even when using low-residue fluxes, it is necessary to have a robust cleaning process after soldering to ensure that no flux residues remain on the assembly (Figure 6
). Remember, low-residue flux is not benign when not fully reacted and, therefore, must be properly cleaned.
QFP rework—the decision process
When deciding which process to use when reworking a QFP (Figure 7), the first thing to determine is if the device has to be removed. Regardless of which soldering method is used, any time a QFP has to be removed it should be done with some type of hot air system. The control of these systems eliminates most of the risk of damage from the removal process. It then can be processed manually by point to point or drag soldering, or through the use of a hot air rework system. When only touch-up is required, one of the two manual methods should be selected. Generally, the quantity of leads to be reworked will determine whether point to point or drag soldering is used. Whenever contemplating drag soldering, always consider the effects on cleaning after rework and whether (when necessary) it can produce acceptable Class 3 results. If solder bridging is the problem, an empty "mini wave" tip can be used to remove the excess solder forming the bridge or even redistribute the solder forming the bridge across the remainder of the leads, eliminating the problem.
Figure 7: The decision process flow chart.
The reason that so many different methods available for soldering and reworking QFPs is because not one method works in all circumstances. In most cases, drag soldering looks like a "silver bullet" for all QFP soldering needs and most of the time it is. This is why it pays to know the pros and cons before selecting the right process for your particular application. Proper technique and training are, as always, imperative for high quality, high yield and low cost.
Bob Doetzer, Circuit Technology Inc., may be contacted at 340 Raleigh St., Holly Springs, NC 27540; 919-552-3434; E-mail: