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Knocking Down the Bone Pile: Soldering for QFPs and Other Gull Wing Leaded Parts

There are multiple methods for hand soldering QFPs as outlined in the IPC 7711.21 Rework and Repair of Printed Circuit Assemblies process guidelines document. QFPs have several challenges related to their hand soldering especially when the component has a high lead count. The methods for hand soldering QFPs listed under IPC 7711 5.5 include the following:

• Top of the lead
• Point-to-point method
• Solder paste method hot air system
• Hook tip wire layover
• Blade tip with wire solder
• Adhesive-backed stencil with solder paste and hot air method

Point-to-Point Soldering
The point-to-point method is the one with the most history behind it. Point-to-point soldering is a manual soldering process in which individual solder connections are soldered one connection at a time (Figure 1). The method usually consists of using a chisel tip connected to the soldering hand piece. First, locate the correct tip. The size of the chisel tip should be such that it can properly create the “heat bridge” between the land and the component lead. A good rule of thumb is to have the tip 60–70% of the pad width. The component should be properly inspected to make sure the leads are coplanar and that they are not damaged and bent. Upon proper inspection, the component should be aligned with the pads and be held with a pair of tweezers or an orangewood stick. Next, liquid flux appropriate for the assembly is applied at two opposite corners and the component is tack-soldered in place such that it does not move off the pads. Flux is applied to the remaining lead/land rows and creates a heat bridge between the land on the PCB, the soldering iron tip, and the component lead adding in solder to form the proper solder fillet. Finally, the fillets are inspected per the class required of the assembly.

Drag Soldering
Drag soldering is a manual soldering process that uses a special soldering iron tip with a concave surface “spoon” or “hoof” shape to hold molten solder (Figure 2). This ball of molten solder is then “dragged” across the leads of the QFP, letting the surface tension and natural wetting forces of the solder deposit the correct amount of solder on each of the leads. When using the drag soldering technique, it is most effective with multi-leaded, very-fine pitched components. As with the point-to-point soldering technique, make sure the opposite corners of a package are tacked into place such that the component is aligned and cannot be moved off the pads. Apply enough solder to cover approximately one-third of the tip. For finer pitch parts apply less solder. Now flux the leads on each side of the component. Start from a row that has not been tacked in position. Bring the tip down at an angle to the point where the tip of the lead meets the land so that the edge with the solder on it is on the land, but the face is tilted away from the component. Hold the tips so the shaft runs parallel to the row of the leads. The angle between the side of the tip and the side of the component can be up to 30 degrees (Figure 3) depending on operator preferences. Start running the tip down the toes of the leads. 

The variables to control are pressure (very light) and speed. The operator should glide across the leads with no pressure, with the speed determined by the thermal mass of the board. Follow this process on all sides, cleaning and inspecting after soldering to the appropriate inspection criteria and class. Make sure the tip is tinned prior to putting it back into the holster. 

Adhesive-backed Stencil

In some applications, less experienced soldering technicians can place an adhesive-backed stencil and solder a fine-pitched gull wing device in place (Figure 4). To begin, make sure the site location is clean and dry prior to placing the adhesive-backed stencil. Peel the stencil from the release liner and align it with the lands on the PCB. Roll solder paste through the apertures of the stencil by using a small handheld squeegee (Figure 5). The squeegee can be moved back and forth several times before removing the stencil. Now carefully place the component onto the bricks of the solder paste printed at the rework location. Reflow the solder paste using a hot air or infrared heat source. Clean and inspect per the guidelines of the work.

Summary
In the point-to-point soldering technique, a technician will be soldering each connection individually, which allows for adjustment of the applied heat, dwell time, and solder volume to suit each particular connection. This technique gives the technician a lot of discretion regarding the rework process, but also reduces the uniformity and consistency of the solder connections. The downside of the point-to-point soldering method is that it is time-consuming and tends to burn out tips more quickly.

In drag soldering, a highly skilled technician can move through components much more quickly. However, it is imperative to have a high-quality solder station and iron with active tip temperature monitoring and control, as well as plenty of liquid flux (may not be suitable for all applications). When done properly, the drag soldering technique results in more uniform solder fillets.

Finally, the stencil approach requires the use of a custom-made stencil while reducing the soldering skill level of the technician. Each of these methods can be used to effectively solder a QFP in place. The method chosen is a function of the soldering skill level of the technician and the pitch and lead count of the component.

Resources

1. IPC 7711.21 Rework and Repair of Printed Circuit Assemblies, Revision C 2020.

This column originally appeared in the August 2022 issue of SMT007 Magazine.