Through Hole Fill: Vertical Fill and Solder Volume Control

Through hole fill is the proportion of a plated barrel that is occupied by solder after the joint has formed. Barrel fill, more formally called vertical fill, is expressed as a percentage of the board thickness, and it is specified separately from the fillet that appears on the surface. A joint can look perfect from above and still fail the fill requirement, which is why the measurement is made on a section rather than by eye.

What Fill Percentage Means

The reference is the full thickness of the board at the hole, from the top pad to the bottom pad. A fill of 75 percent means that three quarters of that distance is occupied by solid solder, measured from the source side of the solder, which may be the top for reflow or the bottom for a wave.

The requirement is normally written as a minimum, often 75 percent for a standard product and higher for a high-reliability one, with the remaining volume allowed to contain flux residue or a void. The measurement is taken at the centre line of the hole, and the percentage is quoted together with the direction of fill.

Vertical Fill and Its Specification

Vertical fill is the figure that appears on a drawing or in a workmanship standard. It is a minimum rather than a target because the soldering process has a natural maximum: solder cannot easily fill beyond the point at which the gas in the barrel has somewhere to go and the flux has been displaced.

A hole that is filled to 100 percent is unusual and not always desirable, because the solder contracts on cooling and the joint may show a shrinkage depression at the surface. The useful target is a consistent figure above the minimum, with the variation between holes in a lot as small as the process allows.

Barrel Geometry and Aspect Ratio

Fill becomes harder as the barrel gets longer relative to its diameter. The gas in the barrel has to escape through the same opening that the solder enters, and a narrow, deep hole leaves little room for that exchange. The aspect ratio therefore sets the difficulty of the joint before any process parameter is considered.

The finished hole diameter is set by the component lead and the clearance allowed around it, so the designer controls the geometry and the process has to work within it. Where the aspect ratio is high, the reliable approach is a slower soldering cycle that allows the gas to escape rather than a hotter one. The barrel condition itself matters too, and the requirements for the plated hole have to be met before fill can be achieved.

Solder Volume and Paste Deposit

In reflow soldering the solder comes from the paste deposit, and the volume printed must be enough to fill the barrel plus form a fillet on both sides. The stencil aperture area is normally the pad, and the paste volume is increased by using a thicker foil, a stepped stencil or by printing an additional deposit on the top side.

Where the deposit is too small, no process setting will produce the fill, because the solder simply is not present. The calculation is straightforward: barrel volume plus two fillets, converted to a paste volume using the metal fraction of the paste. Doing it before the stencil is ordered avoids a recurring yield problem.

Process Settings That Drive Fill

In wave soldering the drivers are the contact time, the wave height, the board temperature at the wave and the hole geometry. Contact time and preheat are the two that are usually adjustable, and both work by giving the solder more time to rise through the barrel while the gas escapes.

In reflow the drivers are the profile and the paste volume. A profile with a slow ramp and an adequate time above liquidus allows the solder to flow into the barrel before it begins to freeze, while a fast profile can freeze the surface while the interior is still filling.

Thermal Mass and Board Thickness

A thick board with internal planes takes heat away from the barrel from both ends, so the solder in the hole cools before it has travelled the full depth. This is the mechanism behind the common observation that fill is good on thin boards and poor on thick ones at the same settings.

The remedy is more heat and more time, delivered in a way that does not damage the components. Preheating from the solder side, using a longer contact time in a wave, or in reflow using a soak that brings the whole assembly to a uniform temperature before the peak, all address the same problem.

Common Causes of Incomplete Fill

The four common causes are insufficient solder volume, insufficient heat, blocked gas escape and poor solderability in the barrel. The first shows as a partial fill with a clean barrel above; the second as fill that stops at a consistent depth; the third as a void or a blowhole at a particular location in the barrel.

Loss of solderability is distinguished by the appearance of the barrel wall, which will show dewetting or a discontinuous fillet rather than a clean boundary. The test for it is a solderability check on a coupon from the same lot rather than an adjustment to the profile.

Inspection and Measurement

Fill is measured on a microsection taken through the hole axis, with the percentage reported against the board thickness and the direction of the fill recorded. The section also shows voids, blowholes and the condition of the barrel, so it is the single most informative inspection for through hole joints.

Non-destructive methods exist but are indirect. X-ray can show the solder profile in the barrel if the geometry is favourable, and it is useful for detecting grossly unfilled holes in production, but the percentage figure that a specification asks for still comes from a section.

Records and Process Control

The record should carry the board thickness, the hole diameter, the paste volume or the wave settings, the profile measured on the board and the fill percentage from the section. With those fields, a fill failure can be attributed to the design, the volume or the thermal budget without a trial.

Fill should be monitored as a trend rather than as a pass or fail result. A slow decline across lots points to a change in the paste, the stencil or the board supplier, while a step change points to a specific process event such as a stencil replacement or a profile change. The barrel condition and the hole wall should be reviewed at the same time, since a change in the plating affects both.

FAQ

What vertical fill percentage is required? 75 percent is a common minimum for standard products, with 100 percent vertical fill demanded in some high-reliability specifications. The requirement should be stated on the drawing rather than assumed.

Why does fill get worse on thicker boards? Because the solder in the barrel loses heat to the surrounding laminate and planes, and the gas in the barrel has a longer path to escape. More heat and more time are needed, not more solder alone.

Can X-ray measure fill percentage? It can show whether a barrel is grossly unfilled, but the percentage that a specification asks for is normally measured on a section. X-ray is a screening tool for production, not a substitute for a section.