The PIH (Paste-In-Hole) process involves directly printing solder paste onto the Plating Through Holes (PTHs) of a PCB (Printed Circuit Board). Subsequently, Through-Hole Devices (THDs) are inserted directly into these holes with pre-applied solder paste. At this point, some of the solder paste adheres to the leads of the component, while most remains on the PCB’s solder pads. When these assemblies pass through a high-temperature reflow oven, the solder paste on the leads and PCB pads melts, securely attaching the component to the circuit board. This method can effectively replace traditional processes like wave soldering or hand soldering.
This process is also known by other names such as “Pin-In-Paste (PIP),” “Intrusive Reflow Soldering (IRS),” and “Reflow Of Through-hole (ROT).”
The advantages of this method are manifold. It eliminates manual soldering or unstable processes like wave soldering, saving labor hours and improving soldering quality while reducing the chances of solder shorts or non-wetting.
However, the PIH process has inherent conditions and limitations:
PIH components must withstand reflow high temperatures.
Typically, standard Through-Hole Devices (THDs) use materials with lower heat resistance compared to Surface Mount Devices (SMDs) because only the soldering leads need to come into contact with the melted solder. In contrast, the PIH process requires traditional THDs to go through the reflow process alongside typical SMDs, meaning they must meet the temperature resistance requirements of reflow. Lead-free components are now mandated to withstand temperatures of up to 260°C for 10 seconds or 250°C for 20 seconds.
PIH components are best packaged in tape-on-reel format.
In order to enable SMT automatic machines to pick up and place components automatically, a sufficient flat surface at top end of components and reel packaging need to be designed to achieve maximum automation. Without this design and packaging, the SMT production line would require an additional operator for manual placement of components, which not only fails to save labor hours but may also lead to quality inconsistencies in SMT, as manual placement might inadvertently interfere with other correctly positioned components. Alternatively, the use of trays with odd-form placement machines for automation can be considered, although it involves higher costs.
PIH components must have a standoff design between their body and the PCB pads.
In general PIH processes, the solder paste is printed larger than the outline of the pad on PCB. This is done to increase the solder paste volume to achieve at least a 75% through-hole soldering fill requirement. Without a standoff design, it’s not possible to increase the solder paste volume by expanding the aperture size in the stencil, which significantly affects the solder fill rate. Moreover, without standoff design, molten solder paste during reflow can flow along the gap between the component and the PCB, leading to excess solder dross and beads. This may impact future electrical quality adversely.
PIH components are best placed on the second side (for two-sided SMT processes).
When PIH components are soldered on the first side of the PCB, there is a risk of solder paste flowing backward into the internals of these components when the Surface Mount Technology (SMT) process continues on the second side. This can lead to potential issues with internal shorts of the components or the risk of them falling into the reflow oven due to gravity. It’s particularly critical to exercise caution with connector components.
For further reading, you might be interested in understanding the differences and effects of converting SMD components to the Paste-In-Hole (PIH) process.
Additionally, solder volume is a significant challenge with the PIH process. IPC-A-610 sets an acceptable standard that the solder fill percentage for through-hole soldering must be greater than 75% of the board thickness, with some requirements allowing for as low as 50%. (Please refer to the diagram below, and for detailed specifications, consult IPC-A-610 section 7.3.5).
Workingbear has noticed that interpreting the requirements for the vertical fill of supported holes in IPC-A-610 (Supported Holes – Solder – Vertical Fill) may not be straightforward for everyone. The graphical representation of vertical fill might lead people to misunderstand. In reality, the soldering termination area in the PIH process should not have solder on the top side of the PCB, contrary to what the diagram seems to show. This is explained in Table 7.4 “Plated-Through Holes with Component Leads – Minimum Acceptable Solder Conditions,” which might appear in different sections in various versions of IPC-A-610.
Table 7.4, Item B: It explains that the acceptable wetting of the Leads and Barrel (hole walls) is 180° (Class 2) and 270° (Class 3). This indicates that the specification pertains to wetting of the hole walls and pins/leads, not whether the through-hole termination area is filled.
Table 7.4, Item C: It states that the percentage of land area covered with wetted solder on solder destination side is 0%. This means that solder is not required in the solder termination area.
Table 7.4, Item D: It explains that the acceptable wetting on solder source side of lead and barrel is 270° (Class 2) and 330° (Class 3).
Table 7.4, Item E: It states that the percentage of land area covered with wetted solder on solder source side is 75% at least.This implies that there should be at least 75% solder coverage in the through-hole from the solder start area.
As for calculating the solder paste amount, you can use the maximum hole diameter minus the minimum pin diameter to calculate the circular area (πr²), then multiply it by the PCB thickness. Remember to multiply by 2 because flux in the solder paste occupies 50% of the volume. After reflow, only half the volume of the original printed solder paste remains. K is an empirical value. Solder paste quantity depends on the aperture open area, aperture shape, the thickness of the stencil, surface roughness of the aperture vertical sides, and factors related to the squeegee such as speed, pressure, material, and angle. Usually, it’s impossible to achieve 100% solder transfer.
The required solder paste volume ≧ π × [(maximum hole diameter / 2)² – (minimum pin diameter / 2)²] × PCB thickness × 2 × K
Note: This formula does not consider the volume of fillets that form on both sides of the PCB when there is enough solder.
If you want to understand the solder quantity calculation for through-holes in more detail, it’s recommended to refer to the article “A Visual Guide to Calculating Solder Paste Volume in SMT Paste-In-Hole (PIH) Reflow Soldering Process” for a detailed explanation.
To increase the solder volume in the through-holes in a Paste-In-Hole (PIH) process, consider the following methods:
Reserve Space for Over Printing:
It’s advisable to discuss this with the PCB Layout Engineer to ensure there is enough space around these plating holes of PIH component for solder paste overprint purpose to prevent unpredictable soldering short.
Note: The solder paste overprint space can’t extend indefinitely. You need to consider the solder paste’s cohesive properties; otherwise, the solder paste might not fully retract from the pad, leading to solder balls. besides, consider aligning the direction of solder paste printing with the extension of the annual rings (we can discuss this further later).
Reduce Through-Hole Diameter: Smaller through-hole diameters will require less solder paste, but be cautious not to make them so small that it becomes challenging to insert components.
Use Step-Up or Step-Down Stencils:
These stencils can increase the thickness of solder paste in specific areas, thereby increasing the solder volume. However, these stencils are more expensive than regular ones, typically by around 10%.
Adjust Solder Paste Printer Parameters:
Parameters like solder paste type, squeegee speed, pressure, and angle, stencil type can affect the amount of solder paste printed. Solder paste with lower viscosity will result in a bit more solder paste volume.
Add Solder Paste Manually:
If your SMT line no longer has an automatic dispenser, you can consider using a manual dispenser or manual solder paste application to increase the solder paste in PIH solder pads. However, this may require additional labor.
Use Solder Preforms:
Solder preforms can be employed to add solder to the PIH solder pads.
Use Short-Lead Components: Components with shorter leads are more suitable for PIH, as some of the solder relies on reflowing from the tip of the leads into the through-holes. It’s generally recommended that lead lengths extend beyond the PCB thickness by 0.2mm-0.6mm, while considering solder strength requirements.
Printing Cross-Shaped solder paste upon PTH for Long-Lead Components:
If lead lengths are excessively long and cannot be shortened, you can use a cross-bridging technique in the middle of the through-holes to ensure more consistent solder paste attachment to the lead tips, facilitating quality control.
Slightly Increase the Lower Reflow Zone Temperature for Long-Lead Components:
You can increase the lower reflow zone temperature slightly to encourage reflow of the solder on the lead tips into the solder joints. However, when doing this, be careful to assess any component drop risks on the 1st reflow side and whether certain components can withstand the higher temperatures.
Pre-solder the First Side (For Double-Sided Boards):
In cases where you have double-sided boards with space on the first side, consider pre-soldering the edge of the annual rings on the component leads side during the first reflow process. Ensure that solder paste quantity and placement are controlled so that solder doesn’t flow into the through-holes, affecting component insertion during the second reflow process.
By implementing these methods, you can enhance the solder volume and improve the quality of solder joints in a PIH process.
Discussion about the solder filled percentage in through-hole soldering for PIH process
The discussion around the solder fill rate in through-hole soldering is valid, especially when component lead length is shorter than the thickness of the PCB. For example, in the case of a Micro-USB connector with component lead lengths of 0.8mm and a PCB thickness of 1.6mm, approximately 0.8mm of the through-hole will be without component leads. In such scenarios, achieving a solder fill rate of close to 100% using Paste-In-Hole (PIH) may be nearly impossible, and the fill rate could be below the 75%. The question arises whether it’s necessary to add more solder to completely fill the through-holes.
From the standpoint of practical experience, there’s often a depth of about 0.3mm to 0.5mm within the through-hole that may remain unfilled after soldering, unless a secondary process is employed. However, the question is, is this secondary soldering really necessary? What’s the purpose of requiring a certain percentage of solder fill in the plating hole, and what is the significance of it?
The primary purpose of solder is to provide electrical connectivity. As long as there’s a solder connection, the electrical signal transmission can occur. In this sense, achieving a 100% fill rate might not be mandatory for electrical connectivity.
The secondary purpose of solder is to provide mechanical strength. The soldered joints should be robust enough to withstand mechanical stresses and not easily break or detach. If the component lead length exceeds the PCB thickness, completely filling the through-holes with solder indeed adds strength to the joint. However, when the lead length is less than the PCB thickness, the difference in solder strength between fully filling the through-hole and ensuring that the component lead is entirely covered by solder might not be significant. This is because the majority of the solder in the joint is typically concentrated at the joint interface where the component lead touches the pad, which is sufficient for mechanical integrity.
In summary, while achieving a high solder fill rate is often a goal in electronics manufacturing, especially for components with lead lengths shorter than the PCB thickness, the primary concern is ensuring that the solder joint provides both electrical connectivity and mechanical strength. A fully filled through-hole might not be a strict requirement, as long as the soldered joint meets these key criteria.
Recommended Reading: Clarifying the Concept of Electronic Component Soldering Strength