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Want to eliminate wave soldering and streamline mixed-technology PCB assembly? Pin-in-Paste (Intrusive) Soldering integrates through-hole (THT) and surface-mount (SMT) components in a single reflow pass, boosting automation, reliability, and reducing cost.
This blog will introduce the benefits, application situation, and process of PiP soldering technology. Instructions for designing and solutions to common issues are also explained.
What is Pin-in-Paste (Intrusive) Soldering?
Pin-in-paste soldering, also known as intrusive soldering, combines both surface-mount and through-hole components for PCB assembly. The method consists of soldering paste being printed over plated through holes, component leads being inserted into those holes, and then the board being reflowed in a solder oven. By enabling the soldering of through-hole components during reflow, this eliminates the need for secondary wave soldering, thus improving automation and reliability.
Why Use Intrusive Soldering
Benefits of Intrusive Soldering
The common soldering technologies are wave soldering and reflow soldering, using intrusive soldering is for:
- Compatible with existing processes, reducing the welding process and equipment costs
- Avoid manual welding and wave soldering in the surface mount process
- Compatible with no-clean soldering processes, simplifying cleaning process after soldering
When to Use Pin-in-Paste Soldering?
The use of intrusive soldering is most appropriate for dense PCBs with limited real estate since traditional through-hole soldering is not feasible. This technique is useful for applications requiring mechanically robust joints, such as in connectors, transformers, and other oversized or heavyweight parts.
Repair and Maintenance
Intrusive soldering is also useful during PCB repair or rework. It provides a reliable method for integrating faulty components without the need for full-wave soldering setups.
Heavy or Large Components
Parts like relays, transformers, and heat sinks are better for intrusive soldering as they require structural strength.
Comparative Analysis with Other Types of Soldering Techniques
| Method | Strengths | Limitations | Best Use Case |
| Intrusive Soldering | Streamlined steps, strong joints, suitable for automation | Needs to be tightly controlled | High-reliability mixed assembly |
| Wave Soldering | Capable of processing multiple PTH components at a time | Masking required, excess flux residue | Boards with a heavy PTH component population |
| Hand Soldering | Adaptable and able to fix mistakes | Not scalable, takes a long time | Low-volume repairs and prototypes |
| Selective Soldering | Focus on PTH soldering with precision | Slower than the wave, the setup cost is high | Mixed assemblies with tight spacing |
| SMT Reflow Only | For surface mount technology, it is very efficient | Not compatible with PTH | Purely SMT boards |
The Pin-in-Paste Process Flow
During the standard solder paste printing step, a stencil is used to apply solder paste to the Plating Through Holes(PTH). Then, the paste is filled, and the through-hole components are added. In one step, the full assembly is put through a reflow oven, where the introduced heat bakes the solder paste, hence liquefying it and creating both electrical and mechanical bonds between SMT and through-hole components.
To achieve the best results, specific measures need to be applied:
- Solder paste must be correctly deposited to completely fill the via and create robust fillet bonds on both faces of the board.
- Shrinkage voids and cold joints are generally associated with incomplete wetting; therefore, proper reflow profiles should be set for time-above-liquidus (TAL) to prevent these defects.
- The leads of the components to be used must be thermally compatible with the reflow temperature range as well as with the pin-in-paste procedure to ensure they do not absorb heat too easily.
These practices help achieve the best joint integrity and reliability during construction.
Critical PCB Design Considerations for Pin-in-Paste
For intrusive soldering methods to succeed, there is a need to balance between component selection and the PCB layout design optimally to allow wetting and barrel filling during reflow soldering.
Lead Length and Shape
The length of the lead should ensure adequate electrical contact and mechanical retention, while also inhibiting shorts or interference with adjacent components. An ideal lead protrusion is typically 0.5 mm to 1.0 mm beyond the bottom of the board. Round and square cross-section leads are favorable; however, flat leads can be troublesome in terms of wetting and should only be used after sufficient testing.
Hole Diameter
The space between the component lead and the plated hole edge must be precise. A diametrical clearance (difference between hole diameter and lead diameter) of 0.2 mm to 0.25 mm is commonly recommended. This captures sufficient solder paste while preventing loose joints that can lead to flotation or insufficient wetting.
Pad Design
Pads’ PTHs should have sufficient dimensions to allow stencil paste application, as well as form a proper solder fillet. The size of the pads should follow IPC-2221 or IPC-7351 and take into consideration the specific geometry of the leads and the class of the board, which is either Class 2 or Class 3.
Solder Mask Clearance
There should be an opening from the solder mask around the PTH for proper solder access and to avoid contamination of the mask. Usually, a clearance of 75-100 micrometers is standard. Tenting is not desirable in this case as it would impede the solder flow.
Via Fill Percentage
For high-reliability applications (often classified as IPC Class 3), a minimum solder fill of 75% in the barrel is typically required to ensure robust mechanical strength. Appropriate stencil design and careful consideration of process steps are important to control the volume of paste. Fill levels can also be guaranteed with X-ray inspection or microsection analysis.
Component Orientation and Spacing
Designers must try to position tall through-hole components away from critical SMT sections and minimize thermal shadowing during reflow. Specific distances between PTH elements should be strictly adhered to so that solder paste can be printed and components inserted, especially with robotic placement.
Thermal Relief and Grounding
Thermal reliefs should be placed on plated through holes connected to big copper pours to reduce heat sinking, which can negatively affect soldering. In situations where thermal dissipation is very critical, alterations to the reflow profile might be needed to achieve even heating.
Common Pin-in-Paste Defects & Solutions
Half-Done Hole Filling
Failure to fill a plated through hole completely with solder paste denotes incomplete fill.
You can try:
- Increasing the volume of overprinted solder paste
- Adjusting stencil aperture size
- Using aperture adjustment techniques
Solder Bridge Defects
Solder bridging occurs as a direct result of an excess of paste being applied or a component being out of position, thus causing solder to connect to neighboring pads or leads. This often causes a short circuit.
To prevent it, it is essential to fine-tune stencil design as well as paste application and insertion accuracy.
Voids or Blowholes
Voids or tiny gaps in solder connections are often due to environmental factors, such as trapped flux residues or moisture escaping the assembly during the reflow process. Such voids not only weaken the solder joint but also compromise reliability.
This defect can be minimized by baking PCBs to eliminate moisture and using low-void solder paste.
Cold Joint
Cold joints, which look dull and grainy, occur when insufficient heat adds to a lack of full solder wetting, an often-encountered problem during the reflow phase.
Solving this problem requires optimization of reflow profiles, particularly TAL and peak temperature.
Pin Floating
Pin floating refers to components’ leads rising out of the solder and is often caused by poor wetting, leading to frail and open joints. Looser lead-to-hole fits can also cause this issue.
It can be mitigated by using components with better lead finish compatibility and tighter lead-to-hole ratios.
Real-World Case Study: Diagnosing Intermittent Pin-in-Paste Failures
An electronics assembler at mid-volume scales struggled with joint failure during intrusive soldering. After reflow, the joints were evaluated using thermal profiling, solder volume, and stencil design which suggested that the joints were acceptable, yet the components continued to fail functional tests. The failure drove up reliability concerns along with the rework numbers.
A deeper analysis targeted the quality of the components. To investigate, samples underwent an accelerated simulated reflow exposure (baking at 125°C for 5 minutes), followed by solderability assessment via wetting balance testing(measuring the solder’s ability to flow and adhere to the lead). While unbaked components passed the wetting equilibrium test, baked ones demonstrated poor solder wetting. This suggests that some form of oxidation or degradation occurred at the surface under thermal stress. The oxidation or degradation that occurred was minimal and unobservable, but became important during reheating to flow temperature.
With test results on hand, the manufacturer engaged the component supplier. Change to improved plating systems and better maintained surface finishing ensured that subsequent batches would pass solderability tests. Failure rates significantly dropped after these changes.
Conclusion
Pin-in-Paste soldering effectively marries the mechanical robustness of through-hole connections with the efficiency of surface-mount reflow processing. In addition, it minimizes production steps, saves manual soldering processes, and improves mechanical interlocking, which is why it is preferred in numerous sectors for tightly packed, multi-technology assemblies.
Successfully implementing Pin-in-Paste soldering requires careful design and process control. For complex projects or to optimize your existing process, partnering with experts experienced in DFM (Design for Manufacturability) and advanced soldering techniques like those at ELEPCB can ensure reliable, high-yield production. Contact us to discuss your specific mixed-assembly challenges!
FAQs
A1: Suitability checks must be performed on each component. ELEPCB recommends checking component specifications with DFM engineers to ensure optimum geometry and withstand reflow temperatures.
A2:
For hybrid assemblies, yes. Intrusive soldering removes flux residue and thermal shock, which improves consistency for critical processes.
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