When Should You Use Intrusive Soldering?

 

Introduction to Intrusive Soldering

Intrusive soldering, also known as pin-in-paste soldering, is a soldering technique used to establish reliable electrical connections between components on a printed circuit board (PCB). It involves inserting component pins into solder paste deposits on a PCB and then reflowing the solder to form the solder joints.

Some key advantages of intrusive soldering include:

• Improved solder joint reliability compared to surface mount soldering
• Ability to solder fine pitch components
• Suits high density PCB assemblies
• Allows greater component packing densities
• Reduced risk of tombstoning during reflow

Intrusive soldering is a widely used process in electronics manufacturing. However, it's not suitable for every PCB assembly. Factors like component types, PCB design, production volumes, and product specifications determine when intrusive soldering is the right approach.

When to Use Intrusive Soldering

High Density Interconnect PCBs

One of the main benefits of intrusive soldering is the ability to pack components densely on PCBs. This makes it well-suited for:

High density interconnect (HDI) PCBs - These have trace widths and spacings under 100 microns (0.1 mm). HDI allows greater routing densities and more signal layers. Intrusive soldering can reliably solder fine pitch components on these boards.

Double sided assemblies - Packaging components on both sides of a PCB maximizes surface real estate. The intrusive method works for double-sided placements.

Area array packages - These component packages have leads distributed over the underside surface rather than just the edges. Examples are ball grid arrays (BGAs) and land grid arrays (LGAs). Intrusive soldering allows securely attaching area array packages.

Fine pitch components - Intrusive soldering can be used for fine pitch components, down to 0.3mm pitch BGAs. It helps minimize solder bridging risks.

In general, when PCB space is limited, and components must be packed densely, intrusive soldering allows reliably interconnecting high density assemblies.

Solder Joint Reliability is Critical

For products where solder joint reliability is critical, intrusive soldering has benefits over surface mount soldering:

• The solder joints have higher standoff heights, improving resistance to impacts and vibration
• There is greater contact area between the pin and solder, enhancing joint strength
• The cylindrical pin shape improves capillary action, creating reliable solder joints

Intrusive soldering is recommended for products where solder joints undergo dynamic stress during use or transport. Examples include:

• Military and aerospace avionics
• Automotive electronics
• Medical devices
• Industrial equipment

For these applications, the improved solder joint reliability from intrusive soldering helps prevent field failures.

High Volume Production

For high volume electronics production, intrusive soldering offers advantages over manual soldering:

• Faster assembly process compared to manual soldering
• Improved consistency and repeatability between assemblies
• Lower production costs versus manual soldering and rework
• High throughput using automated assembly lines

Intrusive soldering with solder paste is well suited for automated assembly. Production volumes in the tens of thousands of boards per month justify the initial setup costs for stencils and solder paste deposition equipment.

High mix, low volume production runs can utilize intrusive soldering, but it involves higher costs for frequent stencil and deposition process changes. Manual soldering may be more economical for prototyping and low volume production.

PCB Design Considerations

To benefit from intrusive soldering, the PCB design needs to accommodate the process requirements:

Solder paste deposits - Sufficient solder paste volume must be applied on pads to form reliable solder joints. Stencil thickness and aperture designs need to match component lead configurations.

Component orientations - Components must be oriented so leads insert vertically into solder paste deposits during placement. Non-vertical insertions increase solder bridging risk.

Lead-to-pad clearances - Adequate clearances are required between component leads and adjacent pads or tracks. This prevents solder bridging defects.

Non-collapsing leads - Components must have solid leads that do not bend or collapse when pressed into solder paste. Lead collapse can trap solder paste and create voids.

Pad and stencil aperture designs - These need to be optimized based on component pin-out and lead types to control solder volume and minimize shorts.

With proper PCB designs, intrusive soldered assemblies can be reliably produced and avoid defects.

Process Considerations

To realize the full benefits of intrusive soldering, the assembly process must also be optimized:

Solder paste - Using solder paste matched to the assembly is critical. The alloy, powder particle size, and rheological properties impact results.

Solder paste application - The volume of paste printed per pad, accuracy of deposition locations, and consistency affect joint quality.

Component placement - Components must be accurately placed with leads centered on pads and properly seated into solder paste. Automated pick-and-place systems provide best results.

Reflow profile - An optimal thermal reflow profile ensures good wetting while minimizing defects like bridges, icicles, and tombstones.

Process controls - Tight control of parameters like paste deposition volumes and reflow profiles improve process capability and yields.

With suitable process development and controls, intrusive soldering can reliably interconnect highly dense PCB assemblies.

Limitations of Intrusive Soldering

While intrusive soldering has significant advantages, it also has limitations:

Lead forming requirements - Component leads must be trimmed and formed to extend straight from the body rather than bent under. This requires additional processing of leaded components.

Lower component height clearance - Intrusive soldering requires clearance between board and adjacent components to insert leads into solder paste. This restricts component height compared to surface mount soldering.

Solder voids - Partial collapse or misalignment of leads in solder paste can create solder voids reducing joint strength.

Solder balling - Some surface mount component packages are prone to solder balling which can cause electrical shorts. This may restrict use of intrusive soldering.

Higher defect rates - Intrusive soldering is more prone to certain defects including insufficient fillets, skewed components, and bridging versus surface mount soldering.

Increased costs - Intrusive soldering involves costs for stencils, solder paste, inspection, and process development exceeding those of manual soldering.

These limitations mean intrusive soldering may not be suitable for all component types or products. The higher defect rates require robust process controls to maintain quality.

Component Suitability Considerations

When considering using intrusive soldering, assess component suitability:

Lead types - Prefer components with solid, non-collapsible leads like gull wing devices. Avoid leads that may bend or fold over on insertion.

Lead pitch and count - Choose fine pitch components with lead counts suiting solder paste volumes that can be printed. Too many leads reduces paste volume per joint.

Lead dimensions - Optimal lead diameters for solder wetting are around 0.4 to 0.8 mm. Avoid very thin leads prone to inadequate filleting.

Solderability - Leads should have solder coatings or materials allowing excellent wetting and bonding of solder.

Thermal suitability - Components must withstand reflow temperatures, up to 260°C for lead-free solders without reliability issues or damage.

Board clearance - Pick components with lead standoff heights providing adequate clearance to board and adjacent components.

Assessing these factors early ensures component selections are compatible with intrusive soldering needs.

Summary of When to Use Intrusive Soldering

In summary, key situations where intrusive soldering is advantageous include:

• HDI PCBs requiring high component density
• Double sided assemblies with components on both sides
• Area array package soldering like BGAs and LGAs
• Fine pitch component soldering down to 0.3mm pitch
• Products where solder joint reliability is critical
• High volume production with automated assembly
• Components with suitable leads and board clearances

The process can maximize component densities, enhance reliability, and lower production costs versus manual soldering. But it also has limitations around defect rates and component clearances. Assessing product needs and specifications determines when intrusive soldering is the best approach over other methods.

Intrusive Soldering Process Step-By-Step

PCB Fabrication

To utilize intrusive soldering, the PCB must be designed and fabricated with the process requirements in mind:

• Pad dimensions and spacing suited for components to be soldered
• Adequate clearance between pads for solder paste application
• Non-solder mask defined pads to allow sufficient solder paste volumes

Other PCB attributes like number of layers, trace dimensions, and material thickness follow conventional standards.

Once designed, the PCB fabrication process proceeds as usual through:

• Laminating copper layers and prepreg
• Drilling through holes
• Plating through holes and applying copper finishes
• Printing solder mask
• Printing silkscreen legend
• Final finishes (ENIG, immersion silver)

This produces a finished PCB ready for solder paste and component assembly.

Stencil Design and Fabrication

A stencil is required to apply solder paste deposits in locations corresponding to PCB pads. The stencil thickness and aperture designs are tailored to the components and PCB:

• Stencil thickness is typically 0.1 to 0.15mm. Thicker stencils allow higher print volumes.
• Aperture dimensions match component lead widths and configurations
• Aperture shapes can be adjusted to control print volumes
• Fiducials align the stencil holes to pads during printing

Laser cutting fabricates stencils from stainless steel or nickel alloy sheets. Stencils have a fine surface finish to release paste cleanly. Proper storage avoids oxidation or damage.

Solder Paste Application

Solder paste is printed through the stencil onto PCB pads:

• Solder paste needs be matched to the assembly and process. Sn96.5/Ag3/Cu0.5 is a common lead-free paste.
• Paste is applied to the stencil surface and a squeegee blade presses it into apertures.
• Excess paste is removed as the blade smooths the stencil surface.
• The paste transfers to pads as the stencil releases from the PCB.
• Optimal print speed, pressure, separation speed, and temperature give consistent paste deposits.

Automated printers allow rapid printing of paste onto multiple boards. Volume checks ensure adequate paste height and uniformity.

Component Placement

Components are precisely placed with leads centered on pads and seated into solder paste:

• Pick and place machines use vacuum pickup tools to handle components.
• Deposited flux helps hold components in position.
• Machines place components rapidly and accurately onto adhesive solder paste.
• Board support tooling avoids smearing deposits during placement.
• Optical inspection confirms all components are present and positioned correctly.

For manual assembly, technicians carefully align component leads with a microscope and press devices into paste deposits. Fine pitch ICs may use specialized placement tools.

Reflow Soldering

Heating reflows the solder paste to form solder joints between component leads and PCB pads:

• Convection, infrared, vapor phase, or other heating methods are used.
• A typical SAC305 lead-free reflow profile peaks around 240-250°C.
• The process activates flux, melts solder, fully wets leads, then cools forming solid solder joints.
• Thermal profile parameters are tightly controlled to ensure good wetting and prevent defects.
• Optical inspection checks for any missing or defective solder joints.

The board is cleaned to remove flux residues. This completes the intrusive soldering process.

Advantages and Disadvantages of Intrusive Soldering

Advantages

• Allows higher component packaging densities on PCBs
• Provides excellent mechanical strength and electrical reliability
• Handles fine pitch components down to 0.3 mm pitch
• Solder voids minimized due to paste encapsulating leads
• Reduced tombstoning of small surface mount parts
• Eliminates need to pre-apply solder to component leads
• Suitable for double-sided board assemblies
• Can solder challenging area array packages like BGAs
• Automated assembly lowers manufacturing costs

Disadvantages

• Requires tight process controls to minimize solder defects
• More prone to solder bridging versus surface mount soldering
• Solder paste deposits lack self-centering of plated through-holes
• Needs stencil and solder paste materials not required for other methods
• PCB design requires consideration of solder masking and clearances
• Height clearance requirements restrict component types
• Leaded parts may require lead forming (cut, bend) to be suitable
• Generally higher production costs than manual soldering or wire splicing

In summary, for suitable products, the advantages of higher density, smaller components, and reliability often outweigh the disadvantages of higher costs and defect rates.

Frequently Asked Questions

What is the key benefit of intrusive soldering?

The main benefit is the ability to reliably solder high densities of fine pitch components onto printed circuit boards. This maximizes the component packaging density on the PCB surface.

What are typical components soldered with intrusive soldering?

Typical component packages are fine-pitch quad flat packs (QFP), ball grid arrays (BGA), chip array packages, and leaded devices with gull wing or J leads. Fine pitch under 1mm between leads can be soldered.

Does intrusive soldering require special PCB designs?

Yes, PCB designs need to provide appropriate solder masking, pad dimensions, and clearances between pads. This accommodates solder paste printing and component placement for the process.

How are BGAs soldered with intrusive soldering?

The solder balls on the BGA package are aligned over matching pads on the PCB. Solder paste is printed on the pads through a stencil. With the balls contacting the paste deposits, reflow solders the package in place intrusively.

What defects are most common with intrusive soldering?

Insufficient solder fillets, solder beads, skewed components, and solder bridging between pads are more common versus surface mount soldering. But process optimization minimizes these risks.