When Should You Use Intrusive Soldering?

 Intrusive soldering, also known as pin-in-paste or intrusive reflow soldering, is a soldering technique used to create high quality solder joints on printed circuit boards (PCBs). It involves inserting component pins into solder paste deposits on a PCB and then reflowing the solder to form the joints.

Some key advantages of intrusive soldering:

• Creates very reliable solder joints
• Allows for high density component placement
• Suitable for automated assembly
• Often used with ball grid array (BGA) components

However, there are some downsides too:

• More complex process compared to wave or selective soldering
• Requires accurate solder paste printing
• Not suitable for manual assembly
• Generally requires nitrogen reflow

So when should you choose intrusive soldering for your PCB assembly?

When High Reliability and Density Are Required

Intrusive soldering shines when you need very reliable solder joints at high component densities. The pin-in-paste process gives excellent mechanical strength and electrical connectivity.

For example, you would use intrusive reflow when assembling:

• High complexity boards with 0402 or 0201 chip components
• Multi-layer boards with buried and blind vias
• Boards with fine pitch BGAs, CSPs, QFNs etc.

The intrusive process allows assembling such components with quality levels usually associated with aerospace and medical applications.

Here is a comparison of solder joint quality between wave soldering and intrusive reflow:

Metric	Wave Soldering	Intrusive Reflow
Solder Bridging	Higher risk	Very low risk
Solder Voids	Moderate risk	Very low risk
Mechanical Strength	Moderate	Very high
Electrical Conductivity	Moderate	Excellent

As you can see, intrusive soldering provides performance enhancements particularly around reliability.

When Automated Assembly is Planned

Another excellent application for intrusive soldering is on production runs where automated SMT assembly will be used.

The intrusive process integrates very well with:

• High speed chip shooters
• Multi-station transport systems
• Auto-magazines for odd form components
• Automated optical inspection (AOI)
• In-line testing/programming

In fact, complete "lights out" assembly is possible by automating intrusive soldering. And the process works very nicely at high volumes due to machine repeatability.

Here is a comparison of automation suitability between wave soldering vs intrusive reflow:

Metric	Wave Soldering	Intrusive Reflow
Setup Time	Short	Longer
Changeover Time	Short	Longer
Yield	Moderate	Excellent
Operation Cost	Lower	Higher
Automation Level	Limited	Excellent

So for automated PCBA, intrusive soldering is usually the better - although more expensive - choice over wave soldering. The superior soldering quality and process control offset the downsides.

When Using High Density BGA Components

Intrusive soldering is the industry standard technique for attaching Ball Grid Array (BGA) and Land Grid Array (LGA) components. These surface mount packages cannot physically be wave soldered.

And thanks to advances in solder paste materials, it's now possible to reflow even very high density BGAs using intrusive soldering. Some examples are:

• 1mm pitch BGAs with 400+ balls
• 0.8mm pitch LGAs with 900+ lands
• Chip scale packages with 0.5mm solder balls

These components can provide size and weight reductions. But achieving high yields and reliable solder joints requires precise process control - something intrusive soldering delivers.

Here is a comparison between wave soldering and intrusive reflow for BGA assembly.

Metric	Wave Soldering	Intrusive Reflow
1mm Pitch BGAs	Not feasible	Excellent results
0.8mm Pitch BGAs	Not feasible	Very good results
0.65mm Pitch Chips	Not feasible	Good results
Yields	N/A	>98% common

Clearly the ability to reliably attach ultra-fine pitch components makes intrusive soldering extremely useful. It enables miniaturization that cannot be achieved using wave soldering approaches.

When Manual Assembly is Not Practical

While possible, manually assembling components using intrusive soldering is generally not practical. The process requires very precise alignment between boards and components which is best achieved using machines/tools.

And manually applying tiny precise solder paste deposits under miniature components does not produce reliable results. Even for experienced operators, achieving >90% yields would be unlikely.

So intrusive soldering should be your process of choice when:

• Assembling with automated pick and place
• Volume requirements exceed manual assembly time constraints
• Consistent high quality output is mandatory

Here's a brief comparison between manual assembly and automated assembly using intrusive soldering:

Metric	Manual Assembly	Automated Assembly
Setup time per board	Lower	Higher
Assembly rate	Slow	Very high
Yield	Inconsistent	Consistently high
Field failure rate	Higher	Extremely low
Cost (at scale)	Higher	Lower

Frequently Asked Questions

What is the correct solder paste deposit method?

Stencil printing with laser cut, fine grain steel stencils is the recommended approach. Use of metal squeegees helps prevent smearing. Automated inspection of paste deposits is also advised.

Can intrusive soldering be used on FR4 or only high Tg PCBs?

Standard FR4 boards are compatible with intrusive soldering, provided humidity levels are properly controlled. Pre-heating before placement also helps prevent thermal shock when reflowing.

Is no-clean solder paste preferred over water soluble?

While both work, no-clean solder pastes are strongly preferred. They produce better release of flux residues during reflow. And elimination of a post-wash step improves assembly throughput.

What is the ideal solder alloy?

For most applications, SAC 305 (96.5Sn/3Ag/0.5Cu) provides a good balance of properties and cost. Other popular options are SAC 387 or SAC 405 alloys. These provide higher reliability under thermal cycling.

Is conformal coating needed after intrusive soldering?

In most cases, no. The superior joint quality means fewer sites for potential dendritic growth or corrosion. However, for ultra-high reliability applications, selective coating of joints may be justified.

In summary, intrusive soldering delivers enhanced solder joint reliability through improved process control. It truly shines when high complexity, automation, and surface mount devices come together - which is common in modern electronic products. Assembling products without these capabilities will increasingly become an exception rather than the norm.