IPC-7621 Explained: Low Pressure Molding (LPM) Encapsulation Guide for Electronics

Protecting electronic assemblies from harsh environments has always been a challenge. Traditional potting compounds work, but they come with significant drawbacks—long cure times, heavy weight, difficult rework, and the potting vessel becoming a permanent part of the assembly. For many applications, there has to be a better way.

Low Pressure Molding (LPM) offers that alternative. Using thermoplastic materials injected at low pressure, LPM encapsulates electronic assemblies quickly, creates lighter packages, and uses reusable tooling. The automotive industry discovered this years ago for sensor modules and connectors, but broader adoption was limited by the lack of industry standards.

IPC-7621 changes that. Released in 2018, this standard provides the guidance engineers need to design for LPM, select appropriate materials, and establish acceptability criteria. Whether you’re evaluating LPM for a new product or already using it and need standardized quality criteria, IPC-7621 is the reference document you need.

What Is IPC-7621?

IPC-7621, officially titled Guideline for Design, Material Selection and General Application of Encapsulation of Electronic Circuit Assembly by Low Pressure Molding with Thermoplastics, is a 40-page guidance document developed by the IPC Low Pressure Molding Task Group (5-33g).

The standard defines encapsulation in the LPM context as a thermoplastic material (typically polyamide) that is heated to a liquid state, injection molded around an electronic assembly at low pressure, and quickly cooled to form a durable, pliable protective barrier.

What IPC-7621 Covers

Who Needs IPC-7621?

The standard acknowledges that acceptability criteria may vary by application—what’s acceptable for an internal assembly may not meet aesthetic requirements for a visible housing. IPC-7621 provides baseline criteria while allowing for application-specific modifications agreed between parties.

Low Pressure Molding vs Traditional Potting

Understanding why LPM exists requires understanding the limitations of traditional potting that it addresses.

Process Comparison

When to Choose LPM Over Potting

IPC-7621 indicates LPM should be considered when you need to:

• Protect assemblies from shock, vibration, or mechanical stress
• Seal against moisture, humidity, or corrosive environments
• Dampen resonance in high-vibration applications
• Achieve faster cycle times than potting allows
• Reduce part weight compared to potting
• Use reusable tooling for cost efficiency
• Maintain the possibility of rework (limited)

When Potting May Still Be Preferred

Traditional potting may be better suited when:

• Maximum chemical resistance is required
• Continuous high-temperature exposure exceeds thermoplastic limits
• Complete void-free encapsulation is critical
• Tooling investment isn’t justified by volume
• Thermal conductivity requirements favor filled epoxies

Thermoplastic Materials for LPM

IPC-7621 covers the thermoplastic materials commonly used in LPM encapsulation, with polyamide (PA) being the most prevalent.

Common LPM Material Types

Key Material Properties to Consider

IPC-7621 identifies critical properties engineers must evaluate when selecting LPM materials:

Material Selection Guidelines

IPC-7621 emphasizes selecting materials with the end environment in mind:

The standard recommends consulting material supplier datasheets and Safety Data Sheets (SDS) for detailed specifications. Materials should comply with applicable regulations including RoHS, REACH, and WEEE where required.

Design Guidelines for LPM Encapsulation

Successful LPM requires designing the assembly with the encapsulation process in mind. IPC-7621 provides specific guidance.

PCB Design Considerations

Flow Hole Design

Flow holes are critical for ensuring material reaches all areas of the assembly. IPC-7621 provides guidance on:

• Hole placement for optimal material distribution
• Hole size relative to board thickness
• Number of holes based on board area
• Positioning away from sensitive components

Without adequate flow holes, the bottom side of the PCB may have voids or incomplete coverage, compromising environmental protection.

Component Density and Clearances

Dense component placement creates challenges for LPM. The standard addresses:

Tooling Design Principles

IPC-7621 covers mold tooling considerations:

• Cavity design for proper material flow
• Gate location and size
• Venting to prevent air entrapment
• Ejector pin placement
• Temperature control (heating/cooling)
• Clamping force requirements

The reusability of LPM tooling is a significant cost advantage over potting vessels, but tooling must be designed correctly to achieve consistent results.

Read more IPC Standards:

IPC Standards Explained: A Comprehensive Guide to PCB & Electronics Assembly Standards

IPC 2615 Standard Explained: PCB Dimensioning & Tolerancing Requirements

MIL-STD-2000A: Military Soldering Standards for Electronic Assemblies

MIL-STD-1686: ESD Control Program Requirements for Military Electronics

IPC 1791 Standard: Everything You Need to Know About Trusted PCB Certification

MIL-STD-883: Complete Guide to Microelectronics Test Methods

J-STD-048 Explained: Complete Guide to Product Discontinuance Notifications

MIL-STD-750: Semiconductor Device Test Methods Explained

MIL-STD-454: General Requirements for Military Electronic Equipment

IPC 9194: Complete Guide to SPC for PCB Assembly Manufacturing

MIL-STD-202: Electronic Component Testing Methods & Procedures

IPC 4110: Complete Guide to Cellulose Paper Specs for Printed Circuit Boards

Boundary Scan Testing (JTAG) in PCB Design: A Complete DFT Guide

Axiomtek IPC-950 Review: Specs, Features & Performance [2026 Guide]

MIL-PRF-55681: Military Ceramic Chip Capacitor Specification Guide

LPM Process Parameters

IPC-7621 describes the process parameters that affect encapsulation quality.

Critical Process Variables

Low Pressure Advantage

The “low pressure” in LPM (typically under 40 bar, often under 20 bar) is critical for electronics encapsulation. Traditional injection molding operates at hundreds of bar, which would damage sensitive electronic components. IPC-7621 emphasizes that pressure must be kept low enough to prevent:

• Component displacement
• Wire bond damage
• Solder joint stress
• PCB flexure or cracking

Process Monitoring

IPC-7621 recommends monitoring and controlling:

• Material temperature consistency
• Shot-to-shot weight variation
• Cycle time consistency
• Mold temperature stability

Consistent process parameters are essential for achieving consistent encapsulation quality.

Acceptability Criteria in IPC-7621

One of IPC-7621’s most valuable contributions is establishing visual and dimensional acceptability criteria for LPM parts.

Common LPM Defects

Acceptability Classifications

IPC-7621 acknowledges that acceptability depends on application:

The standard states that acceptability criteria “were chosen with the intent that the printed board assemblies would not be seen by the end user.” When LPM serves as final housing, more restrictive aesthetic criteria may apply and should be agreed between parties.

Critical Defects

Certain defects are typically unacceptable regardless of application:

• Short-shots exposing uninsulated conductors
• Voids compromising environmental seal
• Flash on connector mating surfaces
• Cracks or fractures in encapsulation
• Contamination embedded in material

Application Areas for LPM Encapsulation

IPC-7621 addresses application-specific considerations for several industries.

Automotive Electronics

The automotive industry has driven much of LPM adoption due to demanding production volumes and environmental requirements.

Marine and Outdoor Applications

For marine and oceanic applications, IPC-7621 notes requirements for:

• Salt spray resistance
• UV stability for exposed applications
• Long-term moisture barrier integrity
• Wide temperature cycling tolerance

Avionics and Space

The standard acknowledges that avionics and space applications may have unique requirements for:

• Outgassing specifications
• Flammability ratings (UL 94)
• Extended temperature ranges
• Specific regulatory compliance

Industrial and Consumer Electronics

LPM finds application in:

• Industrial sensor assemblies
• IoT device enclosures
• LED drivers and modules
• Power supply protection
• Battery management systems

Related Standards and Resources

Official IPC Sources

Related IPC Standards

Industry Resources

LPM equipment manufacturers and material suppliers offer technical resources including:

• Material selection guides
• Process parameter recommendations
• Design for LPM guidelines
• Application engineering support

Major LPM material suppliers include Henkel (Technomelt), Bostik, and specialty compounders who can provide application-specific formulations.

Frequently Asked Questions About IPC-7621

What is the difference between LPM encapsulation and traditional potting?

Traditional potting uses thermoset materials (epoxy, silicone, urethane) that cure through chemical reaction, often requiring minutes to hours. The potting vessel becomes part of the finished assembly. LPM uses thermoplastic materials injected into reusable molds at low pressure, solidifying in seconds through cooling rather than chemical cure. LPM typically produces lighter parts with faster cycle times and the possibility of rework, while potting may offer superior chemical resistance and void-free encapsulation for certain applications.

What materials are covered by IPC-7621?

IPC-7621 primarily addresses thermoplastic materials used in low pressure molding, with polyamide (PA) being the most common. The standard also covers polyolefin, polyester, and specialty blends. These materials are selected for their melt flow characteristics, adhesion properties, flexibility, and compatibility with electronics assembly. Material selection must consider the end-use environment including temperature range, chemical exposure, and mechanical requirements.

How does IPC-7621 define acceptable vs. rejectable defects?

IPC-7621 provides visual acceptability criteria for common LPM defects including short-shots, flashing, bubbles, voids, and surface imperfections. The standard distinguishes between defects that compromise function (generally rejectable) and cosmetic issues that may be acceptable depending on application. Critical defects like exposed conductors or compromised seals are unacceptable regardless of application. The standard allows for application-specific criteria to be agreed between user and supplier.

What pressure range qualifies as “low pressure” molding?

IPC-7621 addresses molding processes typically operating between 1.5 and 40 bar (approximately 20-600 psi), which is significantly lower than traditional injection molding that operates at hundreds of bar. This low pressure is essential for electronics encapsulation because higher pressures would damage sensitive components, displace wires, or stress solder joints. The specific pressure used depends on material viscosity, part geometry, and component sensitivity.

Can LPM encapsulated assemblies be reworked or repaired?

Unlike thermoset potting which is essentially permanent, thermoplastic LPM encapsulation can theoretically be removed by re-melting, allowing limited rework capability. However, IPC-7621 acknowledges that rework is not always practical—the process may damage components or affect their reliability. Reworkability should be evaluated on a case-by-case basis considering the specific material, assembly design, and quality requirements. For most applications, LPM parts are treated as non-reworkable once encapsulated.

Implementing IPC-7621 in Your Organization

Adopting LPM encapsulation requires coordination between design, manufacturing, and quality functions. IPC-7621 provides the framework, but successful implementation depends on proper planning.

Start with design. Involve encapsulation requirements early in product development. Flow holes, component clearances, and connector positioning are much easier to address during initial design than through redesign later.

Select materials carefully. Work with material suppliers who understand electronics encapsulation. Request samples and conduct compatibility testing with your specific components and substrates before committing to production.

Develop your process systematically. LPM equipment suppliers typically offer application engineering support. Take advantage of their experience to establish baseline parameters, then optimize for your specific application.

Establish clear acceptance criteria. Use IPC-7621 as your foundation, but document any application-specific requirements. Ensure all parties—design, manufacturing, quality, and any external suppliers—agree on what constitutes acceptable quality before production begins.

Monitor and control. Like any manufacturing process, LPM requires ongoing monitoring to maintain consistency. Establish process windows, track key variables, and respond quickly when parameters drift outside acceptable ranges.

LPM offers real advantages for electronics protection, but realizing those advantages requires understanding the process, selecting appropriate materials, and maintaining process control. IPC-7621 gives you the guidance to do it right.

This article provides an overview of IPC-7621 principles. For complete guidelines, material specifications, and visual acceptability criteria, purchase the standard directly from IPC at www.ipc.org.