IPC-2225: Complete Guide to MCM-L & Bare Die Assembly

When you move from packaged components to bare die assembly, everything changes. Standard SMT processes no longer apply. Your substrate becomes the package. Wire bonding pads need specific metallization, die attach requires thermal analysis, and encapsulation becomes critical for reliability. Without proper guidance, bare die projects turn into expensive learning experiences.

IPC-2225 provides that guidance. Officially titled “Sectional Design Standard for Organic Multichip Modules (MCM-L) and MCM-L Assemblies,” this standard establishes the design requirements for mounting bare semiconductor dice directly onto organic laminate substrates. Whether you’re designing single chip modules, multichip assemblies, or chip-on-board applications, IPC-2225 covers the thermal, electrical, electromechanical, and mechanical considerations that determine success or failure.

What Is IPC-2225?

IPC-2225 is a sectional design standard within the IPC-2220 family that addresses the unique requirements of organic multichip modules. Unlike traditional PCB design where packaged components handle their own interconnection challenges, MCM-L designs place bare semiconductor dice directly on laminate substrates—requiring the designer to manage everything the package normally handles.

The standard covers Single Chip Modules (SCM-L), Multichip Modules (MCM), and MCM-L assemblies where the “L” designates laminate (organic) substrate technology.

IPC-2225 Standard Overview

Key Concepts in IPC-2225

Understanding MCM Technology Types

Before diving into IPC-2225 specifics, understanding where MCM-L fits in the broader multichip module landscape helps contextualize the standard’s requirements.

MCM Technology Comparison

MCM-L offers the lowest cost entry point for bare die assembly because it leverages existing PCB manufacturing infrastructure. The organic substrate uses familiar materials—FR-4, polyimide, BT resin—processed with standard lamination and photolithography equipment.

MCM-L vs Chip-on-Board (COB)

IPC-2225 addresses both MCM-L and what’s commonly called Chip-on-Board (COB) technology. The distinction is subtle but important.

IPC-2225 uses the term Application Specific Electronic Module (ASEM) for MCM-L devices, distinguishing them from complete application assemblies.

How IPC-2225 Works with IPC-2221

Like other sectional standards in the IPC-2220 family, IPC-2225 doesn’t stand alone. It works in conjunction with IPC-2221, the generic design standard that establishes baseline requirements for all printed board designs.

IPC-2220 Family Hierarchy

When designing an MCM-L assembly, start with IPC-2221 for generic requirements (conductor spacing, material selection, performance classes), then apply IPC-2225 for bare die-specific guidance (wire bond pads, die attach, encapsulation).

Die Attach Methods per IPC-2225

How you attach bare dice to the substrate fundamentally affects thermal performance, reliability, and rework capability. IPC-2225 addresses multiple attachment methods.

Die Attach Technology Comparison

Die Attach Material Selection Factors

For most MCM-L applications, silver-filled epoxy provides the best balance of thermal performance, process compatibility, and cost. The organic substrate’s glass transition temperature (Tg) typically limits attachment to adhesive or low-temperature solder processes.

Die Mounting Pad Requirements

Wire Bonding Design Requirements

Wire bonding remains the dominant interconnection method for MCM-L assemblies. IPC-2225 establishes design rules for successful wire bond attachment.

Wire Bond Pad Specifications

Wire Bond Layout Guidelines

Wire Types for MCM-L

Gold wire on gold-plated pads remains the industry standard for MCM-L wire bonding. The gold-to-gold interface forms reliable intermetallic compounds during thermosonic bonding at 150-200°C.

Flip-Chip Design Considerations

For higher I/O density and better electrical performance, flip-chip attachment eliminates wire bonds entirely. IPC-2225 addresses flip-chip design considerations for organic substrates.

Flip-Chip vs Wire Bond Comparison

Flip-Chip Bump Requirements

Underfill Requirements

Underfill is non-negotiable for flip-chip on organic substrates. The CTE mismatch between silicon die (~3 ppm/°C) and organic laminate (~15-18 ppm/°C) would destroy solder joints without underfill’s stress redistribution.

Substrate Design for MCM-L

The organic substrate in MCM-L serves as both interconnect platform and package. IPC-2225 addresses substrate-specific design requirements.

MCM-L Substrate Material Options

Surface Finish Selection for Bare Die Assembly

For MCM-L assemblies requiring wire bonding, ENIG (Electroless Nickel Immersion Gold) or electrolytic soft gold are the only practical choices. The gold thickness must meet wire bonding requirements (≥0.5 μm) while remaining soft enough for reliable ball bond formation.

Layer Stackup Considerations

Read more IPC Standards:

MIL-PRF-38534: Hybrid Microcircuits Military Specification Guide for Engineers

MIL-PRF-31032: The Definitive Guide to Military-Grade PCB Requirements

MIL-PRF-19500: Military Semiconductor Device Specification Explained

MIL-PRF-123: High-Reliability Ceramic Capacitor Military Standard

MIL-I-46058CMIL-I-46058C: Military Conformal Coating Specification Complete Guide

MIL-HDBK-263: ESD Control Handbook & Implementation Guide

J-STD-075 Guide: PSL Classification for Passive Components & Assembly Process Limits

J-STD-035 Guide: Acoustic Microscopy for IC Package Inspection & Defect Detection

J-STD-033 Guide: MSD Handling, Floor Life, Baking & Dry Storage Requirements

J-STD-032 Explained: Ball Grid Array Ball Construction & Performance Specs

J-STD-027 Guide: Mechanical Outline Standard for Flip Chip & CSP Packages

J-STD-020 Guide: MSL Classification, Reflow Profiles & Moisture Sensitivity Testing

IPC/WHMA-A-620 Explained: Complete Guide to Cable & Wire Harness Standards

IPC-SG-141: Complete Guide to S-Glass Fabric Specifications for PCB Laminates

IPC-QF-143: Complete Guide to Quartz Fabric Specifications for RF/Microwave PCB Laminates

Thermal Management in MCM-L Design

Without the thermal mass of traditional packages, MCM-L assemblies require careful thermal design. Bare dice dissipate heat directly into the substrate.

Thermal Via Design

Thermal Resistance Comparison

For power devices, cavity-down mounting (die facing a metal heatsink through a cutout in the substrate) provides the best thermal performance, as referenced in IPC-2225 design guidance.

Encapsulation and Protection

Bare dice require protection from mechanical damage, moisture, and contamination. IPC-2225 addresses encapsulation requirements for MCM-L reliability.

Encapsulation Methods

Glob Top Material Properties

IPC-2225 Current Status and Modern Context

Here’s something important: IPC-2225 is no longer actively maintained by IPC. The standard was published in 1998 and hasn’t been revised since. This doesn’t mean it’s irrelevant—the fundamental principles still apply—but the semiconductor industry has evolved significantly.

Why IPC-2225 Matters Today

Modern advanced packaging—chiplets, system-in-package (SiP), fan-out wafer-level packaging—builds directly on MCM-L concepts. AMD’s Ryzen processors, Apple’s M-series chips, and high-bandwidth memory (HBM) stacks all use multichip module principles that trace back to IPC-2225 foundations.

Related Modern Standards

Tools and Resources for IPC-2225

Official Documentation

Related IPC Standards

Industry Resources

Frequently Asked Questions About IPC-2225

Is IPC-2225 still valid if it’s no longer maintained?

Yes, the technical content remains valid for MCM-L design fundamentals. However, for cutting-edge applications, supplement IPC-2225 with IPC-2226 (HDI design), IPC-7094 (flip-chip), and current JEDEC standards. The core principles—die attach, wire bonding, thermal management—haven’t changed, but feature sizes and material options have expanded significantly since 1998.

What’s the difference between MCM-L and standard PCB assembly?

In standard PCB assembly, components come in packages that handle interconnection (leads, balls, pads) and protection (molding, encapsulation). In MCM-L, bare semiconductor dice mount directly to the substrate—the substrate becomes the package. This requires wire bonding or flip-chip for interconnection, careful thermal design, and encapsulation that standard PCB assembly doesn’t need.

Do I need IPC-2225 for chip-on-board (COB) LED assemblies?

IPC-2225 concepts apply, though LED-specific guidance exists elsewhere. COB LEDs use bare die on organic substrates—exactly what IPC-2225 addresses. The die attach, wire bonding, and encapsulation guidance translates directly. However, LED thermal and optical requirements may need additional references beyond IPC-2225.

What surface finish should I specify for wire bonding on MCM-L?

ENIG (Electroless Nickel Immersion Gold) or electrolytic soft gold are the standard choices. Gold thickness should be 0.5-1.25 μm for reliable thermosonic ball bonding. The underlying nickel barrier (3-5 μm) prevents copper diffusion into the gold. ENEPIG adds a palladium layer that improves both wire bonding and solder joint reliability if the substrate has mixed assembly requirements.

How does IPC-2225 relate to modern chiplet and advanced packaging?

Modern chiplet architectures—AMD’s Zen processors, Intel’s Foveros, Apple’s M-series—are essentially advanced MCM designs. They use the same fundamental principles IPC-2225 established: bare dice on interconnect substrates with wire bonding or flip-chip attachment. The difference is scale (finer features), complexity (heterogeneous integration), and substrate technology (silicon interposers alongside organic substrates). IPC-2225 provides the foundational understanding; modern designs layer additional complexity on top.

Designing Successful MCM-L Assemblies

IPC-2225 documents the accumulated knowledge of bare die assembly on organic substrates—knowledge that remains relevant even as the standard itself ages. The principles of die attach thermal management, wire bond pad design, flip-chip requirements, and encapsulation selection don’t change just because feature sizes shrink.

For engineers entering bare die assembly, IPC-2225 combined with IPC-2221 provides the foundation. Start with proper die attach selection based on thermal requirements. Design wire bond pads with correct metallization and spacing. Plan thermal via arrays under power devices. Specify appropriate encapsulation for your reliability environment.

The MCM-L concepts in IPC-2225 have become mainstream in ways the 1998 authors might not have anticipated. Every advanced processor, every system-in-package, every heterogeneous integration project builds on multichip module principles. Understanding IPC-2225 isn’t just about legacy technology—it’s about understanding the foundation of modern advanced packaging.