Standards & Compliance

Cross-Section Analysis

Q: How thick must the cross-section sample be ground before final polish?

IPC-TM-650 method 2.1.1 requires that grinding stop within 25 microns of the target plane (the center of the hole or feature of interest) before fine polishing begins. Coarse grinding typically uses 240 to 320 grit SiC paper to remove the bulk of the encapsulant and reach roughly 0.1 mm short of target, followed by 600 and 1200 grit. Final polishing uses 6 micron, 3 micron, and 1 micron diamond or alumina suspensions on napped cloth. Over-grinding past the centerline distorts plating thickness readings because a hole barrel measured off-center reads thicker than the true wall, so technicians watch the hole diameter approach its maximum as the indicator that the plane is centered.

Q: What plated through-hole copper thickness does IPC Class 3 require?

For IPC-6012 Class 3 (high-reliability) rigid boards, the minimum average copper wall thickness in a plated through-hole is 25 microns (0.001 inch) with a thin-area minimum of 20 microns, measured on a cross-section. Class 2 allows 20 microns average. Surface conductor copper for Class 3 is typically 20 microns minimum after plating. These values are read directly from the polished microsection at 200x to 400x using a calibrated reticle or image-analysis software. Class 3 also limits wrap plating, requires at least 5 microns of wrap copper over the foil at the knee of the hole, and caps allowable etchback and negative etchback that the cross-section reveals.

Q: How is intermetallic compound thickness measured in a solder joint cross-section?

After polishing, the joint is lightly etched (for tin-lead, a 2 percent nital or a 5 percent HCl-alcohol etch) to reveal the Cu6Sn5 and Cu3Sn intermetallic layers at the copper-to-solder interface. At 1000x under oil immersion or on an SEM, the scalloped Cu6Sn5 layer is measured at several points and averaged. A healthy reflow joint shows 1 to 3 microns of intermetallic; aging or excess thermal exposure thickens it toward 5 microns or more, which embrittles the joint. A continuous Cu3Sn layer thicker than about 1.5 micron, often accompanied by Kirkendall voids, signals over-aging and is a reliability concern flagged during destructive physical analysis.

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A destructive metallographic inspection in which a coupon or assembly is potted in epoxy, ground, and polished to expose an internal plane, then examined under a microscope to verify hidden features that no external view can reach. The polished face reveals plated through-hole barrel wall thickness, solder joint fillet geometry, intermetallic layers, laminate registration, and void content, with each measurement compared against the acceptance limits in IPC-A-610 and IPC-6012. Because the sample is consumed, cross-sectioning is performed on production coupons or sacrificial units as part of destructive physical analysis, qualifying a build process rather than screening every shipped board.

Category: Standards & Compliance

Class 3 PTH Cu: ≥ 25 μm avg

Healthy IMC: 1 to 3 μm

How a Microsection Reveals Hidden Defects

Most plating and soldering defects live inside a feature where no surface inspection can find them. A barrel crack, a thin wall on one side of a plated hole, a resin-recession crack, or a starved fillet hidden behind a component body all sit below the surface. Cross-section analysis solves this by physically cutting through the feature of interest, mounting the cut sample in clear acrylic or epoxy, and progressively grinding and polishing until the inspection plane reaches the center of the feature. The result is a mirror-bright face that, under reflected-light microscopy at 100x to 400x, shows the true internal structure with micron-level measurement resolution.

The procedure is standardized in IPC-TM-650 method 2.1.1 for boards and method 2.1.1.2 for evaluating plated holes. A typical sequence is rough cut on a precision saw 2 to 3 mm away from the target, vacuum or pressure encapsulation to fill voids and prevent edge rounding, coarse grind through 240 and 320 grit silicon carbide, fine grind through 600 and 1200 grit, then polish with 6, 3, and 1 micron diamond. Stopping within 25 microns of the centerline is critical: a hole barrel measured off-center always reads thicker than the true wall, so a grinder watches the hole diameter widen to its maximum as the cue that the plane is centered.

Once polished, the sample is often etched to bring out features that a bright polish hides. A microetch separates copper grain structure, distinguishes electroless from electrolytic copper, and reveals the scalloped intermetallic compounds at a solder-to-copper interface. The same mount supports both optical microscopy and, where finer resolution is needed, scanning electron microscopy with energy-dispersive spectroscopy to confirm layer composition.

Plating and Intermetallic Measurements

Average barrel wall thickness:
t_avg = ( ∑ t_i ) / n (n ≥ 3 readings around the barrel)

Off-center magnification error (apparent vs. true wall):
t_apparent ≈ t_true / cos(θ), θ = angular offset from hole center

Intermetallic growth with aging:
x(t) ≈ x₀ + k × √t, k ≈ 0.02 to 0.1 μm / √hour at 100 °C

Where t_i = individual wall measurements, θ = how far the polished plane sits from true center, x(t) = Cu₆Sn₅ thickness, x₀ = as-reflowed thickness (≈ 1 μm), and t = thermal aging time. A reflow joint shows 1 to 3 μm intermetallic; beyond 5 μm the joint embrittles.

IPC Acceptance Limits Read from a Cross-Section

Measured Feature	Class 1	Class 2	Class 3	Why It Matters
PTH avg copper wall	≥ 20 μm	≥ 20 μm	≥ 25 μm	Thermal-cycle barrel fatigue
PTH thin-area copper	n/a	≥ 18 μm	≥ 20 μm	Local crack initiation site
Wrap plating at knee	≥ 5 μm	≥ 5 μm	≥ 5 μm	Foil-to-plating continuity
Solder fill (through-hole)	≥ 50%	≥ 75%	≥ 75%	Joint strength and conduction
Intermetallic (Cu₆Sn₅)	< 5 μm	< 5 μm	1 to 3 μm target	Joint embrittlement
BGA ball voiding	≤ 30%	≤ 25%	≤ 25%	Current density, thermal path

Common Questions

Frequently Asked Questions

How thick must the cross-section sample be ground before final polish?

IPC-TM-650 method 2.1.1 requires grinding to stop within 25 μm of the target plane before fine polishing. A typical sequence is 240 to 320 grit to reach about 0.1 mm short of target, then 600 and 1200 grit, then 6, 3, and 1 μm diamond. Watch the hole diameter widen to its maximum: that confirms the plane is centered, since an off-center barrel reads falsely thick.

What plated through-hole copper thickness does IPC Class 3 require?

IPC-6012 Class 3 requires ≥ 25 μm (0.001 in) average copper wall with a 20 μm thin-area minimum, read directly from the microsection at 200x to 400x. Class 2 allows 20 μm average. Class 3 also demands at least 5 μm of wrap copper over the foil at the knee and caps allowable etchback, all of which the polished section reveals.

How is intermetallic compound thickness measured in a solder joint cross-section?

After polishing, the interface is lightly etched (2% nital or 5% HCl-alcohol for tin-lead) to reveal the Cu₆Sn₅ and Cu₃Sn layers. At 1000x or on an SEM, the scalloped layer is measured at several points and averaged. A healthy reflow joint shows 1 to 3 μm; thickening toward 5 μm or a Cu₃Sn layer over 1.5 μm with Kirkendall voids signals over-aging and embrittlement.

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