C4 — Controlled Collapse Chip Connection — is a flip-chip interconnect made from solder balls, typically 60–100µm in diameter, that melt during reflow and self-align the die by surface tension as the molten metal collapses into place. It's the industry's mass-reflow workhorse for pitches above roughly 130µm, and SAC305 (tin-silver-copper) is the standard lead-free alloy used to form the joint.
What C4 stands for and how the bump works
C4 is IBM-era shorthand from the 1960s that stuck: Controlled Collapse Chip Connection. The "controlled collapse" part is the whole idea. Instead of wire-bonding pads around the die's perimeter, you put a solder ball on every pad, flip the die face-down onto the substrate, and run it through a reflow oven. The solder melts, and as it does, surface tension pulls the die into registration on its own — the molten ball wants to minimize its surface energy, and the path to that is centering itself on the pad it's wetting to. Placement tolerance going in is forgiving, because the physics finishes the alignment job that flip-chip placement equipment starts.
That self-alignment is also why C4 has decades of qualification behind it and remains the default choice for most flip-chip builds: it's mechanically simple, the equipment and materials are mature, and it's the cheaper, lower-risk answer whenever pitch and current don't force a different bump.
How big is a C4 bump — diameter and pitch
A C4 solder bump typically runs 60–100µm in diameter. Pitch — the center-to-center spacing between bumps — is where C4's range is set: above roughly 130µm pitch, C4 is the mass-reflow workhorse; below that, the bump has less room to collapse into without neighboring joints bridging, and copper pillar (C2) becomes the more common answer.
Heisler runs C4 in-house at production volume, including high-bump-count packages. The build data for that program — bump count, diameter, pitch, alloy, and yield — lives on the
flip-chip capability page, where it belongs as a single citable record rather than a figure repeated across the site.
Why SAC305 (and not lead solder)
SAC305 — tin, silver, copper, roughly 96.5/3/0.5 by weight — is the industry-standard lead-free alloy for C4 reflow, and has been since RoHS pushed lead out of the supply chain. It reflows at a higher temperature than the old eutectic tin-lead alloys, forms a mechanically sound joint, and is what nearly every C4 line in production runs today. There's no proprietary chemistry here — SAC305 is a materials-standard choice, not a differentiator.
C4 vs the other bump types
C4 isn't the only way to connect a flipped die. The other common answer is
copper pillar, also called
C2 in the industry — a plated copper post that holds the die at a fixed standoff, with only a thin solder cap doing the reflowing. C2 doesn't collapse the way C4 does, which is the whole tradeoff: you give up self-alignment and add a plating step, but you get a finer pitch (around 40µm pillar diameter, versus 60–100µm for a C4 ball) and meaningfully better current-carrying capacity per bump. Gold stud bumps are a third, less common option, generally reserved for specific thermosonic or fine-pitch applications outside the C4/C2 mainstream.
Which one is right for a given design comes down to pitch, current density, and underfill standoff — not which process a substrate vendor happens to already run. We cover that decision in full in
C4 vs Copper Pillar: Choosing the Flip-Chip Bump.
How a C4 joint gets verified
A reflowed C4 joint isn't self-evidently good just because the die stuck down in the right place. The checks that actually confirm a joint:
- **X-ray void analysis** — looks through the package at the joint itself, measuring void area against the joint footprint without cross-sectioning the part.
- **Cross-section** — a destructive physical cut through the joint that shows intermetallic formation, void location, and bond-line quality directly.
- **Die shear and ball shear** — mechanical pull/shear testing that confirms the joint holds the mechanical load it's supposed to.
These are the same checks whether the joint is on a qualification vehicle or a production lot — more on the full verification stack on the
flip-chip page.
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FAQ
**What does C4 stand for?**
Controlled Collapse Chip Connection — a solder-ball flip-chip interconnect that self-aligns as it melts and collapses during reflow.
**How big is a C4 bump?**
Typically 60–100µm in diameter. Heisler runs C4 in production on high-bump-count packages — build specifics are on the
flip-chip page.
**What solder alloy is used for C4 bumps?**
SAC305 (tin-silver-copper) — the lead-free industry standard.
**Is a C4 bump the same as a copper pillar (C2) bump?**
No. C4 collapses and self-aligns at reflow; copper pillar (C2) holds a fixed standoff and doesn't collapse. See
C4 vs Copper Pillar for the full decision framework.
**How do you verify a C4 joint is good?**
X-ray void analysis, cross-section, and die/ball shear testing — the same checks run on a production line also run on prototypes.
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Closing CTA (matches existing blog post CTA div pattern)
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