The popcorn effect: a part or circuit board still has moisture inside; when it enters the reflow oven and heats quickly, the moisture turns to steam and pushes outward, forcing the inside apart.
- ICs: the interfaces between molding compound and die or substrate delaminate; in severe cases the package bulges or cracks — which is where the name “popcorn” comes from.
- Circuit boards: the inner laminate layers separate, commonly called “delamination” or “measling / blistering.”
- Both start the same way: the moisture was already inside before the board entered the oven.
For what MSL levels are and what dry packing includes, see What is MSL? J-STD-033 packing, floor life and baking explained; for storage and baking after opening, see Storage and baking conditions for moisture-sensitive devices. This article starts from “after moist parts go into the oven”: what happens inside, how to confirm it really is moisture, and how to avoid it. Each stage covers what to do and how JIAHON can help; sources and exceptions are in the notes at the end.
Stage 1 | How the moisture gets in
Plastic-packaged ICs and circuit board laminates both absorb water from the air: the longer, damper and warmer the storage, the more they absorb. Absorbed moisture does not leave on its own; it has to be baked out.
| Item | Where the moisture comes from | What to control before reflow |
|---|---|---|
| Plastic-packaged ICs (BGA, QFP, QFN, etc.) | Molding compound absorbs water from the air; damaged moisture barrier bags or too long after opening | Floor life after opening by MSL level; moisture barrier bags with humidity indicator cards |
| Circuit boards | Laminate absorbs water, inner layers too; storage after shipping and time after unpacking both count | Pack for storage; decide before the line whether to bake |
| Connectors and other plastic parts | The plastic body also absorbs water | Storage and baking per the component maker’s documents |
How JIAHON can help:
- Blue six-spot humidity indicator cards: read the card when you open the bag; if it is over the limit, do not send the parts straight into the oven. Six spots show at once — blue means dry, pink means moist.
- Aluminum foil bags (antistatic): flat bags that can go straight on a heat sealer; please ask about the moisture barrier grade.
- Ultra-low humidity dry cabinets: for temporarily storing unused material; please ask about specifications and recovery time after the door is opened.
Stage 2 | What happens inside during those minutes in the oven
| IC | Circuit board | |
|---|---|---|
| Where it fails | Interfaces between molding compound and die, substrate or lead frame; in severe cases the package body | Between inner laminate layers, often at the prepreg-to-core interface |
| What it looks like | Bulging, cracks or bursting of the package; more often the outside looks normal and only the inside is delaminated | Bulges or blisters on the board surface; layer separation visible only in cross-section |
| What it damages | Wire bonds pulled, die attach lifted; electrical failure immediately or later | Layers separate, reducing the board’s heat resistance and reliability |
There are two explanations of the cause, but the practice on the floor is the same:
- Vapor pressure: moisture pressure rises sharply at high temperature and forces the inside apart. This is how J-STD-020 describes it.
- Loss of adhesion: a paper argues that interfaces separate mainly because of thermal stress and “reduced adhesion after moisture absorption,” not purely vapor pressure.
- Either way: keep moisture content low before reflow; do not ramp too steeply, and do not exceed the peak temperature the parts and laminate can withstand. Lead-free processes have higher peaks than tin-lead, so the risk is greater.
- Rework and second-side reflow are also a pass through the oven: by the same logic, confirm the board is not moist before it goes in.
How JIAHON can help: a reflow profiler measures the actual ramp and peak on the board, not just the oven settings; for how to read the results, see section 7 of How to attach thermocouples for reflow profiling.
Stage 3 | How to confirm it really is moisture, not something else
A normal appearance does not mean nothing is wrong: IC delamination often cannot be seen from outside. Check non-destructively first, then destructively.
| Step | IC | Circuit board |
|---|---|---|
| 1. Appearance | Check the package for bulges or cracks under a microscope; then electrical test | Look for bulges or blisters on the board surface |
| 2. Look inside without destroying | Scanning acoustic microscopy for interface delamination (the method J-STD-020 uses); X-ray for solder joints | Grouped comparison within one batch (below) |
| 3. Destructive | Cross-section to see which layer delaminated and whether it cracked through the package | Cross-section to see which layers separated |
Circuit boards: compare four groups from the same batch. Split the boards into four groups — packed or not packed, crossed with baked or not baked — and run each through one reflow. If only the unbaked groups delaminate and the baked groups pass, the cause is moisture absorption, not the reflow process. Also check the batch records: the same board design does not delaminate in every batch; it depends on how long and where that batch was stored.
ICs: a preconditioning failure is not necessarily popcorning. When failures appear “after moisture exposure,” do not assume the popcorn effect; check in order — X-ray first, then cross-section — because the cause may turn out to be something else, such as solder joint defects rather than package cracking.
How JIAHON can help: digital microscopes for checking packages, board surfaces and solder joints, with a built-in measuring system; please ask about models and magnification. Scanning acoustic microscopy, X-ray and cross-sectioning must be sent to a laboratory; JIAHON does not have this equipment.
Stage 4 | How to avoid it
| What to control | What to do | Read next |
|---|---|---|
| IC time after opening | Use within the floor life for its MSL level; reseal or put unused parts in a dry cabinet; bake if over time | Storage and baking conditions for MSDs |
| Circuit board storage | Moisture barrier packaging with desiccant and a humidity indicator card | PCBA moisture barrier packaging |
| Circuit boards before the line | If past the storage period or stored in poor conditions, decide whether to bake | PCB baking |
| Where things are stored | Put temporarily stored material in a dry cabinet, not on open racks | Dry cabinet vs. nitrogen cabinet |
| Reflow profile | Measure the actual board temperature with a profiler; check both ramp and peak | Stage 2 |
How JIAHON can help: the programmable oven JHSXH-480 has a nitrogen flowmeter, a quick-connect nitrogen inlet and multi-segment ramp profiles, with uses including dehumidifying pre-bake of semiconductor components; please ask about temperature range and exhaust specifications. Humidity indicator cards, foil bags and dry cabinets as in stage 1.
For humidity indicator cards, foil bags, dry cabinets, ovens or reflow profilers, please contact JIAHON.
Further reading: Tray vs. tape and reel · How much temperature and humidity control a plant needs · How to inspect BGA solder joints
Notes: sources and exceptions
The following is for readers who want to check; the steps above stand without it. For standard criteria, refer to the official text; where your adopted version, customer specification or component maker’s documents differ, follow them.
Where the two explanations come from
| Explanation | Content | Source |
|---|---|---|
| Vapor pressure | Absorbed moisture rises sharply in pressure at reflow temperature, causing internal delamination, internal cracks and damage to wire bonds and die attach; in severe cases the package bulges and an audible pop can be heard | J-STD-020, chapter 3 Background; the same is described for circuit boards (a CALCE paper on IPC’s technical resources) |
| Thermal stress and adhesion | Interface delamination is caused mainly by thermal stress and moisture-reduced adhesion, not vapor pressure; the paper’s introduction also notes that absorbed moisture has two effects: rapid vaporization and weakening of adhesion between molding compound and interfaces | A paper at the IPC Printed Circuits Expo / APEX 2004 (Binghamton University and others) |
| Factors common to both | Total moisture absorbed, peak temperature, preheat ramp rate, package size and molding compound, interface adhesion. In that paper’s tests, peak temperature had a clear effect, ramp rate less so | The 2004 paper above |
How J-STD-020 checks whether a package has popped — sections of revision F (see the official text for the criteria; this article does not restate them):
| Check | Section |
|---|---|
| Appearance and electrical test before and after reflow | 8.2, 8.3, 8.7, 8.8 |
| Scanning acoustic microscopy after reflow | 8.9 |
| Failure criteria after reflow simulation | 9.1 |
| Delamination needing further evaluation (lead frame and substrate types written separately) | 9.2.1 |
| Body warpage during assembly of moist substrate packages | 9.2.2 |
| Failure verification | 9.3 |
Circuit board notes
- Moisture content limit: IPC-1601 gives an acceptable moisture content limit of about 0.1%–0.5% (by weight, depending on design), measured per IPC-TM-650 2.6.28; the CALCE paper recommends not exceeding 0.1% for lead-free. See also PCBs Are Moisture-Sensitive Devices.
- Material, thickness, copper area: these affect how quickly boards absorb and dry. A 2004 IPC paper groups laminates in three: phenolic and polyimide types (for example CEM-1, CEM-3, flex) absorb water easily; epoxy types (FR-4 etc.) are in between; high-frequency laminates and boards with full copper planes are less affected. It also cites literature that thicker boards, or boards with outer copper planes, need longer and hotter baking.
- Lead-free: the CALCE paper notes that lead-free peaks are about 30–40°C higher than tin-lead, so moisture vapor pressure is higher.
- Cost of baking: for board baking conditions and the OSP exception, see PCB baking.
Standards and papers referenced (refer to the current version from each publisher):
- IPC/JEDEC J-STD-020F, Moisture/Reflow Sensitivity Classification for Nonhermetic Surface Mount Devices (table of contents) — IPC (Global Electronics Association), JEDEC
- IPC/JEDEC J-STD-033D, Handling, Packing, Shipping and Use of Moisture, Reflow, and Process Sensitive Devices (table of contents) — IPC, JEDEC
- Controlling Moisture in Printed Circuit Boards — IPC technical resource; authors at CALCE, University of Maryland
- Moisture and Reflow Sensitivity Evaluations of SMT Packages as a Function of Reflow Profile at Eutectic and Lead Free Temperatures — IPC Printed Circuits Expo / APEX Designers Summit 2004
- PCB Materials Behaviours towards Humidity and Impact of the Design, Finishes, Baking and Assembly Processes on Assembly Quality and Solder Joint Reliability — IPC Printed Circuits Expo / APEX Designers Summit 2004
This is a general overview, not a restatement of any standard or paper, and not an endorsement of your process. Follow the J-STD-020 and J-STD-033 versions you use, customer specifications, component data sheets and your own verification for handling moisture. This is the general case; individual cases vary.