Uploading a Gerber ZIP to JLCPCB feels reassuring: the site parses the files, renders a preview of your board, and shows layer information before you pay. That preview is genuinely useful — and genuinely limited. It shows you that the files can be read and roughly what they contain; it is not a substitute for inspecting the set yourself, layer by layer, with measurements, in a viewer that answers only to you. Gerbv is that offline check: a free, open-source viewer for Gerber RS-274X and Excellon drill files that opens the exact ZIP contents you are about to upload. This guide explains what the upload preview does and does not tell you, the causes behind the problems users most often discover after uploading, and the pre-check workflow that resolves them beforehand. The viewer basics are in how to open and view Gerber files, and Gerbv is on the download page.
What JLCPCB’s viewer shows — and what it cannot
The upload preview’s job is confirmation at a glance: the files parsed, the board’s outline was found, the layers were identified, and the composite looks like a board. Treat it as a receipt, not an inspection. A composite preview flattens exactly the relationships you most need to judge — whether mask openings sit centred on pads, whether the drill pattern truly registers, whether the silkscreen avoids the pads — because in a rendered stack those relationships are hard to read at small scale, and some are only visible with layers isolated or measured. The preview also cannot know your intent: it does not know the board should be a particular size, that a connector pitch is critical, or that the silkscreen text is the wrong revision. Those judgements need the design knowledge only you bring, applied to the files themselves, offline, where every layer can be toggled, recoloured and measured.
The causes behind most upload problems
Problems discovered at or after upload tend to come from a short list of file-level causes, all of them visible in Gerbv beforehand. A missing or ambiguous outline: the board edge is absent, drawn on a layer the importer does not treat as the outline, or left open — the upload then guesses at the board’s shape or flags the set, and any guess risks the wrong edge. Drill mismatch: the drill file’s origin, units or format disagree with the Gerber files, so holes sit offset or at the wrong scale relative to the artwork the preview composites so reassuringly. Layer naming the importer cannot map: files whose names do not make their layer identity clear can be assigned to the wrong layer or left unassigned — a top mask treated as something else produces a board built wrong with complete confidence. Mixed revisions: a set whose files come from different exports, so one layer describes a board the others have already moved on from. None of these requires statistics to take seriously; each is a known, recurring way a Gerber set fails, and each is a five-minute check in a viewer.
The Gerbv pre-check workflow
Extract the exact ZIP you will upload, into its own folder, and open it in Gerbv. Work the set in this order. Inventory: count the layers against your board and confirm each file’s identity from its content, not only its name — if you cannot tell which file is the top solder mask, neither can an importer; rename clearly in your EDA export if needed and re-export. Outline: display the outline layer alone, confirm it is a single closed shape, and measure its extents against the design. Stack and registration: build the layer order from loading and aligning Gerber layers, then overlay the drill file as in viewing drill files and confirm every hit is centred in its pad across both copper sides. Details: silkscreen clear of pads and legible, mask openings where they belong, nothing drawn outside the outline, and the tightest clearances measured rather than assumed. Discipline: if anything fails, fix it in the EDA tool, export a complete fresh set, and check that set — then upload the checked archive itself, not a re-zipped lookalike.
After the check: uploading with the evidence in hand
Upload the verified ZIP, and use the site’s preview for what it is good at: confirming that what arrived matches what you inspected — same outline, same layer count, same overall appearance. If the preview disagrees with your Gerbv check, stop and reconcile before paying; a disagreement means one of the two is looking at different files, and it is usually the upload that picked up the wrong archive. When everything agrees, order with the checked set kept alongside the project as the release record. Ready to order? JLCPCB — referral link: this site may receive coupons when new users register and complete a first order; coupons offset order amounts, cannot be redeemed for cash, and using the link never changes your price (see our Affiliate Disclosure). For the complete gate, the PCB pre-order checklist covers all twelve checks this workflow draws on.
Frequently asked questions
If JLCPCB shows a preview, why check in Gerbv first?
The preview confirms the files parse and renders a composite; it does not isolate layers, measure dimensions, or know your design intent. The failures that matter — drill mismatch, a wrong or missing outline, unclear layer identity, mixed revisions — are far easier to see and prove in an offline viewer before uploading.
What is the single most important check before a JLCPCB upload?
The outline: display it alone in Gerbv, confirm one closed shape, and measure it. A surprising share of upload problems trace back to the board edge being absent, open, or on a layer the importer does not recognise as the outline.
My drill holes look fine in the upload preview but wrong in Gerbv. Which do I trust?
Trust the files, examined closely: overlay the drill layer on each copper layer in Gerbv at magnification. A composite preview can mask a small consistent offset that is obvious when holes and pads are inspected directly — and the factory works from the files, not the preview.