Phantom BOM and phantom assembly: meaning, example and calculator

1. Phantom, stocked sub-assembly, purchased part or kit

In a multi-level BOM, each item that has children can be handled in several ways by the ERP. The chosen type does not change the structure or the quantities, but it changes the work orders and the stock movements:

2. The worked example: 10 carts

We take the cart from the multi-level BOM article: welded frame × 1, wheel assembly × 4, handle × 1; the wheel assembly contains one wheel, one plate and 4 M8 screws. We launch 10 carts. The frame and the handle stay ordinary sub-assemblies with their own orders; only the wheel assembly's handling changes.

Stocked wheel assembly: two orders and a trip through stock

Phantom wheel assembly: a single order

Component quantities are exactly the same: 40 wheels, 40 plates, 160 screws. What disappears is one work order, a stock receipt and a stock issue of 40 wheel assemblies, and an item in stock to count. In return, the shop can no longer build wheel assemblies ahead of time: they are built at the station, within the cart's order.

3. Work order calculator with phantoms

Paste your BOMs as links, one line per component: parent; child; quantity. Enter the product, the quantity to launch and the items to treat as phantoms: the calculator lists the resulting work orders and each one's pick list, as a requirements run with no stock would.

The calculation runs in your browser; nothing is sent. Clear the phantom field to see the orders without phantoms; total component quantities stay the same. For totals per item, use the BOM explosion calculator.

4. When to use a phantom item

• A sub-assembly built at the station and fitted straight away: it physically exists for a few minutes, on the line. Stocking it would cost an order and two stock movements for nothing.
• A grouping created for design: a CAD sub-assembly that helps modelling or reuse but matches no manufacturing step. Declaring it a phantom avoids taking it apart in the manufacturing BOM.
• A module shared by several products: a set of parts used in several BOMs. It is maintained in one place, and its where-used shows every product that uses it.

Conversely, keep a sub-assembly stocked when it is built in batches ahead of time, inspected separately, sold as a spare part or assembled in another shop: it then has a life of its own, with its routing and its stock.

5. In CAD and in the ERP

• CAD: Autodesk Inventor offers a "Phantom" BOM structure for a component or sub-assembly; in SOLIDWORKS, a configuration's "Promote" option brings a sub-assembly's children up into the parent's BOM. In both cases, the sub-assembly stays in the model but disappears from the BOM.
• SAP: the usual setting is special procurement type 50 (phantom assembly) in the item's MRP view; the item becomes a phantom in every BOM it appears in.
• Odoo: the "Kit" BOM type plays this role: its components are consumed directly by the parent's order, or picked together at delivery.
• Other ERP systems: there is a "phantom" flag on the item or on the BOM line. The principle is the same: requirements planning blows through the item instead of creating an order for it.

The CAD phantom and the ERP phantom are two separate decisions: the first concerns the engineering BOM, the second the manufacturing BOM. A sub-assembly visible in the eBOM can very well become a phantom only in the ERP; that is even the cleanest case, see BOM to ERP integration.

6. Pitfalls

1. A phantom with a routing: if building the assembly takes time, that time must appear in the parent's routing, or it vanishes from capacity planning and from the cost. See BOM and ERP.
2. A phantom with stock: a customer return or a surplus can create stock of a phantom item. Depending on the settings, the ERP consumes it first or ignores it; check which, or those parts stay forgotten.
3. A phantom sold as a spare part: if it must be sold on its own, it needs a real flow (order or purchase) for that demand; the phantom alone does not allow it.
4. Misread nested phantoms: a phantom can contain another one; everything goes up to the first non-phantom parent. The result is correct, but the pick list gets long: read it through once at the station.

7. FAQ

What is a phantom item in a BOM?

A sub-assembly that appears in the product structure but is neither stocked nor built on its own order: during requirements planning, the ERP blows through it and its components are consumed directly by the parent's work order. Also called phantom assembly or phantom BOM.

What is the difference between a phantom and a stocked sub-assembly?

A stocked sub-assembly has its own work order and goes through stock: it is built, stored, then picked by the parent's order. A phantom has neither order nor stock: its components go straight onto the parent order's pick list. Total component quantities are the same.

What is the difference between a phantom item and a kit?

"Kit" covers two things. A sales kit is a sold item whose components are picked and shipped together, with no assembly. A manufacturing kit is a grouping built on the line and never stocked: it is then a phantom. Some ERP systems, such as Odoo, actually call their phantom-type BOM a "kit".

How do you set up a phantom assembly in SAP?

The most common way is special procurement type 50 (phantom assembly) in the item's MRP view: the sub-assembly becomes a phantom in every BOM it appears in. The SAP documentation for your release details the other possible settings.

When should you use a phantom item?

When a group of parts must stay visible in the structure (for design, reuse across products or assembly documentation) without creating a work order or a stock movement: a sub-assembly built at the station and fitted straight away, or a grouping created for CAD.