Introduction
The bill of materials (BOM) is one of the most critical documents in manufacturing and production planning. It provides a detailed parts list for a product along with important data needed to procure, build, and service that product. However, not all BOMs are the same. The level of detail required in a BOM depends greatly on the stage and scale of production.
In this article, we examine the key differences in BOM planning for prototype, low-volume, medium-volume, and high-volume production scenarios. We'll explore how the BOM evolves from basic part lists to comprehensive manufacturing plans aligned to the production strategy. With an understanding of BOM requirements for different volumes, you can develop robust BOMs tailored to your specific production needs.
Prototype and Low Volume BOMs
In the early prototyping and low volume production stages, the BOM can be fairly simple. The focus is identifying all the parts needed to build a functioning prototype or initial production run. Detailed manufacturability data isn't critical.
Prototype BOM
For building a prototype, the BOM contains just the minimal information needed to gather parts and assemble the design. This typically includes:
- Part numbers - Identifies exact parts to purchase
- Part descriptions - High level overview of the part
- Quantity needed - How many of each part to obtain
- Suppliers - Where to source the parts
- Reference designators - Marks part installation locations
A prototype BOM example:
| Part Number | Description | Quantity | Supplier | Reference Designator |
|---|---|---|---|---|
| 4N28-M5X1 | Optocoupler | 3 | Digi-Key | U1, U2, U3 |
| LM324PWRG4 | Op-amp IC | 1 | Mouser | U4 |
| PRTR5V0U2X | 5V regulator | 1 | Mouser | U5 |
This provides just enough data to buy the parts to build the first version of a design. Detailed specifications, pricing, manufacturability notes, etc. are not included.
The goal is to get working prototypes built quickly. The BOM only needs to facilitate sourcing of the necessary components.
Low Volume Production BOM
For low volume production of less than a few hundred units, the BOM can still be fairly basic. It expands on the prototype version with some additional manufacturing-related data:
- Target quantity - Desired production volume
- Cost estimates - Approximate part costs
- Lead times - Procurement time estimates
- Basic lifecycle data - Internal revision codes
- Mount types - Surface mount, through-hole, etc.
Here is an example low volume production BOM:
| Part Number | Description | Target Quantity | Unit Cost | Lead Time | Rev | Mount Type |
|---|---|---|---|---|---|---|
| 4N28-M5X1 | Optocoupler | 500 | $1.25 | 2 weeks | Rev A | Surface Mount |
| LM324PWRG4 | Op-amp IC | 250 | $0.40 | 4 weeks | Rev A | Surface Mount |
| PRTR5V0U2X | 5V regulator | 100 | $0.75 | 6 weeks | Rev A | Through-hole |
The additional data provides estimates for procurement planning and identifying any long lead time parts. This helps guide purchasing decisions for the production run.
At low volumes, detailed manufacturability analysis isn't essential. The focus is sourcing just enough parts to build the required quantity.
Medium Volume BOMs
As production quantities grow into the hundreds or thousands of units, the BOM requirements increase. More details are needed for manufacturability analysis, cost reduction, and production planning.
Medium volume BOMs expand on basic part list data with:
- Lifecycle data - Revision histories, ECOs, etc.
- Supplier details - Preferred sources, minimum order quantities
- Lead times - Accurate procurement times
- Pricing - Negotiated costs and MOQ discounts
- Test specifications - Required tests for components
- Packaging data - Reel sizes, tube counts, etc.
- Manufacturing notes - Assembly requirements
Additional columns can provide this expanded dataset. Here is an example medium volume BOM:
| Part Number | Description | Target Qty | Unit Cost | Lead Time | Rev | Lifecycle | Package | Suppliers | MOQ | Assembly Notes |
|---|---|---|---|---|---|---|---|---|---|---|
| 4N28-M5X1 | Optocoupler | 5000 | $1.10 | 12 wks | Rev B | Active | Reel | Digi-Key | 2500 | Reflow profile 4 |
| LM324PWRG4 | Op-amp IC | 3000 | $0.35 | 4 wks | Rev C | Active | Tube | Mouser | 30 | Hand solder |
| PRTR5V0U2X | 5V regulator | 1000 | $0.60 | 10 wks | Rev C | Active | Reel | Mouser | 1000 | Reflow profile 2 |
The increased detail helps with production planning, cost analysis, and identifying any component-specific manufacturing requirements. This data is needed for seamless ramp up to medium volume production.
High Volume Production BOM
For high volume manufacturing of thousands to millions of units, the BOM becomes much more robust. It serves as a comprehensive manufacturing plan for the product, guiding procurement, test, assembly, rework, and more.
Typical high volume production BOM contents include:
- Approved manufacturer list (AML) - Qualified component sources
- Lifecycle data - Revision histories, discontinuances
- Lead times - Accurate procurement times from AML
- Pricing - Negotiated costs from AML suppliers
- Minimum order quantity (MOQ) - Negotiated minimum order sizes
- Unit of measure (UOM) - Purchase units (reel, tube, tray, etc.)
- Component package data - All relevant package dimensions and specifications
- Assembly and test requirements - Detailed notes for manufacturing
- Approved manufacturer procedures (AMP) - Statistical process control data
Additional columns in the BOM provide space for this expanded dataset required for large-scale production.
Here is an abbreviated example of a high volume BOM format:
| Part Number | Description | Annual Qty | Unit Cost | Lead Time | Rev | Lifecycle | Package | AML Suppliers | MOQ | UOM | Assembly Notes | Test Procedures | AMP |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 4N28-M5X1 | Optocoupler | 500K | $0.95 | 10 wks | Rev C | Active | Reel | CSH,Arrow | 2500 | Reel | Reflow profile 4 | Mil- STD-750 | Yes |
| LM324PWRG4 | Op-amp IC | 100K | $0.25 | 4 wks | Rev B | Active | Tube | TTInc,Mouser | 30 | Tube | Hand solder | JESD57 | Yes |
| PRTR5V0U2X | 5V regulator | 1M | $0.50 | 8 wks | Rev C | Active | Reel | Mouser | 1000 | Reel | Reflow profile 2 | Mil-STD-202 | Yes |
The comprehensive dataset ensures a smooth ramp to high volume production. The BOM acts as the manufacturing plan guiding procurement, assembly, test, and all other production activities.
BOM Planning Best Practices
When planning BOMs at different production volumes, keep these best practices in mind:
- Start basic - Minimal data needed for prototypes and low volumes
- Add details as volumes increase - Expand datasets to meet production requirements
- Align to production processes - Tailor BOM data to specific assembly lines
- Include critical manufacturability data - Assembly, test, packaging, and material info
- Utilize complete lifecycle fields - Rev histories, ECOs, discontinuances
- Standardize formats - Consistent structure across products
- Integrate with ERP/MRP - Smooth data flows to planning systems
- Make BOM easily accessible - User friendly interface for production personnel
With careful planning aligned to production volumes, your BOM will drive efficiency across the manufacturing process.
Frequently Asked Questions
What key details are needed in a low volume production BOM vs. a high volume BOM?
For low volumes, minimal data like basic part descriptors, quantities, and sources is adequate. High volumes require much deeper information including lead times, packaging, approved suppliers, assembly/test notes, SPC data, and full lifecycle details.
When should I engage with suppliers for negotiated pricing?
Initial prototype and low volume BOMs use estimate or list pricing. Once moving to thousands of units, supplier engagement for formal quotes and negotiated pricing discounts is advised. This significantly reduces costs.
What are some key tests that may be specified in a high volume production BOM?
Common tests specified include:
- Solderability testing like JESD22-B102
- Environmental stress testing like MIL-STD-202
- Temperature cycling, autoclave, and HAST
- Die shear and pull force measurements
- Functional testing to device specifications
How often should a BOM be updated for an active product?
For active high volume products, the BOM should be updated continually as part of ongoing lifecycle management. As components change over time, the BOM must be kept current to reflect the as-built configuration.
What are approved manufacturer lists and procedures?
An approved manufacturer list (AML) specifies qualified suppliers. Approved manufacturer procedures (AMP) provide supplier-specific process control data for statistical process control. This ensures quality for high volume production.
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