Before we go further, we must understand what spare components are and why they are required.
Spare/Overage/Scrap component:
Spare/Overage/Scrap Components are extra components purchased beyond the actual quantity required for PCB assembly. For example, if a component with MPN (manufacturer part number): ABCDEFDXXYZ includes designators C1, C2, and C3. means each board requires 3 pieces of this component, an assembly order for 15 boards would necessitate 15 x 3 = 45 pieces of MPN: ABCDEFDXXYZ - Refer Fig.2a. However, it’s crucial to purchase an additional quantity beyond this 45 to account for potential issues, such as defects during assembly, handling losses, or future repairs. This additional quantity is what is referred to as Spare/Overage/Scrap components.
Fig.2a: Represent of actual Qty required of MPN: ABCDEFDXXYZ
But why is this extra quantity necessary, and how much additional stock should be purchased? The answer depends on various factors, including the size and criticality of the components, historical failure rates, and specific operational considerations.
In this blog, we will explore these factors in detail, discussing general strategies for determining how much extra to order. While the specific requirements may vary by plant and operational context, our aim is to provide a foundational approach to managing spare components effectively, ultimately helping to reduce downtime and maintain product reliability.
Introduction:
Spare components help maintain product reliability over time by providing replacements for parts that may fail or degrade. Having spares on hand can reduce downtime during repairs or maintenance, which is especially important in critical applications.
Why is Spare/Overage/Scrap Quantity Necessary:
When assembling PCBs, purchasing extra components beyond the exact number required is essential for several reasons:
1.Manufacturing Variability
During the manufacturing and assembly process, some components may be damaged, defective and loss in machine. Having extra components ensures that you have replacements on hand to avoid delays or interruptions. Moreover, Human or machine errors during assembly might lead to the use of extra components or the need for replacements. Spare components help avoid these issues.
2.Quality Assurance and Testing
Some components are failed while testing or be consumed in the process. Extra quantities allow to test component without risking shortages. In the prototyping phase, additional components are often used to test different configurations and ensure that the final design meets specifications.
3.Operational and Supply Chain Risks
Components can have unpredictable lead times. Extra quantities help to cover potential delays in the supply chain, ensuring that production schedules are not affected. Suppliers may occasionally face issues that delay deliveries. Having spare components mitigates the risk of supply chain disruptions impacting your operations.
4.Supporting Long-Term Maintenance and Repairs
For products with long lifecycles, spare components are essential for repairs and maintenance. They ensure that you can provide ongoing support even as the original components become obsolete. Availability of spare parts is crucial for maintaining customer satisfaction by allowing for quick repairs and minimizing downtime.
5.Compensating for Handling and Inventory Losses
Components can be damaged or lost during handling and storage. Extra quantities help to cover these losses and maintain adequate inventory levels. Discrepancies between recorded and actual inventory can occur. Spare components help ensure that you have enough stock to meet production needs despite these inaccuracies.
Spare, overage, or scrap components are essential for managing the complexities and uncertainties of PCB assembly and manufacturing. They provide a safety net against defects, supply chain disruptions, and future maintenance needs. By incorporating extra components into your strategy, you ensure smoother operations, higher reliability, and better support for your products throughout their lifecycle.
Strategic to Purchasing Spare/Overage/Scrap Components:
1. Identify which components are crucial to the functionality of your PCB. Components with high failure rates or critical functions typically require more spares.
2. Analyse historical data on component usage and failure rates. Components that frequently fail or are subject to high stress during operation may need a larger surplus.
3. Review past data on component failures and replacements. This can provide insights into how often components are likely to be damaged or defective.
4. Use reliability metrics such as Mean Time Between Failures (MTBF) to estimate the likelihood of component failure and determine the appropriate quantity of spares.
5. Evaluate the lead times for ordering components. Components with long lead times may require a larger stock of spares to prevent production delays.
6. Weigh the cost of purchasing additional components against the potential cost of downtime or production halts. Sometimes, investing in extra spares can be more cost-effective than dealing with delays.
7. Allocate a budget for spare components based on the criticality and cost of the components. Ensure that your investment in spares aligns with your overall financial strategy.
There are component Size and cost play important roles in deciding the scrap or Overage component during PCB assembly.
Below explanation of size effects on scrap rates and coverage decisions during surface-mount technology (SMT) assembly and through-hole component soldering, considering component size, quantity, and feeder setup losses.
Small Chip Components (e.g., 0402, 0201 packages) Component Size: <2mm x 2mm:
Loss Rates:
Small quantities: 10% to 50%
Production quantities: 5% to 30%
Reason: Smaller components are more prone to handling errors and losses, especially during setup in tape & reel or cut tape. Loss rates decrease in larger production runs due to optimized processes.
Medium-Sized Components (e.g., SOT, 1212 packages, or components ≥2mm x 2mm):
Loss Rates:
Small quantities: 5% to 25%
Production quantities: 2% to 15%
Reason: Medium-sized components are easier to handle compared to smaller chip components, but losses can still occur during setup, particularly in smaller quantities. Production runs typically see reduced losses due to process efficiency.
Large Components (e.g., ICs ≥4mm x 4mm or larger):
Loss Rates:
Small quantities: 2% to 5%
Production quantities: 1% to 3%
Reason: Larger components are much easier to set up and handle in feeders, resulting in lower losses overall, both in small and production quantities.
Through-Hole Components:
Loss Rates: 0.5% to 5%
Reason: Through-hole components are typically handled manually, leading to minimal losses. These components are also used less frequently in modern designs, further reducing the impact of losses.
The above understanding is for general consideration only and does not guarantee the avoidance of assembly downtime. For accurate scrap/overage requirements, you must implement all critical aspects specific to your project. This is highly dependent on the production line and the machines used. You need the specifications of the machines before considering the overage/scrap.
Due to the rate of scrap used by the assembler, you may receive quotes with significant differences in component costs at times
Beyond size considerations, when dealing with smaller quantities, assemblers may take component costs into account during prototyping to determine scrap rates. Cost is a key factor that impacts budgets, so the percentage of actual scrap may be reduced when the budget is critical.
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