Category Archives: Tips and Techniques

Two Approaches to Managing Alternate Parts


I have a part (actually multiple parts) that can be made by 2 different manufacturers: in-house and an outside source. Parts from both sources are functionally identical and treated as interchangeable in assemblies. What is the best way to handle this? Is it possible to have a part with multiple manufacturers? Could I create two parts and have Aligni treat them as interchangeable?
– H.D. at Overwatch Imaging


The short answer is to use Aligni’s Part Alternates feature. Here’s a bit more detail…

You’ll get the most out of Aligni if you maintain your database to be consistent with Aligni’s Manufacturer / Vendor relationships model. In summary:

  • Every item (part) has a unique Aligni P/N (SKU)
  • Each item has a single manufacturer
  • Each item’s manufacturer P/N is unique to that manufacturer
  • Each manufacturer has one or more vendors

In this post, we consider two distinct situations in the current case because the two situations justify different treatment within Aligni:

  1. Two commodity parts are manufactured by two different manufacturers that are functionally equivalent. An example of this would be electronic resistors.
  2. One part is specified but is manufactured by two different service providers. An example of this would be a machined part built-to-spec by a machine house.
Part Alternates are shown and managed on the details tab of the Part page. This part in our online demo database is a good example with three part alternates defined.

First Case: Different Manufacturers

In the first case, let’s consider two electronic resistors that are manufactured by two different manufacturers. In the electronics industry, these are commodity parts that are available from many sources. They are sometimes colloquially referred to as “jelly bean” parts and are often considered “generic”. However, on our Part Alternates documentation page, we caution against configuring these parts in Aligni as generic.

So per our suggestion, these two parts would get separate Aligni part records, each associated with its respective manufacturer. You would then associate the parts to each other using Aligni’s Part Alternates. You have two options for this association:

  • Equivalent – This means that two parts are suitable for any installation. They have identical parameters and differ only by manufacturer.
  • Alternate – This means that two parts are similar, but substitution requires consideration.

At this point, the choice becomes industry or application specific — your organization will need to decide to what level “functionally identical” may be interpreted. For example, components intended for spaceborne instruments may be functionally identical to others for terrestrial applications, but may require special consideration for use in space.

Second Case: Generic Manufacturers

Let’s consider a different class of part, machined parts, and suggest one possible way to set things up for that scenario. Here, you’re the designer of the part and you provide drawings, schematics, and other specifications about how the part will be constructed. You provide this information to a machine shop and they build the part to your specifications.

To best fit the Aligni relationships model, we’ll create a manufacturer called Generic Machined and we may have multiple vendors (machine shops) associated with this manufacturer. With the relationships defined this way, RFQs and POs entered in Aligni can be routed correctly to your machine shop vendors when you have parts for this Generic Machined manufacturer.

When setup this way, there really is no need to use Aligni’s Part Alternates feature. In fact, it cannot be used since there is only one part in consideration.

Inventory, SKUs, and Nuance

The astute reader will recognize an important nuance in the results achieved by the two approaches above. Notably, in the first approach, we end up with two Aligni parts (SKUs) that will be inventoried separately whereas, in the second approach, we have two vendors for the same Aligni part and therefore only one SKU.

In the first case, we decided that it was important to keep the items separated. This is open to debate, but we feel that it is not good practice to use generics in this situation. Here are some reasons to consider:

  • Buyers should not be making engineering decisions. While the specifications may be identical to a buyer’s eye, an engineer may think differently. The results of an incorrect application can range from benign to havoc in the final product: loss of performance, field failures, reduced lifetime, expensive rework, or scrapped material.
  • Separate parts should be lifecycle managed separately. Two manufacturers operate differently and likely have very different schedules for obsolescence or other lifecycle events such as engineering change notifications. As separate parts, these can be monitored more easily and reliably.
  • Partner communication is more precise. When generics are used, there can be some ambiguity when communicating a BOM to manufacturing partners. Since parts and packaging is typically marked with the manufacturer’s information it makes sense to have BOMs reflect these markings.
  • Traceability. With two separate part records, everything from RFQ to purchasing to builds and inventory is tracked more accurately. If you ever need to review or audit for usage, you’ll know exactly which parts were used for which assemblies.

The second case is a good example of a legitimate and unambiguous use of generics. In this case, the generic manufacturer is an abstract concept used to fit operational behaviors to the Aligni relationship model but without compromising efficiency or clarity. If the two machine shops can genuinely create parts that are form, fit, and function compatible, then their outputs can be mixed in inventory and they should fall under the same SKU.

The BOM : Necessary and Sufficient Items

This blog entry also appears as a guest blog with our friends at Octopart. Thanks, Janine!

In the electronics, mechanical, and chemical industries, a Bill of Materials (BOM) is a list of the raw materials that are required to manufacture or formulate an end product. A BOM is most often used to communicate material requirements to manufacturing partners and typically accompanies other data such as schematic drawings, procedural documents, or pick and place data for electronic assemblies.

Necessary Items

A Bill of Materials (BOM) has three necessary components:

  • Manufacturer and Manufacturer P/N – These are used to uniquely identify the item in the world of all components.
  • Quantity and Units – How many parts (and the corresponding unit of measure) are used on your assembly.
  • Reference Designators – Where the parts go on your assembly.
  • These are the essential components. There can be no compromises here. If any of these components is missing from the BOM, it is incomplete.

    But this is also a sufficient set for a BOM. Any additional data can be acquired from other sources and is not required to completely describe the BOM. In fact, including additional data may be detrimental unless the BOM is well-maintained with timely data.

    This is minimal set because if anything is left out, the BOM is incomplete. Let’s go through these one by one.

    Manufacturer and Manufacturer P/N

    The manufacturer and manufacturer part number (MPN) are required to uniquely identify the part. It is common to cheat and use a description for some generic components. For example, some folks find it acceptable to say “10kOhm, 5%, 0402 resistor”. While this may be acceptable for some applications, it is certainly not always acceptable. For any given resistor, the datasheet for that resistor includes a number of parameters. To thoroughly specify the lowly, mundane resistor, you’d have to attach a datasheet. Save everyone some grief and just pick a widely available part and be specific about the manufacturer and MPN. Use PLM software and other methods to specify and link acceptable alternates and substitutes. That way, if the part isn’t available, you can be specific about what to use in its stead.

    In the PLM world, the MPN is basically a SKU (stock keeping unit). It uniquely describes an item of identical form, fit, function, or formulation. If any of these four change, a new SKU is necessarily created.

    What is PLM?

    PLM is, in many ways, the hardware equivalent of the software world’s Source Code Management (SCM). These days, even the most rudimentary software projects are managed under SCM systems such as Git, Subversion, Perforce, and others. SCM puts source code into a much richer context. Tracking changes, branch management, merges, code reviews, and more are all possible with SCM. These things are possible with folders and files but at a significant cost and reduction in operational efficiency. So anyone even remotely serious about software development is using SCM.

    Similarly, PLM puts the hardware design into a richer structure that improves operational efficiency. Part and supplier information are managed in database records to avoid duplication of data. Assembly part lists (BOMs) link to these records and are also managed in database records so that comparison is easily done. Maintenance of project cost and inventory availability is all a free side effect of curating your entire part database.

    Quantity and Units

    Your purchasing folks need to know how many (or how much) of a particular item to purchase for each assembly. Most items will have the units of “each” but some items will require a broader set such as milliliters (adhesives, thermal paste, sealants, formulations), centimeters (wire, tape, heat shrink tubing), and so on.

    Reference Designators

    The term “reference designator” is common in electronics manufacturing and refers to the typical way to call out an instance of a part on a schematic. C1, C2, C3 often refer to capacitors 1, 2, and 3 on the schematic. The prefix (C, R, U, etc) is a convenience and mostly irrelevant, but the designators themselves must be unique within the BOM.

    Mechanical drawings have a similar style, but typically reference drawings of the part rather than the abstract representations seen on electronics schematics. In either case, the result is the same – each part in the CAD of the assembly is uniquely identified and references a unique part on the BOM.

    Sufficient? (or When a BOM Becomes a Time Bomb)

    BOMs can get pretty loaded with additional information. Some things often seen on BOMs are:

    • Item description
    • Component pricing (at various quantities)
    • Supplier information such as distributor(s) and distributor inventory
    • Lead time
    • Vendor part numbers
    • Datasheet links
    • Alternative parts for substitutions

    All of this information is undoubtedly useful in some context. It’s often helpful to know know additional information about an item on a BOM. Purchasing folks would like to know which items will require the longest lead time and what they can expect to pay for them. Engineers would like additional details about the part and reference datasheets. Manufacturers may like to know which parts they could substitute in case they are short on inventory.

    The question is, are any of these required on the BOM and, if not required, should they be there anyway? I argue that they are not required. This is all information that lives elsewhere and is maintained elsewhere. If you’re using a good PLM & MRP software, this information lives in your database. If not, the information lives in a variety of other places such as manufacturer websites, supplier websites, and email interactions. Yes, it’s cumbersome to refer to these other places, but it’s also cumbersome to maintain a BOM that has captured all this data. (hint: this is why good software is helpful)

    With this additional information, a BOM has a half-life and as that information expires, becomes less useful and more tedious to maintain. In fact, stale information can actually be detrimental and more costly than the absence of information.

    Case Study Challenge – Packaging

    Packaging presents a bit of a minor (maybe just annoying) challenge to defining a BOM. A widely seen example is in the distribution of small electronic components where the same fundamental part is sold is different packaging and quantities. Note that I’m not referring to the device package (like SOT-23 vs. QFN). I’m referring to the way the devices are packaged for sale such as plastic tubes, 250-pc tape & reel, or 3,000-pc tape & reel.

    Texas Instruments is a large, well-known electronics component manufacturer. They use a suffix on their MPN to indicate delivery packaging which has absolutely nothing to do with the underlying part. For example, let’s take their TPS2552DBV, a precision adjustable power distribution switch. This device is available in 250-pc tape & reel (TPS2552DBVT) or 3,000-pc tape & reel packaging (TPS2552DBVR). Same part. Same device package. Different spool quantity.

    Why is this a challenge? Because no one from engineering cares (nor do they need to care) what size the delivery packaging is. But when someone places an order with them, they need to know which packaging to send them. If you place an order for 3,000 TPS2552DBV, do you mean you want one TPS2552DBVR or twelve TPS2552DBVT?

    And even the supply chain folks can get confused. Look at the Digi-Key prices for these two different part numbers.



    These are the same part. They have the same form, fit, function, and formulation. No one (even at Texas Instruments) could tell any functional difference between these two parts and yet their price in Digi-Key cut tape quantities is different by about 13%. Consider a situation where your supplier is out of stock on one of these two part numbers with a lead time of 8 weeks but has the other part number in stock. Without this institutional knowledge, you could have a line-down situation for 8 weeks.


    TPS2552DBVHow can this be resolved? This is a common situation that quickly gets out of hand when working with spreadsheet BOMs, but is rather mundane for software to handle. The TPS2552DBV needs to present as a single component to your engineers, but provide additional depth to purchasing folks. Your part database needs to present a unified front: one part that your entire organization can agree is the same part, regardless of how it is packaged for sale.

    TPS2552DBV-VPNOne solution is to allow a single part record to link to multiple vendor part numbers with a brief description of what each one means. For example, the table to the right captures various vendors and their corresponding internal part number representations.

    With this data available, quoting and purchasing software can use this to resolve any ambiguities. More importantly, since this is a known and documented software feature, the personnel handling purchasing know that they’re free to make decisions based on this information, confident that will not affect form, fit, function, or formulation of the final product.