In the previous post we discussed the panel requirements of fab houses and how designs can be optimized for efficiency. If you missed the previous post you can find it below. In this post we’ll take a closer look at the requirements the CM may have to be able to assemble the board. It’s worthwhile to first discuss what the array features will be used for. The grip rails are the dark green features in figure 1 at the top and bottom of the board.
Figure 1: 10-up array with 2 sided grip rails which include tooling holes and fiduals
The main purpose of the grip rails is a place to put the tooling holes and global fiducials on the array without having them on one of the boards. In conveyor type pick and place systems the grip rails provide surface clear of parts where the board edges can sit on the rubber conveyor tracks while it is moved through the assembly system. The tooling holes are used as both a mechanical alignment and way to hold the board firmly in place while the pick and place is stuffing the board. In some machines an operator will load the board into the machine with pins that go through the tooling holes. In some conveyor type systems locator pins come up and secure the board automatically once the array reaches an end stop within the machine.
The global fiducials (small white dots near the tooling holes) are optical markers that can be observed by the pick and place machine and gives the machine an idea of where each individual board is located (and helps compensate any expansion or contraction of the board to help find local fiducials). The mechanical alignment is the first step in finding the global fiducials and the global fiducials is the first step in finding the local fiducials on each board. Global fiducials can be easier for the machine to locate as there aren’t other features on the grip rails that can confuse the machine vision as is more likely on the local fiducials. A minimum of three global fiducials is required by the pick and place machine for proper XY alignment of the machine.
Designing the Features for the Pick and Place Machine
Typically ½” grip rails and ¼” tooling holes are a safe bet to start with when panelizing a board as most CMs can accommodate this. You could then call the CM and find out if the features can be smaller and what their process can accommodate. Inefficiencies due to these manufacturing features are a key reason to panelizing – one set of grip rails for a bunch of boards is more efficient then a set for each board. It also saves time in manufacturing as the machine can be loaded by the array instead of by the board which saves time. In any case reducing the size of the features required for manufacturing results in more boards per panel which decreases board cost. For further information many CMs and fab shops may have existing documentation that provides their standard practices and processes such as design for manufacturing (DFM), design for assembly (DFA) and design for test (DFT) etc.
To V-Score or Not To V-Score
The board in figure 1 as shown includes a v-score to segregate the grip rails and also each individual board. When manufacturing is complete the boards can be broken apart by using a machine that resembles a pizza cutter. The v-score feature for separating the boards provides maximum efficiency in the array because no material (other than the grip rails) is wasted. It does come at a cost though and is not suitable for all situations. The main issue with v-scoring is that during de-panelization the board bends near the v-score and this can damage components. Ceramic capacitors are the most likely to suffer this demise but as a general rule of thumb all components should be at least 5mm from a v-score and more if possible. This keep back distance could reduce efficiencies unless clear edges are required in the design as is the case when the board is mounted in a slotted enclosure. The other option to v-score is tab and route as shown in figure 2.
Figure 2: An array using tab and route instead of v-scoring
In this approach the boards are routed apart as shown in black in the picture and the tab material holds the panel together. The breakaway at the tabs could be v-score or mouse bites and reduces the de-panelization stresses. This approach allows components to be closer to the edge of the board and is also the go to method for boards of an unusual shape. V-scoring alone is reserved for boards that have straight edges on all sides. In cases where over hanging connectors are used it is common to use a mix of route outs and v-scores, route outs on the sides with connectors and v-score on the sides that are straight. One benefit of v-score over mouse bites is that it is possible to create minor delamination of the board during de-panelization and this could lead to premature failure of the board over time due to environmental stresses.
These are some of the many reasons why it’s important for hardware designers to consider the panelization process during the board design. While the fab shop will know how to panelize the design properly, their decisions will be based partly on the features that are on the board. If some components come up to the edge of the board then the panelization approach will change. If the designer keeps the panelization in mind while designing the board it is more likely to be panelization friendly leading to high yields and more efficient use of panel real estate.
Getting a Quote from the Fab Shop
When discussing a board build with the fab shop two considerations are important when trying to get the highest value from a design. The first is allowing X-outs. When the fab shop completes a panel they test the panel using a flying probe tester. If you have never seen one of these things in action it’s worth a visit to Youtube [https://www.youtube.com/watch?v=fjmjYVNuLEE]. The flying probe tester checks for continuity at the end of traces to ensure the etching process didn’t etch through a trace. It also looks for shorts to ensure the etching process successfully removed all the material that it should have. Often times the tester may find a problem in a panel so the fab company will scratch an X through the effected board to indicate that it should not be populated. Allowing X-outs in the panels typically saves 30%-50% on the cost of the fab. The flip side of this is that it is hard to plan small builds as the purchased parts quantity may never match the number of boards available. This means the parts will need to be saved for the next build unless the parts are kitted after the fab is completed and you know how many boards will be available for populating.
A side note about saving on fab costs during prototype builds is getting a min4max quote. Typically these days, every board stack-up is custom which means the fab house can’t use leftover panel area to put other designs on. Getting a min4max quote means you’ll get the whole panel filled with your board and the yield received from the panel is the yield you get. The fab shop charges for the cost of the panel(s) on a per PCB basis. Thus often the overall cost is exactly the same if you were to use only a portion of the panel or the whole panel. If you are lucky enough and your first prototypes are good, you end up with some ‘free’ bare boards you can populate at a later time.
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Trevor Smouter – Fidus Hardware Designer