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March 2, 2015 AT 8:46 pm

Secret Tools of the Old Crow: Tab Routing and You

Hey folks, the Old Crow here.  When a project requires multiple instances of a circuit board, the designer will often elect to use a “panel array.”  This means the CAM files generated by the CAD software have been set to include information that allows a board house to create multiple copies of the board in an X-Y fashion on a given piece of board stock.  There are a number of advantages to requesting boards as an array, but the chief among those is it makes automated assembly much more efficient as the program a pick/place machine operator enters to place parts can simply be set to “step and repeat” the placement profile to a reference location of each board in the array.  Then the populated board array can be taken through the reflow oven to solder many boards at once.

When it comes time to depanelize the boards, that is, cut them into their individual units, the boards are either cut with a depanelizing blade in the case of edge-scored board arrays or with a depanelizing punch in the case of what is known as “tab-routing.”  Edge-scoring uses a fine-tip milling bit to score each side of the board material according to the dimensions of each board unit. Tab routing  uses a milling bit to route a channel around each board in the array in order to create the individual units, but in order to maintain it as an array small ‘tabs’ are left at various points around each unit.  This way the array dimensions are still accurate but depanelizing is simple.  To get an idea of the hardware involved, here are my blade and punch as well as examples of scored and tab-routed board panels.  There are some partially depanelized boards on the table in front of the tools as well.

1_depanel_detail_lg

2_scoredroute

3_N200-PCB-Depaneling-Punch

In this (blurry) image the large blue device is a scored-route cutting blade: you set the backstop to align a score, and move the blade head to the other side.  Do this for each panel and score line.  For tab-routed board the punch is used, which is the silver cylinder to the right.  The punch is air-operated: the blue air hose connects the shop air system to a floor pedal, one sets the panel on the punch deck (flat silver surface above the cylinder), align the punch hook with a tab, and give the floor pedal a nudge.  The punch cuts the tab away and this is repeated for all tabs.

There are a couple of caveats to consider when panelizing boards.  One is to make sure there is enough clearance to allow the depanelizing tools to operate, which means parts placement in the layout phase is a factor.  Also, while most board houses can determine during manufacture where to optimally locate tabs for the tab route specification, sometimes the designer needs to step in to avoid problems such as this one I had in 2011.

4_tabxtal

This board had a 25% failure rate when being tested, and it was discovered the clock generator was not starting.  The crystal turned out to be bad as replacing it cured the issue.  The problem was in trying to determine why it failed.  As it turns out, the close proximity of the tab to the crystal was the problem as the shock of the punch cutting the tab caused the crystal to crack internally 25% of the time.  This tab was board vendor-placed, so a custom tab placement request was submitted and the problem went away. So, even something as innocuous as a board tab can cause problems if given the right (or perhaps wrong) setup.  Cheers!

 


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