Heatsinks absorb and dissipate heat from an electronic component through contact. with a metal base or thermal interface material. If the pastemask openings are too large in the heat sink. once the solderpaste melts it could cause components to float off the pads. To prevent this, reduce how much paste put down on a heat pad—instead of one large paste mask opening. try breaking it up into several, smaller paste mask openings. This will help to ensure components will not float and crash into other parts. during the oven process. which can shorts.
Pastemask openings are an essential part of DFM checks. Before sending off to manufacture. ask yourself: Do all the component leads on the PCB have the proper pastemask. opening (and size) for the stencil?
Take care not to create etch traps when routing circuit traces. Determine 15 shows types of etch capture routing recommendations. Another problem with fine pitch PCB design is to ensure traces and vias do not draw heat from the solder pad.
Physique 15. Etch Trap Routing Guidelines
Small BGA pads don't have much solder and rely on uniform heating system for a good joint. Below are a few tips to ensure that traces and vias do not provide as heatsinks.
•Keep the track smaller than the pad or via.
•Do not gang up pads with a copper put such as around a floor plane without thermal reliefs
•Use person traces to interconnect the pads.
TI applications processors are relatively low-power devices therefore the smaller traces work.
Other parts in PCB design
When we are discussing Acid Traps. we are describing acute or odd angles of copper features on the printed circuit board. which cause acid to pool during the PCB creation process. This issue occurs before the wash process. where acid traps allow for residual acid to get caught in these areas and not get cleared away. As a result. the wanted copper features the Gerber file. included on the board begin to erode, creating “opens”. or lost copper connections.
Avoiding acid traps are especially important with the 4 or 5 mil traces on today’s designs. Since they are so thin, they can become opens (openings created. within wanted copper due to trapped acid). Some software has built in checks for these. but, if your software does not, you will have to test the board for anything that could cause this.
How to prevent Acid Traps: Avoid placing the trace coming into a pad at an acute or odd angle. Keep angles 45 or 90 degrees next to the pad.
Slivers and Islands
Slivers and islands are free floating copper on many plane layers . that can create some serious problem in the acid tank. Several issues can occur because of this. The small specks of copper have known to float off the PCB panel. and find their way to other etch on the panel creating a short. Or, if they are large enough to not be able to float off it becomes your antenna. which can cause noise and other interference. within the board. (because its copper with nothing grounding it—it becomes a signal gatherer). Some software can search for these within your design, but. if your software does not have this capability. you will have to find and remove them from the board design.
Solder Bridges Forming Between Pins
With etch traces being so fine-lined and pin-pitch so tight. it is essential to include soldermasks within a PCB design. Excluding soldermasks can result. in a large blob of solder (especially between pins) during assembly–resulting in a short. Besides. it can also cause reduced corrosion protection on other copper on the outside layers. To help prevent these issues, be sure to check the alignment. soldermask to soldermask (webbing) spacing. from pad to etch lines and to shapes. Also, be sure there is no soldermask covering pins—your board house can. inform you on the minimum webbing space and alignment they will allow.
Cold Solder Joints or No Solder Connection
Checks for vias within pads are essential—if the via is not placed. it could allow solder paste to flow down into the via. This will result in a cold solder joint or no real solder connection. You need to find out: What percentage of the via allowed in the pad before plugging the via is a need.
Most software should be capable of checking for these issues. but if yours does not, you will have to check your design to be sure. it is in compliance with your assembly house criteria.
Test Points are Not Included
It is important to include a way to test your final product once they are off the assembly lines. by including test points in your initial design. you are providing a way to audit the success or failure of a board once it has completed. DFM checks must include test point to component clearances, pad size, under components. and a way to lock down these locations once a fixture built.
Test point data is then used to create a fixture known as the Bed of Nails Tester. A bed of nails tester is a software system that locks your test point locations down within the design. They provide you with the ability to allow for a design change to reworked into this test fixture. saving money.
If you wait to include test points until after your prototype has completed. you could alter the electronics on the board. (which could create crosstalk, noise, and a whole host of other issues). and so not test the board’s true functionality. You will be changing the design and how the board operates. By incorporating the test points into your board during the design phase. it provides you with the ability to lock down what’s there and only modify any changes.
Items to keep in mind when adding test points to your design: Are they accessible? Does the DM checker make sure your points are not hidden? How is the pin spacing (be sure they are not too close together)?
Copper to Board Edge
Part of the PCB creation process includes the motorized. transport of boards into acid. and wash baths. The copper to board edge is the space on the grip the side of the PCB panels[CB14]. used to transport the boards throughout the manufacturing process. If the proper spacing between the copper to board edge. is not set true manufacturing issues can created. If the copper is too close to the edge of the board, shorts can created. when the electrical current applied to the panel during the etching processes.
The consequences of manufacturing failure are not only frustrating, but costly. Planning for the future by designing for manufacture is one of the. many ways to avoid running into any DFM issues. The good news is, many of the issues listed above can identified using software (OrCAD 17.2. comes armed with OrCAD DesignTrueTM In-Design DFM Technology). but, if your software does not have DFM checking capabilities. you will have to identify and resolve them yourself.
The last thing any engineer wants is to receive “the call” from. their manufacturer stating their board has failed their DFM checks. so it is important to look for the above issues in your final checks. whether that’s through automated or manual checks.
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