LegendPolarity Marking

IPC does not recommend any standardlegend polarity marking and it’s up to the PCB designer to choose what is bestfor their personal practice.

The goal of the Polarity Marking Legend is to aid assembly toavoid polarized component packages from being inverted during the assemblymachine setup or manual solder process. Therefore, Polarity Marking is only necessaryon land patterns that require a specific rotation during the assembly process.For very dense part placements, the polarity marker can be placed under thepackage and covered up during the assembly process. However, the best practiceis to locate the polarity indicator outside the package so that it is visibleafter the assembly process to allow the end user to visually validate that theassembly insertion process is correct. This is typically known as “PostAssembly Inspection Process”.

Polarity Markings are unique from company to company. Here aresamples of the most popular shapes.

The size, rotation and location of the Polarity Markings are userdefinable. Here are some recommendations.

The most popular polarity marking is a filled Dot. The size of thePolarity Marking is relevant to the size of the component package and padwidth. However, the polarity dot size is also dependent on the part placementdensity and the assembly shops ability to easily locate the dot to avoidcomponent rotation insertion errors. The smallest visible markings range from0.30 mm – 0.40 mm and are typically used for micro-miniature packages or verydense part placement PCB layouts. The average size ranges are 0.50 mm – 0.80mm. The largest recommended polarity dot is 1.00 mm. Sometimes, a polarity Lineis better than a Dot as it is visible and does not take up PCB real estate space.

This illustration indicates 3 popular locations for a 0.50 mm doton an SOP package. The preferred polarity dot placement is at the end of thepad which is the furthest distance from the component package body. This makesthe post assembly inspection process easy, as the component package may move upor down during assembly reflow but the dot will always be visible. 

The component body outline legend line width should be a minimumof 0.12 mm. The Polarity Marking Symbol and component body legend should have aminimum 0.75 mm gap from the solder mask. The figure below illustrates theanatomy of a land pattern and feature sizes and spaces.

The Triangle Polarity Marking is also popular as it has a uniqueshape that is easily identified.

The Line Polarity Marking is also popular because it takes up theleast amount of PCB real estate and runs along the entire side of the pad thatis visible after assembly.

Here are sample of SOT23 patterns with Polarity Lines:

Note: check with your PCB manufacturer to verify if they can meet yourdimensional requirements for all land pattern features.

Bottom only termination packages – BGA, LGA, CGA, QFN, PQFN, SON,PSON and DFN

The polarity marking size should match the Terminal Width. The gapbetween the body legend and the polarity marker should range from 0.15 mm –0.25 mm

Here is an example of Polarized chip capacitors packages.Normally, the plus + sign is used to indicate a Capacitor, but if you arestandardizing on a Dot polarity marker, you may consider the Filled Dot.However, the Line polarity marker takes up less space for high density designs.To obtain the highest level of symmetry in your PCB layouts, all polarity markingshapes, sizes and locations should be documented as your company standard.

The Polarity Line can be the same line thickness as the bodylegend outlines or thicker, but not thinner.

AssemblyPolarity Marking

The Assembly Drawing Polarity Marking can be accomplished by usinga chamfered outline in the corner where pin 1 is located:

Here is an example of a custom connector Legend and Assemblypolarity marking. The black lines are Legend and the Yellow lines are Assembly.

A round filled dot is also an acceptable Assembly Polarity Marker.