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Recommendations 


Desplegar Recommendations for design and manufacture printed circuit boards with S.M.D.

Printed Circuit Board:
Alco LogoArea soldering (types of finishes)
Alco LogoMounting holes
Alco Logo Special marks
Alco LogoMinimum distances to borders card
Alco Logo Panelize
Alco LogoExterior finish of cards (cut)

Printed Circuit:

It is the physical support for the electronic components placing both conventional as SMD

Typically, the circuit boards that are to be assembled by automated machine, should be prepared and designed thinking that will move through conveyors or rails, the outer edges being those who are regularly exposed to these elements.


Finish welding areas (types of finishes) [TOP]

Both SMT reflow, as for welding, wave soldering machine, the surface of the weld area must be very flat. Uneven surfaces in welding areas cause the components do not, in many cases, contact with the pad and allow no welding is correct. In many cases, these points will unsoldered.

 



No seat pin with solder paste (defective welds, low reliability)

 

Of all the systems that exist to tin the solder pad, we recommend not to use the so-called impregnation roll tin. This system causes the tin does not deposited uniformly on the solder pad.

Any other system technologies to deposit tin uniformly valid for this process, being most recommended methods for electrochemical deposition (Sn, Ni, Au, Ag Chemical) than mecónica nature, like HAL (Hot Air Leveling).


Mounting holes [TOP]

It is advisable to consider include two to four holes of 3 to 4 mm in diameter, separated from the outside edge between 5 and 15mm center, and if possible, spaced from one another along the same line.

If the card has already because of some component albeit different hole size, and is more or less at the edge of the plate, they could also be used as mounting holes.


Fiducial Marks [TOP]

Normally, SMD machines may use different fiducial marks for quality assurance on assembly and loss of materials. These marks are usually several types, circles, squares and crosses, the most reliable the circle.

 






Example fiducial mark type circle, within the circuit area

 

This type of markings should be located in an area of ​​the pcb where no reflections occur for nodes that may be nearby. The inner circle must be metallized, while remaining annulus unmetallized should go without mask.

This mark is used so that the machine can correct board to board any deviations that occur because in most cases the irregular cut the boards in the factory circuit. This causes the reference distances vary and machine in these cases mount components with slight displacements.

In each card must be MINIMO two marks of this type of equal size and features, located in one of the diagonals of the card and as far away from each other.

It must be taken into consideration the outer diameter of the fiducial for this does not invade the "displacement zone" of 3 or 3.5 mm. from edge.


Minimum distance to edge of card [TOP]

As a rule, in their own designs, avoid that no component less than 4 mm from the outer edges of the card. The reason is as described in the first part. Recommended in case you need to add connectors or components that can not meet this standard, try putting on parallel sides and no sides that generate contiguous storage problems and Welding.


Panelization or paneling [TOP]

This system is used to mount the same time, a greater number of cards of relatively small size and to reduce the unit cost of assembly.

The measure recommended maximum is approximately 300 mm. wide by 400 mm long, these being the maximum external boundaries between the most distant nodes. An optimum size panel is close to a sheet A4 + / - 25 mm.

The Scoring (precut) panel is recommended to prevent breakage easily both in assembly and welding.

If size, there are components that do not keep the minimum edge distance (3 mm.) Is recommended to add to the additional strips edges of between 5 and 10 mm. wide. These strips come in handy for fixing holes recommended above points.

It is also necessary to include strips of material between the circuits, when the components of the circuit can stand "and invade" the space of the adjacent circuit. This often occurs in those circuits using elbow connectors.

 



Photo of a panel of 4 circuits. The totally flat surface finish (Sn Chemical), the outer edges, the pre-cut type scoring, the fixing holes on the flanges, and the fiducial marks on two opposite corners is observed. Flanges between circuits are not necessary.


Vias [TOP]

For multilayer interconnect circuits in any board, the use of side pathways or vias are necessary. By putting discovered and unmasked side of Pads way, the well known "sink effect" occurs: the liquid solder is "strained" by the drill due to capillarity and gravity. This causes defective and unreliable welds having no meniscus to ensure sufficient rigidity and durability.

 



Picture routes with high risk of causing the sink effect on adjacent pads. You need them away from the pad, or at least reduce its diameter and cover with anti-weld mask..

 

 

 

 

 

 

 

 

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