FAQ about printed circuit boards

Below we have listed some of the most common questions we receive and everyday discussion topics. You can also ask your PCB question here.


What is a microvia?

According to the new definition within IPC-T-50M a microvia is a blind structure with a maximum aspect ratio of 1:1, terminating on a target land with a total depth of no more than 0.25mm measured from the structure’s capture land foil to the target land.

What is meant by a blind via hole?

It is a hole that runs from an outer layer to the inner layer, but not through the entire PCB. These holes can be drilled mechanically or using laser technology. The image shows a laser drilled blind via.

What is meant by a buried via hole?

This is a hole that runs between one or more inner layers. They are normally mechanically drilled.


What type of via hole plugging is recommended?

The preferred type of plugging for standard product (not including capped via hole) is IPC 4761 type VI filled and covered, with target being complete fill. The image below shows type VI with liquid soldermask coverage.

Single sided plugging is not recommended (including type II tented and covered) due to concerns over entrapment of chemistry or likelihood of solderballs being present with HASL finishes (LF and SnPb).


Do I have to use an FR4 material with a high Tg (Tg = glass transition temperature) for lead-free soldering?

No, not necessarily. There are many factors to be taken into account, e.g. how many layers, the thickness of the PCB and also a good understanding of the assembly process (number of soldering cycles, time above 260 degrees, etc.). Some research has shown that a material with a “standard” Tg value has even performed better than some materials with a higher Tg value. Note that even with “leaded” soldering the Tg value is exceeded.
What is of most importance is how the material behaves at temperatures above the Tg value (post Tg) so knowing the temperature profiles the board will be subjected to will help you look evaluate the necessary performance characteristics.

Which PCB surface is best for lead-free soldering?

There is no “best surface;” all surfaces have their pros and cons. Which one you should choose depends on many factors. Please consult our technicians or review the information on surface finishes within this section of the website.

How many reflow cycles can FR4 materials withstand?

It is hard to give a precise answer, but we have made tests with material with up to 22 reflows, four of these with a peak temperature of 270C°. The stress after 22 reflows is considerable and material can degrade, but all connections remained functional. Our recommendation is to choose a higher grade material where there are more than 6 layers and thicker than 1.6 mm.


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