In general opens are caused by debris or flaws being imaged. These will
generally decrease when the exposure is increased due to undercutting,
refraction, diffusion and reflections.
Shorts are typically the result of light undercutting the original or an
out of contact between the film and the material being exposed. As the
exposure increases or vacuum decreases this will generally increase.
There are several variables to consider:
CLEANLINESS of the exposure area. This includes the dust flaws in the
artwork master, and imperfections in the glass.
CONTACT is the distance between the film and the material to be exposed.
It is affected by the texture of the materials being exposed, the flatness of
the exposure frame, flatness of the work, the cleanliness of the exposure
surfaces and the vacuum draw. It is important to note that more vacuum is not
necessarily better. High vacuum levels tend to cause the materials to seal
around the perimeter and trap air, also in glass tool frames high vacuum can
cause the glass to distort. Even 1/3 vacuum .33 bar or 10" produces a force of
5 lb/in^2 or over 2000 lb over a 18 x 24" panel almost a 1000 kg.
The texture of the materials impacts the ability of air to escape.
Phototools often contain small grain particles on the surface to assist the
air to escape.
CIRCUIT DENSITY: As the circuit density increases the exposure process
window decreases. This further increases the sensitivity to contact and/or
cleanliness.
The PHOTOSENSITIVE MATERIAL being exposed. Dry films and liquids respond
differently. In general liquids have closer contact and are thinner; both of
these characteristics increase the ability to resolve images. Dry films
resolving capabilities vary with thickness of the film, thickness of the cover
sheet and formulation differences. It is also worth noting that the dry film
tends to diffuse the illumination energy that falls on it.
The SPECTRUM and INTENSITY of the light source. Some materials change
characteristics with the spectral profile of the light source. The spectrum
affects the resolution, depth of cure and tackiness.
It has been observed primarily in dry films that the intensity of the
light source plays a role. Higher intensities tend to improve resolution up to
a point where the increased intensity no longer plays a role in increased
resolution and short exposures begin to pose a problem for control.
The OPTICAL SIZE of the light source. Exposure printers fall into three
categories, flood light source, collimated and point light source. Flood
sources tend to undercut debris but are very sensitive to exposure level and
vacuum contact. Collimated light sources are less exposure critical and less
sensitive to vacuum contact but are very sensitive to debris and flaws in the
film and glass. Point source systems provide a compromise that improves yield
and productivity.
The TREATMENT of the surface under the photosensitive material affects the
resolution. As the exposure energy strikes the copper surface it is reflected
and diffused. This surface plays a role.
DEVELOPING and ETCHING play a role in the process window and ultimate
resolution. From and exposure point of view consider the partially cured
border around features that results from soft illumination.
-------------------------------
More detailed comparison of the printer types.
- Flood Systems -
Have reflectors as large as the exposure frame that contain the lamp. The
reflectors are made from a number of flat facets of aluminum that are mirror,
brush, or dimple finish. The lamps are mercury capillary, or medium pressure
arc. These systems produce soft illumination. They are the least sensitive to
dust and debris but are limited in resolution, are very sensitive to vacuum
contact and have a significant difference in performance across the exposure
plane. This is due to the differences in optical sharpness from the center as
compared with the reverse light angles in the edges caused by the side
reflectors. Capillary lamps have from 10 - 15k exposures life.
- Collimated Systems -
Use high pressure short arc lamps in an elliptical reflector that refocuses
the energy on an optical integrator, the rays are then bounced from a
parabolic collimating mirror before striking the exposure plane. The system
has the least sensitivity to out of contact exposures. The tight optical
angles tends to reproduce flaws in the glass, and the artwork. There is also a
high sensitivity to dust and debris. These systems are the more costly to
purchase, and have a higher operating cost due to lamp cost and life, power
consumption, air conditioning load and the required cleanliness of the
environment. Yields are often a trade of shorts for opens. Most of the
benefits of the collimation is lost due to the optical diffusion of the
photosensitive materials and reflections from the copper. Lamps have a 750h to
1000h life, there is an explosion possibility if lamps are run too long.
- Point Source Systems -
Have a compact optical system, lamp plus reflector that is typically 25 cm
square. These systems use medium pressure additive lamps that can be
manufactured with a spectrum which optimizes the resolution of the
photoresist. They provide a compromise between flood and collimation in
optical sharpness. On a scale of 1 to 10 with collimation being 1 and flood
systems rating a 10, the optical sharpness of the point source is 3. The
higher intensity and favorable spectrum enhance the resolution. As a result of
the increased resolution over flood systems in combination with a reduced
sensitivity to flaws and debris of collimated system, the point system tends
to produce at higher yields. Lamps are the least costly and have a 1000 -
2500 hour life this relates to about 50,000 - 250,000 exposure depending on
the production rate.
----------
From: indal
Sent: Friday, April 11, 1997 9:29 PM
To: [log in to unmask]
Subject: OPENS AND SHORTS
During PCB fabrication opens and shorts are major defects observed at ET. we
conducted a DOE on a specific partnumber and found that every step change at
primary imaging has a great degree of corrilation to Opens and Shorts. However
the photoresist manufacturer specs are very wide 5 to 8 SST or 12 to 18
RST.Why are we finding a huge quality difference inside the specification ?
How are you taking care of the above?
PLEASE COMMENT AT [log in to unmask]
***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]> *
* with <subject: subscribe/unsubscribe> and no text in the body. *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] *
***************************************************************************
|
